The Unstable Book

Welcome to the Unstable Book! This book consists of a number of chapters, each one organized by a "feature flag." That is, when using an unstable feature of Rust, you must use a flag, like this:

#![feature(coroutines, coroutine_trait)]

use std::ops::{Coroutine, CoroutineState};
use std::pin::Pin;

fn main() {
    let mut coroutine = || {
        yield 1;
        return "foo"
    };

    match Pin::new(&mut coroutine).resume(()) {
        CoroutineState::Yielded(1) => {}
        _ => panic!("unexpected value from resume"),
    }
    match Pin::new(&mut coroutine).resume(()) {
        CoroutineState::Complete("foo") => {}
        _ => panic!("unexpected value from resume"),
    }
}

The coroutines feature has a chapter describing how to use it.

Because this documentation relates to unstable features, we make no guarantees that what is contained here is accurate or up to date. It's developed on a best-effort basis. Each page will have a link to its tracking issue with the latest developments; you might want to check those as well.

Compiler flags

branch-protection

This option lets you enable branch authentication instructions on AArch64. This option is only accepted when targeting AArch64 architectures. It takes some combination of the following values, separated by a ,.

  • pac-ret - Enable pointer authentication for non-leaf functions.
  • leaf - Enable pointer authentication for all functions, including leaf functions.
  • b-key - Sign return addresses with key B, instead of the default key A.
  • bti - Enable branch target identification.

leaf and b-key are only valid if pac-ret was previously specified. For example, -Z branch-protection=bti,pac-ret,leaf is valid, but -Z branch-protection=bti,leaf,pac-ret is not.

Rust's standard library does not ship with BTI or pointer authentication enabled by default. In Cargo projects the standard library can be recompiled with pointer authentication using the nightly build-std feature.

cf-protection

The tracking issue for this feature is: #93754.


This option enables control-flow enforcement technology (CET) on x86; a more detailed description of CET is available here. Similar to clang, this flag takes one of the following values:

  • none - Disable CET completely (this is the default).
  • branch - Enable indirect branch tracking (IBT).
  • return - Enable shadow stack (SHSTK).
  • full - Enable both branch and return.

This flag only applies to the LLVM backend: it sets the cf-protection-branch and cf-protection-return flags on LLVM modules. Note, however, that all compiled modules linked together must have the flags set for the compiled output to be CET-enabled. Currently, Rust's standard library does not ship with CET enabled by default, so you may need to rebuild all standard modules with a cargo command like:

$ RUSTFLAGS="-Z cf-protection=full" cargo +nightly build -Z build-std --target x86_64-unknown-linux-gnu

Detection

An ELF binary is CET-enabled if it has the IBT and SHSTK tags, e.g.:

$ readelf -a target/x86_64-unknown-linux-gnu/debug/example | grep feature:
      Properties: x86 feature: IBT, SHSTK

Troubleshooting

To display modules that are not CET enabled, examine the linker errors available when cet-report is enabled:

$ RUSTC_LOG=rustc_codegen_ssa::back::link=info rustc-custom -v -Z cf-protection=full -C link-arg="-Wl,-z,cet-report=warning" -o example example.rs
...
/usr/bin/ld: /.../build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-d73f7266be14cb8b.rlib(std-d73f7266be14cb8b.std.f7443020-cgu.12.rcgu.o): warning: missing IBT and SHSTK properties

codegen-backend

The tracking issue for this feature is: #77933.


This feature allows you to specify a path to a dynamic library to use as rustc's code generation backend at runtime.

Set the -Zcodegen-backend=<path> compiler flag to specify the location of the backend. The library must be of crate type dylib and must contain a function named __rustc_codegen_backend with a signature of fn() -> Box<dyn rustc_codegen_ssa::traits::CodegenBackend>.

Example

See also the hotplug_codegen_backend test for a full example.

use rustc_codegen_ssa::traits::CodegenBackend;

struct MyBackend;

impl CodegenBackend for MyBackend {
   // Implement codegen methods
}

#[no_mangle]
pub fn __rustc_codegen_backend() -> Box<dyn CodegenBackend> {
    Box::new(MyBackend)
}

Unstable codegen options

All of these options are passed to rustc via the -C flag, short for "codegen". The flags are stable but some of their values are individually unstable, and also require using -Z unstable-options to be accepted.

linker-flavor

In addition to the stable set of linker flavors, the following unstable values also exist:

  • ptx: use rust-ptx-linker for Nvidia NVPTX GPGPU support.
  • bpf: use bpf-linker for eBPF support.
  • llbc: for linking in llvm bitcode. Install the preview rustup componentsllvm-bitcode-linker and llvm-tools to use as a self-contained linker by passing -Zunstable-options -Clink-self-contained=+linker together with -Clinker-flavor=llbc. Can currently only be used for Nvidia NVPTX targets (nvptx64-nvidia-cuda).

Additionally, a set of more precise linker flavors also exists, for example allowing targets to declare that they use the LLD linker by default. The following values are currently unstable, and the goal is for them to become stable, and preferred in practice over the existing stable values:

  • gnu: unix-like linker with GNU extensions
  • gnu-lld: gnu using LLD
  • gnu-cc: gnu using a C/C++ compiler as the linker driver
  • gnu-lld-cc: gnu using LLD and a C/C++ compiler as the linker driver
  • darwin: unix-like linker for Apple targets
  • darwin-lld: darwin using LLD
  • darwin-cc: darwin using a C/C++ compiler as the linker driver
  • darwin-lld-cc: darwin using LLD and a C/C++ compiler as the linker driver
  • wasm-lld: unix-like linker for Wasm targets, with LLD
  • wasm-lld-cc: unix-like linker for Wasm targets, with LLD and a C/C++ compiler as the linker driver
  • unix: basic unix-like linker for "any other Unix" targets (Solaris/illumos, L4Re, MSP430, etc), not supported with LLD.
  • unix-cc: unix using a C/C++ compiler as the linker driver
  • msvc-lld: MSVC-style linker for Windows and UEFI, with LLD
  • em-cc: emscripten compiler frontend, similar to wasm-lld-cc with a different interface

This flag generally controls whether the linker will use libraries and objects shipped with Rust instead of those in the system. The stable boolean values for this flag are coarse-grained (everything or nothing), but there exists a set of unstable values with finer-grained control, -Clink-self-contained can accept a comma-separated list of components, individually enabled (+component) or disabled (-component):

  • crto: CRT objects (e.g. on windows-gnu, musl, wasi targets)
  • libc: libc static library (e.g. on musl, wasi targets)
  • unwind: libgcc/libunwind (e.g. on windows-gnu, fuchsia, fortanix, gnullvm targets)
  • linker: linker, dlltool, and their necessary libraries (e.g. on windows-gnu and for rust-lld)
  • sanitizers: sanitizer runtime libraries
  • mingw: other MinGW libs and Windows import libs

Out of the above self-contained linking components, linker is the only one currently implemented (beyond parsing the CLI options).

It refers to the LLD linker, built from the same LLVM revision used by rustc (named rust-lld to avoid naming conflicts), that is distributed via rustup with the compiler (and is used by default for the wasm targets). One can also opt-in to use it by combining this flag with an appropriate linker flavor: for example, -Clinker-flavor=gnu-lld-cc -Clink-self-contained=+linker will use the toolchain's rust-lld as the linker.

control-flow-guard

The tracking issue for this feature is: #68793.


The rustc flag -Z control-flow-guard enables the Windows Control Flow Guard (CFG) platform security feature.

CFG is an exploit mitigation designed to enforce control-flow integrity for software running on supported Windows platforms (Windows 8.1 onwards). Specifically, CFG uses runtime checks to validate the target address of every indirect call/jump before allowing the call to complete.

During compilation, the compiler identifies all indirect calls/jumps and adds CFG checks. It also emits metadata containing the relative addresses of all address-taken functions. At runtime, if the binary is run on a CFG-aware operating system, the loader uses the CFG metadata to generate a bitmap of the address space and marks those addresses that contain valid targets. On each indirect call, the inserted check determines whether the target address is marked in this bitmap. If the target is not valid, the process is terminated.

In terms of interoperability:

  • Code compiled with CFG enabled can be linked with libraries and object files that are not compiled with CFG. In this case, a CFG-aware linker can identify address-taken functions in the non-CFG libraries.
  • Libraries compiled with CFG can linked into non-CFG programs. In this case, the CFG runtime checks in the libraries are not used (i.e. the mitigation is completely disabled).

CFG functionality is completely implemented in the LLVM backend and is supported for X86 (32-bit and 64-bit), ARM, and Aarch64 targets. The rustc flag adds the relevant LLVM module flags to enable the feature. This flag will be ignored for all non-Windows targets.

When to use Control Flow Guard

The primary motivation for enabling CFG in Rust is to enhance security when linking against non-Rust code, especially C/C++ code. To achieve full CFG protection, all indirect calls (including any from Rust code) must have the appropriate CFG checks, as added by this flag. CFG can also improve security for Rust code that uses the unsafe keyword.

Another motivation behind CFG is to harden programs against return-oriented programming (ROP) attacks. CFG disallows an attacker from taking advantage of the program's own instructions while redirecting control flow in unexpected ways.

Overhead of Control Flow Guard

The CFG checks and metadata can potentially increase binary size and runtime overhead. The magnitude of any increase depends on the number and frequency of indirect calls. For example, enabling CFG for the Rust standard library increases binary size by approximately 0.14%. Enabling CFG in the SPEC CPU 2017 Integer Speed benchmark suite (compiled with Clang/LLVM) incurs approximate runtime overheads of between 0% and 8%, with a geometric mean of 2.9%.

Testing Control Flow Guard

The rustc flag -Z control-flow-guard=nochecks instructs LLVM to emit the list of valid call targets without inserting runtime checks. This flag should only be used for testing purposes as it does not provide security enforcement.

Control Flow Guard in libraries

It is strongly recommended to also enable CFG checks for all linked libraries, including the standard library.

To enable CFG in the standard library, use the cargo -Z build-std functionality to recompile the standard library with the same configuration options as the main program.

For example:

rustup toolchain install --force nightly
rustup component add rust-src
SET RUSTFLAGS=-Z control-flow-guard
cargo +nightly build -Z build-std --target x86_64-pc-windows-msvc
rustup toolchain install --force nightly
rustup component add rust-src
$Env:RUSTFLAGS = "-Z control-flow-guard"
cargo +nightly build -Z build-std --target x86_64-pc-windows-msvc

Alternatively, if you are building the standard library from source, you can set control-flow-guard = true in the config.toml file.

coverage-options

This option controls details of the coverage instrumentation performed by -C instrument-coverage.

Multiple options can be passed, separated by commas. Valid options are:

  • branch or no-branch: Enables or disables branch coverage instrumentation.

debug-info-for-profiling


Introduction

Automatic Feedback Directed Optimization (AFDO) is a method for using sampling based profiles to guide optimizations. This is contrasted with other methods of FDO or profile-guided optimization (PGO) which use instrumented profiling.

Unlike PGO (controlled by the rustc flags -Cprofile-generate and -Cprofile-use), a binary being profiled does not perform significantly worse, and thus it's possible to profile binaries used in real workflows and not necessary to construct artificial workflows.

Use

In order to use AFDO, the target platform must be Linux running on an x86_64 architecture with the performance profiler perf available. In addition, the external tool create_llvm_prof from this repository must be used.

Given a Rust file main.rs, we can produce an optimized binary as follows:

rustc -O -Zdebug-info-for-profiling main.rs -o main
perf record -b ./main
create_llvm_prof --binary=main --out=code.prof
rustc -O -Zprofile-sample-use=code.prof main.rs -o main2

The perf command produces a profile perf.data, which is then used by the create_llvm_prof command to create code.prof. This final profile is then used by rustc to guide optimizations in producing the binary main2.

default-hidden-visibility

The tracking issue for this feature is: https://github.com/rust-lang/compiler-team/issues/656


This flag can be used to override the target's default_hidden_visibility setting. Using -Zdefault_hidden_visibility=yes is roughly equivalent to Clang's -fvisibility=hidden cmdline flag.

direct_access_external_data

The tracking issue for this feature is: https://github.com/rust-lang/compiler-team/issues/707


Option -Z direct-access-external-data controls how to access symbols of external data.

Supported values for this option are:

  • yes - Don't use GOT indirection to reference external data symbols.
  • no - Use GOT indirection to reference external data symbols.

If the option is not explicitly specified, different targets have different default values.

dump-mono-stats


The -Z dump-mono-stats compiler flag generates a file with a list of the monomorphized items in the current crate. It is useful for investigating compile times.

It accepts an optional directory where the file will be located. If no directory is specified, the file will be placed in the current directory.

See also -Z dump-mono-stats-format and -Z print-mono-items. Unlike print-mono-items, dump-mono-stats aggregates monomorphized items by definition and includes a size estimate of how large the item is when codegened.

See https://rustc-dev-guide.rust-lang.org/backend/monomorph.html for an overview of monomorphized items.

dump-mono-stats-format


The -Z dump-mono-stats-format compiler flag controls what file format to use for -Z dump-mono-stats. The default is markdown; currently JSON is also supported. JSON can be useful for programmatically manipulating the results (e.g. to find the item that took the longest to compile).

dwarf-version

This option controls the version of DWARF that the compiler emits, on platforms that use DWARF to encode debug information. It takes one of the following values:

  • 2: DWARF version 2 (the default on certain platforms, like macOS).
  • 4: DWARF version 4 (the default on certain platforms, like Linux).
  • 5: DWARF version 5.

dylib-lto

This option enables using LTO for the dylib crate type. This is currently only used for compiling rustc itself (more specifically, the librustc_driver dylib).

emit-stack-sizes

The tracking issue for this feature is: #54192


The rustc flag -Z emit-stack-sizes makes LLVM emit stack size metadata.

NOTE: This LLVM feature only supports the ELF object format as of LLVM 8.0. Using this flag with targets that use other object formats (e.g. macOS and Windows) will result in it being ignored.

Consider this crate:

#![crate_type = "lib"]

use std::ptr;

pub fn foo() {
    // this function doesn't use the stack
}

pub fn bar() {
    let xs = [0u32; 2];

    // force LLVM to allocate `xs` on the stack
    unsafe { ptr::read_volatile(&xs.as_ptr()); }
}

Using the -Z emit-stack-sizes flag produces extra linker sections in the output object file.

$ rustc -C opt-level=3 --emit=obj foo.rs

$ size -A foo.o
foo.o  :
section                                 size   addr
.text                                      0      0
.text._ZN3foo3foo17he211d7b4a3a0c16eE      1      0
.text._ZN3foo3bar17h1acb594305f70c2eE     22      0
.note.GNU-stack                            0      0
.eh_frame                                 72      0
Total                                     95

$ rustc -C opt-level=3 --emit=obj -Z emit-stack-sizes foo.rs

$ size -A foo.o
foo.o  :
section                                 size   addr
.text                                      0      0
.text._ZN3foo3foo17he211d7b4a3a0c16eE      1      0
.stack_sizes                               9      0
.text._ZN3foo3bar17h1acb594305f70c2eE     22      0
.stack_sizes                               9      0
.note.GNU-stack                            0      0
.eh_frame                                 72      0
Total                                    113

As of LLVM 7.0 the data will be written into a section named .stack_sizes and the format is "an array of pairs of function symbol values (pointer size) and stack sizes (unsigned LEB128)".

$ objdump -d foo.o

foo.o:     file format elf64-x86-64

Disassembly of section .text._ZN3foo3foo17he211d7b4a3a0c16eE:

0000000000000000 <_ZN3foo3foo17he211d7b4a3a0c16eE>:
   0:   c3                      retq

Disassembly of section .text._ZN3foo3bar17h1acb594305f70c2eE:

0000000000000000 <_ZN3foo3bar17h1acb594305f70c2eE>:
   0:   48 83 ec 10             sub    $0x10,%rsp
   4:   48 8d 44 24 08          lea    0x8(%rsp),%rax
   9:   48 89 04 24             mov    %rax,(%rsp)
   d:   48 8b 04 24             mov    (%rsp),%rax
  11:   48 83 c4 10             add    $0x10,%rsp
  15:   c3                      retq

$ objdump -s -j .stack_sizes foo.o

foo.o:     file format elf64-x86-64

Contents of section .stack_sizes:
 0000 00000000 00000000 00                 .........
Contents of section .stack_sizes:
 0000 00000000 00000000 10                 .........

It's important to note that linkers will discard this linker section by default. To preserve the section you can use a linker script like the one shown below.

/* file: keep-stack-sizes.x */
SECTIONS
{
  /* `INFO` makes the section not allocatable so it won't be loaded into memory */
  .stack_sizes (INFO) :
  {
    KEEP(*(.stack_sizes));
  }
}

The linker script must be passed to the linker using a rustc flag like -C link-arg.

// file: src/main.rs
use std::ptr;

#[inline(never)]
fn main() {
    let xs = [0u32; 2];

    // force LLVM to allocate `xs` on the stack
    unsafe { ptr::read_volatile(&xs.as_ptr()); }
}
$ RUSTFLAGS="-Z emit-stack-sizes" cargo build --release

$ size -A target/release/hello | grep stack_sizes || echo section was not found
section was not found

$ RUSTFLAGS="-Z emit-stack-sizes" cargo rustc --release -- \
    -C link-arg=-Wl,-Tkeep-stack-sizes.x \
    -C link-arg=-N

$ size -A target/release/hello | grep stack_sizes
.stack_sizes                               90   176272

$ # non-allocatable section (flags don't contain the "A" (alloc) flag)
$ readelf -S target/release/hello
Section Headers:
  [Nr]   Name              Type             Address           Offset
       Size              EntSize            Flags  Link  Info  Align
(..)
  [1031] .stack_sizes      PROGBITS         000000000002b090  0002b0f0
       000000000000005a  0000000000000000   L       5     0     1

$ objdump -s -j .stack_sizes target/release/hello

target/release/hello:     file format elf64-x86-64

Contents of section .stack_sizes:
 2b090 c0040000 00000000 08f00400 00000000  ................
 2b0a0 00080005 00000000 00000810 05000000  ................
 2b0b0 00000000 20050000 00000000 10400500  .... ........@..
 2b0c0 00000000 00087005 00000000 00000080  ......p.........
 2b0d0 05000000 00000000 90050000 00000000  ................
 2b0e0 00a00500 00000000 0000               ..........

Author note: I'm not entirely sure why, in this case, -N is required in addition to -Tkeep-stack-sizes.x. For example, it's not required when producing statically linked files for the ARM Cortex-M architecture.

env-set

The tracking issue for this feature is: #118372.


This option flag allows to specify environment variables value at compile time to be used by env! and option_env! macros. It also impacts tracked_env::var function from the proc_macro crate.

This information will be stored in the dep-info files. For more information about dep-info files, take a look here.

When retrieving an environment variable value, the one specified by --env-set will take precedence. For example, if you want have PATH=a in your environment and pass:

rustc --env-set PATH=env

Then you will have:

#![allow(unused)]
fn main() {
assert_eq!(env!("PATH"), "env");
}

It will trigger a new compilation if any of the --env-set argument value is different. So if you first passed:

--env-set A=B --env X=12

and then on next compilation:

--env-set A=B

X value is different (not set) so the code will be re-compiled.

Please note that on Windows, environment variables are case insensitive but case preserving whereas rustc's environment variables are case sensitive. For example, having Path in your environment (case insensitive) is different than using rustc --env-set Path=... (case sensitive).

export-executable-symbols

The tracking issue for this feature is: #84161.


The -Zexport-executable-symbols compiler flag makes rustc export symbols from executables. The resulting binary is runnable, but can also be used as a dynamic library. This is useful for interoperating with programs written in other languages, in particular languages with a runtime like Java or Lua.

For example on windows:

#[no_mangle]
fn my_function() -> usize {
    return 42;
}

fn main() {
    println!("Hello, world!");
}

A standard cargo build will produce a .exe without an export directory. When the export-executable-symbols flag is added

export RUSTFLAGS="-Zexport-executable-symbols"
cargo build

the binary has an export directory with the functions:

The Export Tables (interpreted .edata section contents)

...

[Ordinal/Name Pointer] Table
    [   0] my_function
    [   1] main

(the output of objdump -x on the binary)

Please note that the #[no_mangle] attribute is required. Without it, the symbol is not exported.

The equivalent of this flag in C and C++ compilers is the __declspec(dllexport) annotation or the -rdynamic linker flag.

--extern Options

  • Tracking issue for --extern crate modifiers: #98405
  • Tracking issue for noprelude: #98398
  • Tracking issue for priv: #98399
  • Tracking issue for nounused: #98400
  • Tracking issue for force: #111302

The behavior of the --extern flag can be modified with noprelude, priv or nounused options.

This is unstable feature, so you have to provide -Zunstable-options to enable it.

Examples

Use your own build of the core crate.

rustc main.rs -Z unstable-options --extern noprelude:core=libcore.rlib

To use multiple options, separate them with a comma:

rustc main.rs -Z unstable-options --extern noprelude,priv,nounused:mydep=mydep.rlib

Options

  • noprelude: Do not add the crate to the external prelude. If used, it will need to be imported using extern crate. This is used by the build-std project to simulate compatibility with sysroot-only crates.
  • priv: Mark the crate as a private dependency for the exported_private_dependencies lint.
  • nounused: Suppress unused-crate-dependencies warnings for the crate.
  • force: Resolve the crate as if it is used, even if it is not used. This can be used to satisfy compilation session requirements like the presence of an allocator or panic handler.

external-clangrt

This option controls whether the compiler links in its own runtime library for sanitizers. Passing this flag makes the compiler not link its own library. For more information, see the section in the sanitizers doc on working with other languages.

function-return

The tracking issue for this feature is: https://github.com/rust-lang/rust/issues/116853.


Option -Zfunction-return controls how function returns are converted.

It is equivalent to Clang's and GCC's -mfunction-return. The Linux kernel uses it for RETHUNK builds. For details, see LLVM commit 2240d72f15f3 ("[X86] initial -mfunction-return=thunk-extern support") which introduces the feature.

Supported values for this option are:

  • keep: do not convert function returns.
  • thunk-extern: convert function returns (ret) to jumps (jmp) to an external symbol called __x86_return_thunk.

Like in Clang, GCC's values thunk and thunk-inline are not supported.

Only x86 and non-large code models are supported.

instrument-xray

The tracking issue for this feature is: #102921.


Enable generation of NOP sleds for XRay function tracing instrumentation. For more information on XRay, read LLVM documentation, and/or the XRay whitepaper.

Set the -Z instrument-xray compiler flag in order to enable XRay instrumentation.

  • -Z instrument-xray – use the default settings
  • -Z instrument-xray=skip-exit – configure a custom setting
  • -Z instrument-xray=ignore-loops,instruction-threshold=300 – multiple settings separated by commas

Supported options:

  • always – force instrumentation of all functions
  • never – do no instrument any functions
  • ignore-loops – ignore presence of loops, instrument functions based only on instruction count
  • instruction-threshold=10 – set a different instruction threshold for instrumentation
  • skip-entry – do no instrument function entry
  • skip-exit – do no instrument function exit

The default settings are:

  • instrument both entry & exit from functions
  • instrument functions with at least 200 instructions, or containing a non-trivial loop

Note that -Z instrument-xray only enables generation of NOP sleds which on their own don't do anything useful. In order to actually trace the functions, you will need to link a separate runtime library of your choice, such as Clang's XRay Runtime Library.

link-native-libraries

This option allows ignoring libraries specified in #[link] attributes instead of passing them to the linker. This can be useful in build systems that manage native libraries themselves and pass them manually, e.g. with -Clink-arg.

  • yes - Pass native libraries to the linker. Default.
  • no - Don't pass native libraries to the linker.

linker-features


The -Zlinker-features compiler flag allows enabling or disabling specific features used during linking, and is intended to be stabilized under the codegen options as -Clinker-features.

These feature flags are a flexible extension mechanism that is complementary to linker flavors, designed to avoid the combinatorial explosion of having to create a new set of flavors for each linker feature we'd want to use.

For example, this design allows:

  • default feature sets for principal flavors, or for specific targets.
  • flavor-specific features: for example, clang offers automatic cross-linking with --target, which gcc-style compilers don't support. The flavor is still a C/C++ compiler, and we don't want to multiply the number of flavors for this use-case. Instead, we can have a single +target feature.
  • umbrella features: for example, if clang accumulates more features in the future than just the +target above. That could be modeled as +clang.
  • niche features for resolving specific issues: for example, on Apple targets the linker flag implementing the as-needed native link modifier (#99424) is only possible on sufficiently recent linker versions.
  • still allows for discovery and automation, for example via feature detection. This can be useful in exotic environments/build systems.

The flag accepts a comma-separated list of features, individually enabled (+features) or disabled (-features), though currently only one is exposed on the CLI:

  • lld: to toggle using the lld linker, either the system-installed binary, or the self-contained rust-lld linker.

As described above, this list is intended to grow in the future.

One of the most common uses of this flag will be to toggle self-contained linking with rust-lld on and off: -Clinker-features=+lld -Clink-self-contained=+linker will use the toolchain's rust-lld as the linker. Inversely, -Clinker-features=-lld would opt out of that, if the current target had self-contained linking enabled by default.

llvm-module-flag


This flag allows adding a key/value to the !llvm.module.flags metadata in the LLVM-IR for a compiled Rust module. The syntax is

-Z llvm_module_flag=<name>:<type>:<value>:<behavior>

Currently only u32 values are supported but the type is required to be specified for forward compatibility. The behavior element must match one of the named LLVM metadata behaviors

location-detail

The tracking issue for this feature is: #70580.


Option -Z location-detail=val controls what location details are tracked when using caller_location. This allows users to control what location details are printed as part of panic messages, by allowing them to exclude any combination of filenames, line numbers, and column numbers. This option is intended to provide users with a way to mitigate the size impact of #[track_caller].

This option supports a comma separated list of location details to be included. Valid options within this list are:

  • file - the filename of the panic will be included in the panic output
  • line - the source line of the panic will be included in the panic output
  • column - the source column of the panic will be included in the panic output

Any combination of these three options are supported. Alternatively, you can pass none to this option, which results in no location details being tracked. If this option is not specified, all three are included by default.

An example of a panic output when using -Z location-detail=line:

panicked at 'Process blink had a fault', <redacted>:323:0

The code size savings from this option are two-fold. First, the &'static str values for each path to a file containing a panic are removed from the binary. For projects with deep directory structures and many files with panics, this can add up. This category of savings can only be realized by excluding filenames from the panic output. Second, savings can be realized by allowing multiple panics to be fused into a single panicking branch. It is often the case that within a single file, multiple panics with the same panic message exist -- e.g. two calls to Option::unwrap() in a single line, or two calls to Result::expect() on adjacent lines. If column and line information are included in the Location struct passed to the panic handler, these branches cannot be fused, as the output is different depending on which panic occurs. However if line and column information is identical for all panics, these branches can be fused, which can lead to substantial code size savings, especially for small embedded binaries with many panics.

The savings from this option are amplified when combined with the use of -Zbuild-std, as otherwise paths for panics within the standard library are still included in your binary.

move_size_limit


The -Zmove-size-limit=N compiler flag enables large_assignments lints which will warn when moving objects whose size exceeds N bytes.

Lint warns only about moves in functions that participate in code generation. Consequently it will be ineffective for compiler invocation that emit metadata only, i.e., cargo check like workflows.

no-jump-tables

The tracking issue for this feature is #116592


This option enables the -fno-jump-tables flag for LLVM, which makes the codegen backend avoid generating jump tables when lowering switches.

This option adds the LLVM no-jump-tables=true attribute to every function.

The option can be used to help provide protection against jump-oriented-programming (JOP) attacks, such as with the linux kernel's IBT.

RUSTFLAGS="-Zno-jump-tables" cargo +nightly build -Z build-std

no-parallel-llvm


This flag disables parallelization of codegen and linking, while otherwise preserving behavior with regard to codegen units and LTO.

This flag is not useful for regular users, but it can be useful for debugging the backend. Codegen issues commonly only manifest under specific circumstances, e.g. if multiple codegen units are used and ThinLTO is enabled. Serialization of these threaded configurations makes the use of LLVM debugging facilities easier, by avoiding the interleaving of output.

no-unique-section-names


This flag currently applies only to ELF-based targets using the LLVM codegen backend. It prevents the generation of unique ELF section names for each separate code and data item when -Z function-sections is also in use, which is the default for most targets. This option can reduce the size of object files, and depending on the linker, the final ELF binary as well.

For example, a function func will by default generate a code section called .text.func. Normally this is fine because the linker will merge all those .text.* sections into a single one in the binary. However, starting with LLVM 12, the backend will also generate unique section names for exception handling, so you would see a section name of .gcc_except_table.func in the object file and potentially in the final ELF binary, which could add significant bloat to programs that contain many functions.

This flag instructs LLVM to use the same .text and .gcc_except_table section name for each function, and it is analogous to Clang's -fno-unique-section-names option.

profile

The tracking issue for this feature is: #42524.


This feature allows the generation of code coverage reports.

Set the -Zprofile compiler flag in order to enable gcov profiling.

For example:

cargo new testgcov --bin
cd testgcov
export RUSTFLAGS="-Zprofile -Ccodegen-units=1 -Copt-level=0 -Clink-dead-code -Coverflow-checks=off -Zpanic_abort_tests -Cpanic=abort"
export CARGO_INCREMENTAL=0
cargo build
cargo run

Once you've built and run your program, files with the gcno (after build) and gcda (after execution) extensions will be created. You can parse them with llvm-cov gcov or grcov.

Please note that RUSTFLAGS by default applies to everything that cargo builds and runs during a build! When the --target flag is explicitly passed to cargo, the RUSTFLAGS no longer apply to build scripts and procedural macros. For more fine-grained control consider passing a RUSTC_WRAPPER program to cargo that only adds the profiling flags to rustc for the specific crates you want to profile.

profile-sample-use


-Zprofile-sample-use=code.prof directs rustc to use the profile code.prof as a source for Automatic Feedback Directed Optimization (AFDO). See the documentation of -Zdebug-info-for-profiling for more information on using AFDO.

remap-cwd-prefix

The tracking issue for this feature is: #87325.


This flag will rewrite absolute paths under the current working directory, replacing the current working directory prefix with a specified value.

The given value may be absolute or relative, or empty. This switch takes precedence over --remap-path-prefix in case they would both match a given path.

This flag helps to produce deterministic output, by removing the current working directory from build output, while allowing the command line to be universally reproducible, such that the same execution will work on all machines, regardless of build environment.

Example

# This would produce an absolute path to main.rs in build outputs of
# "./main.rs".
rustc -Z remap-cwd-prefix=. main.rs

remap-path-scope

The tracking issue for this feature is: #111540.


When the --remap-path-prefix option is passed to rustc, source path prefixes in all output will be affected by default. The --remap-path-scope argument can be used in conjunction with --remap-path-prefix to determine paths in which output context should be affected. This flag accepts a comma-separated list of values and may be specified multiple times, in which case the scopes are aggregated together. The valid scopes are:

  • macro - apply remappings to the expansion of std::file!() macro. This is where paths in embedded panic messages come from
  • diagnostics - apply remappings to printed compiler diagnostics
  • debuginfo - apply remappings to debug informations
  • object - apply remappings to all paths in compiled executables or libraries, but not elsewhere. Currently an alias for macro,debuginfo.
  • all - an alias for all of the above, also equivalent to supplying only --remap-path-prefix without --remap-path-scope.

Example

# This would produce an absolute path to main.rs in build outputs of
# "./main.rs".
rustc --remap-path-prefix=$(PWD)=/remapped -Zremap-path-scope=object main.rs

report-time

The tracking issue for this feature is: #64888


The report-time feature adds a possibility to report execution time of the tests generated via libtest.

This is unstable feature, so you have to provide -Zunstable-options to get this feature working.

Sample usage command:

./test_executable -Zunstable-options --report-time

Available options:

--report-time
                Show execution time of each test.
                Threshold values for colorized output can be
                configured via
                `RUST_TEST_TIME_UNIT`, `RUST_TEST_TIME_INTEGRATION`
                and
                `RUST_TEST_TIME_DOCTEST` environment variables.
                Expected format of environment variable is
                `VARIABLE=WARN_TIME,CRITICAL_TIME`.
                Not available for --format=terse
--ensure-time
                Treat excess of the test execution time limit as
                error.
                Threshold values for this option can be configured via
                `RUST_TEST_TIME_UNIT`, `RUST_TEST_TIME_INTEGRATION`
                and
                `RUST_TEST_TIME_DOCTEST` environment variables.
                Expected format of environment variable is
                `VARIABLE=WARN_TIME,CRITICAL_TIME`.
                `CRITICAL_TIME` here means the limit that should not be
                exceeded by test.

Example of the environment variable format:

RUST_TEST_TIME_UNIT=100,200

where 100 stands for warn time, and 200 stands for critical time.

Examples

cargo test --tests -- -Zunstable-options --report-time
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s
     Running target/debug/deps/example-27fb188025bec02c

running 3 tests
test tests::unit_test_quick ... ok <0.000s>
test tests::unit_test_warn ... ok <0.055s>
test tests::unit_test_critical ... ok <0.110s>

test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

     Running target/debug/deps/tests-cedb06f6526d15d9

running 3 tests
test unit_test_quick ... ok <0.000s>
test unit_test_warn ... ok <0.550s>
test unit_test_critical ... ok <1.100s>

test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

sanitizer

Sanitizers are tools that help detect and prevent various types of bugs and vulnerabilities in software. They are available in compilers and work by instrumenting the code to add additional runtime checks. While they provide powerful tools for identifying bugs or security issues, it's important to note that using sanitizers can introduce runtime overhead and might not catch all possible issues. Therefore, they are typically used alongside other best practices in software development, such as testing and fuzzing, to ensure the highest level of software quality and security.

The tracking issues for this feature are:


This feature allows for use of one of following sanitizers:

  • Those intended for testing or fuzzing (but not production use):

  • Those that apart from testing, may be used in production:

    • ControlFlowIntegrity LLVM Control Flow Integrity (CFI) provides forward-edge control flow protection.
    • DataFlowSanitizer a generic dynamic data flow analysis framework.
    • KernelControlFlowIntegrity LLVM Kernel Control Flow Integrity (KCFI) provides forward-edge control flow protection for operating systems kernels.
    • MemTagSanitizer fast memory error detector based on Armv8.5-A Memory Tagging Extension.
    • SafeStack provides backward-edge control flow protection by separating the stack into safe and unsafe regions.
    • ShadowCallStack provides backward-edge control flow protection (aarch64 only).

To enable a sanitizer compile with -Zsanitizer=address, -Zsanitizer=cfi, -Zsanitizer=dataflow,-Zsanitizer=hwaddress, -Zsanitizer=leak, -Zsanitizer=memory, -Zsanitizer=memtag, -Zsanitizer=shadow-call-stack, or -Zsanitizer=thread. You might also need the --target and build-std flags. If you're working with other languages that are also instrumented with sanitizers, you might need the external-clangrt flag. See the section on working with other languages.

Example:

$ RUSTFLAGS=-Zsanitizer=address cargo build -Zbuild-std --target x86_64-unknown-linux-gnu

Additional options for sanitizers can be passed to LLVM command line argument processor via LLVM arguments using llvm-args codegen option (e.g., -Cllvm-args=-dfsan-combine-pointer-labels-on-load=false). See the sanitizer documentation for more information about additional options.

AddressSanitizer

AddressSanitizer is a memory error detector. It can detect the following types of bugs:

  • Out of bound accesses to heap, stack and globals
  • Use after free
  • Use after return (runtime flag ASAN_OPTIONS=detect_stack_use_after_return=1)
  • Use after scope
  • Double-free, invalid free
  • Memory leaks

The memory leak detection is enabled by default on Linux, and can be enabled with runtime flag ASAN_OPTIONS=detect_leaks=1 on macOS.

AddressSanitizer is supported on the following targets:

  • aarch64-apple-darwin
  • aarch64-unknown-fuchsia
  • aarch64-unknown-linux-gnu
  • x86_64-apple-darwin
  • x86_64-unknown-fuchsia
  • x86_64-unknown-freebsd
  • x86_64-unknown-linux-gnu

AddressSanitizer works with non-instrumented code although it will impede its ability to detect some bugs. It is not expected to produce false positive reports.

See the Clang AddressSanitizer documentation for more details.

Examples

Stack buffer overflow:

fn main() {
    let xs = [0, 1, 2, 3];
    let _y = unsafe { *xs.as_ptr().offset(4) };
}
$ export RUSTFLAGS=-Zsanitizer=address RUSTDOCFLAGS=-Zsanitizer=address
$ cargo run -Zbuild-std --target x86_64-unknown-linux-gnu
==37882==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffe400e6250 at pc 0x5609a841fb20 bp 0x7ffe400e6210 sp 0x7ffe400e6208
READ of size 4 at 0x7ffe400e6250 thread T0
    #0 0x5609a841fb1f in example::main::h628ffc6626ed85b2 /.../src/main.rs:3:23
    ...

Address 0x7ffe400e6250 is located in stack of thread T0 at offset 48 in frame
    #0 0x5609a841f8af in example::main::h628ffc6626ed85b2 /.../src/main.rs:1

  This frame has 1 object(s):
    [32, 48) 'xs' (line 2) <== Memory access at offset 48 overflows this variable
HINT: this may be a false positive if your program uses some custom stack unwind mechanism, swapcontext or vfork
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow /.../src/main.rs:3:23 in example::main::h628ffc6626ed85b2
Shadow bytes around the buggy address:
  0x100048014bf0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100048014c00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100048014c10: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100048014c20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100048014c30: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x100048014c40: 00 00 00 00 f1 f1 f1 f1 00 00[f3]f3 00 00 00 00
  0x100048014c50: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100048014c60: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100048014c70: f1 f1 f1 f1 00 00 f3 f3 00 00 00 00 00 00 00 00
  0x100048014c80: 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1
  0x100048014c90: 00 00 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
  Shadow gap:              cc
==37882==ABORTING

Use of a stack object after its scope has already ended:

static mut P: *mut usize = std::ptr::null_mut();

fn main() {
    unsafe {
        {
            let mut x = 0;
            P = &mut x;
        }
        std::ptr::write_volatile(P, 123);
    }
}
$ export RUSTFLAGS=-Zsanitizer=address RUSTDOCFLAGS=-Zsanitizer=address
$ cargo run -Zbuild-std --target x86_64-unknown-linux-gnu
=================================================================
==39249==ERROR: AddressSanitizer: stack-use-after-scope on address 0x7ffc7ed3e1a0 at pc 0x55c98b262a8e bp 0x7ffc7ed3e050 sp 0x7ffc7ed3e048
WRITE of size 8 at 0x7ffc7ed3e1a0 thread T0
    #0 0x55c98b262a8d in core::ptr::write_volatile::he21f1df5a82f329a /.../src/rust/src/libcore/ptr/mod.rs:1048:5
    #1 0x55c98b262cd2 in example::main::h628ffc6626ed85b2 /.../src/main.rs:9:9
    ...

Address 0x7ffc7ed3e1a0 is located in stack of thread T0 at offset 32 in frame
    #0 0x55c98b262bdf in example::main::h628ffc6626ed85b2 /.../src/main.rs:3

  This frame has 1 object(s):
    [32, 40) 'x' (line 6) <== Memory access at offset 32 is inside this variable
HINT: this may be a false positive if your program uses some custom stack unwind mechanism, swapcontext or vfork
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-use-after-scope /.../src/rust/src/libcore/ptr/mod.rs:1048:5 in core::ptr::write_volatile::he21f1df5a82f329a
Shadow bytes around the buggy address:
  0x10000fd9fbe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10000fd9fbf0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10000fd9fc00: 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1
  0x10000fd9fc10: f8 f8 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
  0x10000fd9fc20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x10000fd9fc30: f1 f1 f1 f1[f8]f3 f3 f3 00 00 00 00 00 00 00 00
  0x10000fd9fc40: 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1
  0x10000fd9fc50: 00 00 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
  0x10000fd9fc60: 00 00 00 00 00 00 00 00 f1 f1 f1 f1 00 00 f3 f3
  0x10000fd9fc70: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10000fd9fc80: 00 00 00 00 f1 f1 f1 f1 00 00 f3 f3 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
  Shadow gap:              cc
==39249==ABORTING

ControlFlowIntegrity

The LLVM CFI support in the Rust compiler provides forward-edge control flow protection for both Rust-compiled code only and for C or C++ and Rust -compiled code mixed-language binaries, also known as “mixed binaries” (i.e., for when C or C++ and Rust -compiled code share the same virtual address space), by aggregating function pointers in groups identified by their return and parameter types.

LLVM CFI can be enabled with -Zsanitizer=cfi and requires LTO (i.e., -Clinker-plugin-lto or -Clto). Cross-language LLVM CFI can be enabled with -Zsanitizer=cfi, and requires the -Zsanitizer-cfi-normalize-integers option to be used with Clang -fsanitize-cfi-icall-experimental-normalize-integers option for cross-language LLVM CFI support, and proper (i.e., non-rustc) LTO (i.e., -Clinker-plugin-lto).

It is recommended to rebuild the standard library with CFI enabled by using the Cargo build-std feature (i.e., -Zbuild-std) when enabling CFI.

See the Clang ControlFlowIntegrity documentation for more details.

Example 1: Redirecting control flow using an indirect branch/call to an invalid destination

#![feature(naked_functions)]

use std::arch::asm;
use std::mem;

fn add_one(x: i32) -> i32 {
    x + 1
}

#[naked]
pub extern "C" fn add_two(x: i32) {
    // x + 2 preceded by a landing pad/nop block
    unsafe {
        asm!(
            "
             nop
             nop
             nop
             nop
             nop
             nop
             nop
             nop
             nop
             lea eax, [rdi+2]
             ret
        ",
            options(noreturn)
        );
    }
}

fn do_twice(f: fn(i32) -> i32, arg: i32) -> i32 {
    f(arg) + f(arg)
}

fn main() {
    let answer = do_twice(add_one, 5);

    println!("The answer is: {}", answer);

    println!("With CFI enabled, you should not see the next answer");
    let f: fn(i32) -> i32 = unsafe {
        // Offset 0 is a valid branch/call destination (i.e., the function entry
        // point), but offsets 1-8 within the landing pad/nop block are invalid
        // branch/call destinations (i.e., within the body of the function).
        mem::transmute::<*const u8, fn(i32) -> i32>((add_two as *const u8).offset(5))
    };
    let next_answer = do_twice(f, 5);

    println!("The next answer is: {}", next_answer);
}

Fig. 1. Redirecting control flow using an indirect branch/call to an invalid destination (i.e., within the body of the function).

$ cargo run --release
   Compiling rust-cfi-1 v0.1.0 (/home/rcvalle/rust-cfi-1)
    Finished release [optimized] target(s) in 0.42s
     Running `target/release/rust-cfi-1`
The answer is: 12
With CFI enabled, you should not see the next answer
The next answer is: 14
$

Fig. 2. Build and execution of Fig. 1 with LLVM CFI disabled.

$ RUSTFLAGS="-Clinker-plugin-lto -Clinker=clang -Clink-arg=-fuse-ld=lld -Zsanitizer=cfi" cargo run -Zbuild-std -Zbuild-std-features --release --target x86_64-unknown-linux-gnu
   ...
   Compiling rust-cfi-1 v0.1.0 (/home/rcvalle/rust-cfi-1)
    Finished release [optimized] target(s) in 1m 08s
     Running `target/x86_64-unknown-linux-gnu/release/rust-cfi-1`
The answer is: 12
With CFI enabled, you should not see the next answer
Illegal instruction
$

Fig. 3. Build and execution of Fig. 1 with LLVM CFI enabled.

When LLVM CFI is enabled, if there are any attempts to change/hijack control flow using an indirect branch/call to an invalid destination, the execution is terminated (see Fig. 3).

Example 2: Redirecting control flow using an indirect branch/call to a function with a different number of parameters

use std::mem;

fn add_one(x: i32) -> i32 {
    x + 1
}

fn add_two(x: i32, _y: i32) -> i32 {
    x + 2
}

fn do_twice(f: fn(i32) -> i32, arg: i32) -> i32 {
    f(arg) + f(arg)
}

fn main() {
    let answer = do_twice(add_one, 5);

    println!("The answer is: {}", answer);

    println!("With CFI enabled, you should not see the next answer");
    let f: fn(i32) -> i32 =
        unsafe { mem::transmute::<*const u8, fn(i32) -> i32>(add_two as *const u8) };
    let next_answer = do_twice(f, 5);

    println!("The next answer is: {}", next_answer);
}

Fig. 4. Redirecting control flow using an indirect branch/call to a function with a different number of parameters than arguments intended/passed in the call/branch site.

$ cargo run --release
   Compiling rust-cfi-2 v0.1.0 (/home/rcvalle/rust-cfi-2)
    Finished release [optimized] target(s) in 0.43s
     Running `target/release/rust-cfi-2`
The answer is: 12
With CFI enabled, you should not see the next answer
The next answer is: 14
$

Fig. 5. Build and execution of Fig. 4 with LLVM CFI disabled.

$ RUSTFLAGS="-Clinker-plugin-lto -Clinker=clang -Clink-arg=-fuse-ld=lld -Zsanitizer=cfi" cargo run -Zbuild-std -Zbuild-std-features --release --target x86_64-unknown-linux-gnu
   ...
   Compiling rust-cfi-2 v0.1.0 (/home/rcvalle/rust-cfi-2)
    Finished release [optimized] target(s) in 1m 08s
     Running `target/x86_64-unknown-linux-gnu/release/rust-cfi-2`
The answer is: 12
With CFI enabled, you should not see the next answer
Illegal instruction
$

Fig. 6. Build and execution of Fig. 4 with LLVM CFI enabled.

When LLVM CFI is enabled, if there are any attempts to change/hijack control flow using an indirect branch/call to a function with different number of parameters than arguments intended/passed in the call/branch site, the execution is also terminated (see Fig. 6).

Example 3: Redirecting control flow using an indirect branch/call to a function with different return and parameter types

use std::mem;

fn add_one(x: i32) -> i32 {
    x + 1
}

fn add_two(x: i64) -> i64 {
    x + 2
}

fn do_twice(f: fn(i32) -> i32, arg: i32) -> i32 {
    f(arg) + f(arg)
}

fn main() {
    let answer = do_twice(add_one, 5);

    println!("The answer is: {}", answer);

    println!("With CFI enabled, you should not see the next answer");
    let f: fn(i32) -> i32 =
        unsafe { mem::transmute::<*const u8, fn(i32) -> i32>(add_two as *const u8) };
    let next_answer = do_twice(f, 5);

    println!("The next answer is: {}", next_answer);
}

Fig. 7. Redirecting control flow using an indirect branch/call to a function with different return and parameter types than the return type expected and arguments intended/passed at the call/branch site.

$ cargo run --release
   Compiling rust-cfi-3 v0.1.0 (/home/rcvalle/rust-cfi-3)
    Finished release [optimized] target(s) in 0.44s
     Running `target/release/rust-cfi-3`
The answer is: 12
With CFI enabled, you should not see the next answer
The next answer is: 14
$

Fig. 8. Build and execution of Fig. 7 with LLVM CFI disabled.

$ RUSTFLAGS="-Clinker-plugin-lto -Clinker=clang -Clink-arg=-fuse-ld=lld -Zsanitizer=cfi" cargo run -Zbuild-std -Zbuild-std-features --release --target x86_64-unknown-linux-gnu
   ...
   Compiling rust-cfi-3 v0.1.0 (/home/rcvalle/rust-cfi-3)
    Finished release [optimized] target(s) in 1m 07s
     Running `target/x86_64-unknown-linux-gnu/release/rust-cfi-3`
The answer is: 12
With CFI enabled, you should not see the next answer
Illegal instruction
$

Fig. 9. Build and execution of Fig. 7 with LLVM CFI enabled.

When LLVM CFI is enabled, if there are any attempts to change/hijack control flow using an indirect branch/call to a function with different return and parameter types than the return type expected and arguments intended/passed in the call/branch site, the execution is also terminated (see Fig. 9).

Example 4: Redirecting control flow using an indirect branch/call to a function with different return and parameter types across the FFI boundary

int
do_twice(int (*fn)(int), int arg)
{
    return fn(arg) + fn(arg);
}

Fig. 10. Example C library.

use std::mem;

#[link(name = "foo")]
extern "C" {
    fn do_twice(f: unsafe extern "C" fn(i32) -> i32, arg: i32) -> i32;
}

unsafe extern "C" fn add_one(x: i32) -> i32 {
    x + 1
}

unsafe extern "C" fn add_two(x: i64) -> i64 {
    x + 2
}

fn main() {
    let answer = unsafe { do_twice(add_one, 5) };

    println!("The answer is: {}", answer);

    println!("With CFI enabled, you should not see the next answer");
    let f: unsafe extern "C" fn(i32) -> i32 = unsafe {
        mem::transmute::<*const u8, unsafe extern "C" fn(i32) -> i32>(add_two as *const u8)
    };
    let next_answer = unsafe { do_twice(f, 5) };

    println!("The next answer is: {}", next_answer);
}

Fig. 11. Redirecting control flow using an indirect branch/call to a function with different return and parameter types than the return type expected and arguments intended/passed in the call/branch site, across the FFI boundary.

$ make
mkdir -p target/release
clang -I. -Isrc -Wall -c src/foo.c -o target/release/libfoo.o
llvm-ar rcs target/release/libfoo.a target/release/libfoo.o
RUSTFLAGS="-L./target/release -Clinker=clang -Clink-arg=-fuse-ld=lld" cargo build --release
   Compiling rust-cfi-4 v0.1.0 (/home/rcvalle/rust-cfi-4)
    Finished release [optimized] target(s) in 0.49s
$ ./target/release/rust-cfi-4
The answer is: 12
With CFI enabled, you should not see the next answer
The next answer is: 14
$

Fig. 12. Build and execution of Figs. 10–11 with LLVM CFI disabled.

$ make
mkdir -p target/release
clang -I. -Isrc -Wall -flto -fsanitize=cfi -fsanitize-cfi-icall-experimental-normalize-integers -fvisibility=hidden -c -emit-llvm src/foo.c -o target/release/libfoo.bc
llvm-ar rcs target/release/libfoo.a target/release/libfoo.bc
RUSTFLAGS="-L./target/release -Clinker-plugin-lto -Clinker=clang -Clink-arg=-fuse-ld=lld -Zsanitizer=cfi -Zsanitizer-cfi-normalize-integers" cargo build -Zbuild-std -Zbuild-std-features --release --target x86_64-unknown-linux-gnu
   ...
   Compiling rust-cfi-4 v0.1.0 (/home/rcvalle/rust-cfi-4)
    Finished release [optimized] target(s) in 1m 06s
$ ./target/x86_64-unknown-linux-gnu/release/rust-cfi-4
The answer is: 12
With CFI enabled, you should not see the next answer
Illegal instruction
$

Fig. 13. Build and execution of FIgs. 10–11 with LLVM CFI enabled.

When LLVM CFI is enabled, if there are any attempts to redirect control flow using an indirect branch/call to a function with different return and parameter types than the return type expected and arguments intended/passed in the call/branch site, even across the FFI boundary and for extern "C" function types indirectly called (i.e., callbacks/function pointers) across the FFI boundary, the execution is also terminated (see Fig. 13).

HWAddressSanitizer

HWAddressSanitizer is a newer variant of AddressSanitizer that consumes much less memory.

HWAddressSanitizer is supported on the following targets:

  • aarch64-linux-android
  • aarch64-unknown-linux-gnu

HWAddressSanitizer requires tagged-globals target feature to instrument globals. To enable this target feature compile with -C target-feature=+tagged-globals

See the Clang HWAddressSanitizer documentation for more details.

Example

Heap buffer overflow:

fn main() {
    let xs = vec![0, 1, 2, 3];
    let _y = unsafe { *xs.as_ptr().offset(4) };
}
$ rustc main.rs -Zsanitizer=hwaddress -C target-feature=+tagged-globals -C
linker=aarch64-linux-gnu-gcc -C link-arg=-fuse-ld=lld --target
aarch64-unknown-linux-gnu
$ ./main
==241==ERROR: HWAddressSanitizer: tag-mismatch on address 0xefdeffff0050 at pc 0xaaaae0ae4a98
READ of size 4 at 0xefdeffff0050 tags: 2c/00 (ptr/mem) in thread T0
    #0 0xaaaae0ae4a94  (/.../main+0x54a94)
    ...

[0xefdeffff0040,0xefdeffff0060) is a small allocated heap chunk; size: 32 offset: 16
0xefdeffff0050 is located 0 bytes to the right of 16-byte region [0xefdeffff0040,0xefdeffff0050)
allocated here:
    #0 0xaaaae0acb80c  (/.../main+0x3b80c)
    ...

Thread: T0 0xeffe00002000 stack: [0xffffc28ad000,0xffffc30ad000) sz: 8388608 tls: [0xffffaa10a020,0xffffaa10a7d0)
Memory tags around the buggy address (one tag corresponds to 16 bytes):
  0xfefcefffef80: 00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
  0xfefcefffef90: 00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
  0xfefcefffefa0: 00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
  0xfefcefffefb0: 00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
  0xfefcefffefc0: 00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
  0xfefcefffefd0: 00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
  0xfefcefffefe0: 00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
  0xfefcefffeff0: 00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
=>0xfefceffff000: d7  d7  05  00  2c [00] 00  00  00  00  00  00  00  00  00  00
  0xfefceffff010: 00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
  0xfefceffff020: 00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
  0xfefceffff030: 00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
  0xfefceffff040: 00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
  0xfefceffff050: 00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
  0xfefceffff060: 00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
  0xfefceffff070: 00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
  0xfefceffff080: 00  00  00  00  00  00  00  00  00  00  00  00  00  00  00  00
Tags for short granules around the buggy address (one tag corresponds to 16 bytes):
  0xfefcefffeff0: ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..
=>0xfefceffff000: ..  ..  8c  ..  .. [..] ..  ..  ..  ..  ..  ..  ..  ..  ..  ..
  0xfefceffff010: ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..
See https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html#short-granules for a description of short granule tags
Registers where the failure occurred (pc 0xaaaae0ae4a98):
    x0  2c00efdeffff0050  x1  0000000000000004  x2  0000000000000004  x3  0000000000000000
    x4  0000fffefc30ac37  x5  000000000000005d  x6  00000ffffc30ac37  x7  0000efff00000000
    x8  2c00efdeffff0050  x9  0200efff00000000  x10 0000000000000000  x11 0200efff00000000
    x12 0200effe00000310  x13 0200effe00000310  x14 0000000000000008  x15 5d00ffffc30ac360
    x16 0000aaaae0ad062c  x17 0000000000000003  x18 0000000000000001  x19 0000ffffc30ac658
    x20 4e00ffffc30ac6e0  x21 0000aaaae0ac5e10  x22 0000000000000000  x23 0000000000000000
    x24 0000000000000000  x25 0000000000000000  x26 0000000000000000  x27 0000000000000000
    x28 0000000000000000  x29 0000ffffc30ac5a0  x30 0000aaaae0ae4a98
SUMMARY: HWAddressSanitizer: tag-mismatch (/.../main+0x54a94)

KernelControlFlowIntegrity

The LLVM Kernel Control Flow Integrity (CFI) support to the Rust compiler initially provides forward-edge control flow protection for operating systems kernels for Rust-compiled code only by aggregating function pointers in groups identified by their return and parameter types. (See LLVM commit cff5bef "KCFI sanitizer".)

Forward-edge control flow protection for C or C++ and Rust -compiled code "mixed binaries" (i.e., for when C or C++ and Rust -compiled code share the same virtual address space) will be provided in later work by defining and using compatible type identifiers (see Type metadata in the design document in the tracking issue #89653).

LLVM KCFI can be enabled with -Zsanitizer=kcfi.

LLVM KCFI is supported on the following targets:

  • aarch64-linux-android
  • aarch64-unknown-linux-gnu
  • x86_64-linux-android
  • x86_64-unknown-linux-gnu

See the Clang KernelControlFlowIntegrity documentation for more details.

DataFlowSanitizer

DataFlowSanitizer is a generalised dynamic data flow analysis.

Unlike other Sanitizer tools, this tool is not designed to detect a specific class of bugs on its own. Instead, it provides a generic dynamic data flow analysis framework to be used by clients to help detect application-specific issues within their own code.

DataFlowSanitizer is supported on the following targets:

  • x86_64-unknown-linux-gnu

See the Clang DataFlowSanitizer documentation for more details.

KernelAddressSanitizer

KernelAddressSanitizer (KASAN) is a freestanding version of AddressSanitizer which is suitable for detecting memory errors in programs which do not have a runtime environment, such as operating system kernels. KernelAddressSanitizer requires manual implementation of the underlying functions used for tracking KernelAddressSanitizer state.

KernelAddressSanitizer is supported on the following targets:

  • aarch64-unknown-none
  • riscv64gc-unknown-none-elf
  • riscv64imac-unknown-none-elf
  • x86_64-unknown-none

See the Linux Kernel's KernelAddressSanitizer documentation for more details.

LeakSanitizer

LeakSanitizer is run-time memory leak detector.

LeakSanitizer is supported on the following targets:

  • aarch64-apple-darwin
  • aarch64-unknown-linux-gnu
  • x86_64-apple-darwin
  • x86_64-unknown-linux-gnu

See the Clang LeakSanitizer documentation for more details.

MemorySanitizer

MemorySanitizer is detector of uninitialized reads.

MemorySanitizer is supported on the following targets:

  • aarch64-unknown-linux-gnu
  • x86_64-unknown-freebsd
  • x86_64-unknown-linux-gnu

MemorySanitizer requires all program code to be instrumented. C/C++ dependencies need to be recompiled using Clang with -fsanitize=memory option. Failing to achieve that will result in false positive reports.

See the Clang MemorySanitizer documentation for more details.

Example

Detecting the use of uninitialized memory. The -Zbuild-std flag rebuilds and instruments the standard library, and is strictly necessary for the correct operation of the tool. The -Zsanitizer-memory-track-origins enables tracking of the origins of uninitialized memory:

use std::mem::MaybeUninit;

fn main() {
    unsafe {
        let a = MaybeUninit::<[usize; 4]>::uninit();
        let a = a.assume_init();
        println!("{}", a[2]);
    }
}
$ export \
  RUSTFLAGS='-Zsanitizer=memory -Zsanitizer-memory-track-origins' \
  RUSTDOCFLAGS='-Zsanitizer=memory -Zsanitizer-memory-track-origins'
$ cargo clean
$ cargo run -Zbuild-std --target x86_64-unknown-linux-gnu
==9416==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x560c04f7488a in core::fmt::num::imp::fmt_u64::haa293b0b098501ca $RUST/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/src/rust/src/libcore/fmt/num.rs:202:16
...
  Uninitialized value was stored to memory at
    #0 0x560c04ae898a in __msan_memcpy.part.0 $RUST/src/llvm-project/compiler-rt/lib/msan/msan_interceptors.cc:1558:3
    #1 0x560c04b2bf88 in memory::main::hd2333c1899d997f5 $CWD/src/main.rs:6:16

  Uninitialized value was created by an allocation of 'a' in the stack frame of function '_ZN6memory4main17hd2333c1899d997f5E'
    #0 0x560c04b2bc50 in memory::main::hd2333c1899d997f5 $CWD/src/main.rs:3

MemTagSanitizer

MemTagSanitizer detects a similar class of errors as AddressSanitizer and HardwareAddressSanitizer, but with lower overhead suitable for use as hardening for production binaries.

MemTagSanitizer is supported on the following targets:

  • aarch64-linux-android
  • aarch64-unknown-linux-gnu

MemTagSanitizer requires hardware support and the mte target feature. To enable this target feature compile with -C target-feature="+mte".

See the LLVM MemTagSanitizer documentation for more details.

SafeStack

SafeStack provides backward edge control flow protection by separating the stack into data which is only accessed safely (the safe stack) and all other data (the unsafe stack).

SafeStack can be enabled with the -Zsanitizer=safestack option and is supported on the following targets:

  • x86_64-unknown-linux-gnu

See the Clang SafeStack documentation for more details.

ShadowCallStack

ShadowCallStack provides backward edge control flow protection by storing a function's return address in a separately allocated 'shadow call stack' and loading the return address from that shadow call stack.

ShadowCallStack requires a platform ABI which reserves x18 as the instrumentation makes use of this register.

ShadowCallStack can be enabled with -Zsanitizer=shadow-call-stack option and is supported on the following targets:

  • aarch64-linux-android

A runtime must be provided by the application or operating system.

See the Clang ShadowCallStack documentation for more details.

ThreadSanitizer

ThreadSanitizer is a data race detection tool. It is supported on the following targets:

  • aarch64-apple-darwin
  • aarch64-unknown-linux-gnu
  • x86_64-apple-darwin
  • x86_64-unknown-freebsd
  • x86_64-unknown-linux-gnu

To work correctly ThreadSanitizer needs to be "aware" of all synchronization operations in a program. It generally achieves that through combination of library interception (for example synchronization performed through pthread_mutex_lock / pthread_mutex_unlock) and compile time instrumentation (e.g. atomic operations). Using it without instrumenting all the program code can lead to false positive reports.

ThreadSanitizer does not support atomic fences std::sync::atomic::fence, nor synchronization performed using inline assembly code.

See the Clang ThreadSanitizer documentation for more details.

Example

static mut A: usize = 0;

fn main() {
    let t = std::thread::spawn(|| {
        unsafe { A += 1 };
    });
    unsafe { A += 1 };

    t.join().unwrap();
}
$ export RUSTFLAGS=-Zsanitizer=thread RUSTDOCFLAGS=-Zsanitizer=thread
$ cargo run -Zbuild-std --target x86_64-unknown-linux-gnu
==================
WARNING: ThreadSanitizer: data race (pid=10574)
  Read of size 8 at 0x5632dfe3d030 by thread T1:
    #0 example::main::_$u7b$$u7b$closure$u7d$$u7d$::h23f64b0b2f8c9484 ../src/main.rs:5:18 (example+0x86cec)
    ...

  Previous write of size 8 at 0x5632dfe3d030 by main thread:
    #0 example::main::h628ffc6626ed85b2 /.../src/main.rs:7:14 (example+0x868c8)
    ...
    #11 main <null> (example+0x86a1a)

  Location is global 'example::A::h43ac149ddf992709' of size 8 at 0x5632dfe3d030 (example+0x000000bd9030)

Instrumentation of external dependencies and std

The sanitizers to varying degrees work correctly with partially instrumented code. On the one extreme is LeakSanitizer that doesn't use any compile time instrumentation, on the other is MemorySanitizer that requires that all program code to be instrumented (failing to achieve that will inevitably result in false positives).

It is strongly recommended to combine sanitizers with recompiled and instrumented standard library, for example using cargo -Zbuild-std functionality.

Working with other languages

Sanitizers rely on compiler runtime libraries to function properly. Rust links in its own compiler runtime which might conflict with runtimes required by languages such as C++. Since Rust's runtime doesn't always contain the symbols required by C++ instrumented code, you might need to skip linking it so another runtime can be linked instead.

A separate unstable option -Zexternal-clangrt can be used to make rustc skip linking the compiler runtime for the sanitizer. This will require you to link in an external runtime, such as from clang instead.

Build scripts and procedural macros

Use of sanitizers together with build scripts and procedural macros is technically possible, but in almost all cases it would be best avoided. This is especially true for procedural macros which would require an instrumented version of rustc.

In more practical terms when using cargo always remember to pass --target flag, so that rustflags will not be applied to build scripts and procedural macros.

Symbolizing the Reports

Sanitizers produce symbolized stacktraces when llvm-symbolizer binary is in PATH.

Additional Information

self-profile


The -Zself-profile compiler flag enables rustc's internal profiler. When enabled, the compiler will output three binary files in the specified directory (or the current working directory if no directory is specified). These files can be analyzed by using the tools in the measureme repository.

To control the data recorded in the trace files, use the -Zself-profile-events flag.

For example:

First, run a compilation session and provide the -Zself-profile flag:

$ rustc --crate-name foo -Zself-profile

This will generate three files in the working directory such as:

  • foo-1234.events
  • foo-1234.string_data
  • foo-1234.string_index

Where foo is the name of the crate and 1234 is the process id of the rustc process.

To get a summary of where the compiler is spending its time:

$ ../measureme/target/release/summarize summarize foo-1234

To generate a flamegraph of the same data:

$ ../measureme/target/release/inferno foo-1234

To dump the event data in a Chromium-profiler compatible format:

$ ../measureme/target/release/crox foo-1234

For more information, consult the measureme documentation.

self-profile-events


The -Zself-profile-events compiler flag controls what events are recorded by the self-profiler when it is enabled via the -Zself-profile flag.

This flag takes a comma delimited list of event types to record.

For example:

$ rustc -Zself-profile -Zself-profile-events=default,args

Event types

  • query-provider

    • Traces each query used internally by the compiler.
  • generic-activity

    • Traces other parts of the compiler not covered by the query system.
  • query-cache-hit

    • Adds tracing information that records when the in-memory query cache is "hit" and does not need to re-execute a query which has been cached.
    • Disabled by default because this significantly increases the trace file size.
  • query-blocked

    • Tracks time that a query tries to run but is blocked waiting on another thread executing the same query to finish executing.
    • Query blocking only occurs when the compiler is built with parallel mode support.
  • incr-cache-load

    • Tracks time that is spent loading and deserializing query results from the incremental compilation on-disk cache.
  • query-keys

    • Adds a serialized representation of each query's query key to the tracing data.
    • Disabled by default because this significantly increases the trace file size.
  • function-args

    • Adds additional tracing data to some generic-activity events.
    • Disabled by default for parity with query-keys.
  • llvm

    • Adds tracing information about LLVM passes and codegeneration.
    • Disabled by default because this significantly increases the trace file size.

Event synonyms

  • none

    • Disables all events. Equivalent to the self-profiler being disabled.
  • default

    • The default set of events which stikes a balance between providing detailed tracing data and adding additional overhead to the compilation.
  • args

    • Equivalent to query-keys and function-args.
  • all

    • Enables all events.

Examples

Enable the profiler and capture the default set of events (both invocations are equivalent):

$ rustc -Zself-profile
$ rustc -Zself-profile -Zself-profile-events=default

Enable the profiler and capture the default events and their arguments:

$ rustc -Zself-profile -Zself-profile-events=default,args

shell-argfiles


The -Zshell-argfiles compiler flag allows argfiles to be parsed using POSIX "shell-style" quoting. When enabled, the compiler will use shlex to parse the arguments from argfiles specified with @shell:<path>.

Because this feature controls the parsing of input arguments, the -Zshell-argfiles flag must be present before the argument specifying the shell-style argument file.

src-hash-algorithm

The tracking issue for this feature is: #70401.


The -Z src-hash-algorithm compiler flag controls which algorithm is used when hashing each source file. The hash is stored in the debug info and can be used by a debugger to verify the source code matches the executable.

Supported hash algorithms are: md5, sha1, and sha256. Note that not all hash algorithms are supported by all debug info formats.

By default, the compiler chooses the hash algorithm based on the target specification.

temps-dir


The -Ztemps-dir compiler flag specifies the directory to write the intermediate files in. If not set, the output directory is used. This option is useful if you are running more than one instance of rustc (e.g. with different --crate-type settings), and you need to make sure they are not overwriting each other's intermediate files. No files are kept unless -C save-temps=yes is also set.

tiny-const-eval-limit


The -Ztiny-const-eval-limit compiler flag sets a tiny, non-configurable limit for const eval. This flag should only be used by const eval tests in the rustc test suite.

tls_model

The tracking issue for this feature is: None.


Option -Z tls-model controls TLS model used to generate code for accessing #[thread_local] static items.

Supported values for this option are:

  • global-dynamic - General Dynamic TLS Model (alternatively called Global Dynamic) is the most general option usable in all circumstances, even if the TLS data is defined in a shared library loaded at runtime and is accessed from code outside of that library. This is the default for most targets.
  • local-dynamic - model usable if the TLS data is only accessed from the shared library or executable it is defined in. The TLS data may be in a library loaded after startup (via dlopen).
  • initial-exec - model usable if the TLS data is defined in the executable or in a shared library loaded at program startup. The TLS data must not be in a library loaded after startup (via dlopen).
  • local-exec - model usable only if the TLS data is defined directly in the executable, but not in a shared library, and is accessed only from that executable.
  • emulated - Uses thread-specific data keys to implement emulated TLS. It is like using a general-dynamic TLS model for all modes.

rustc and LLVM may use a more optimized model than specified if they know that we are producing an executable rather than a library, or that the static item is private enough.

ub-checks

The tracking issue for this feature is: #123499.


The -Zub-checks compiler flag enables additional runtime checks that detect some causes of Undefined Behavior at runtime. By default, -Zub-checks flag inherits the value of -Cdebug-assertions.

All checks are generated on a best-effort basis; even if we have a check implemented for some cause of Undefined Behavior, it may be possible for the check to not fire. If a dependency is compiled with -Zub-checks=no but the final binary or library is compiled with -Zub-checks=yes, UB checks reached by the dependency are likely to be optimized out.

When -Zub-checks detects UB, a non-unwinding panic is produced. That means that we will not unwind the stack and will not call any Drop impls, but we will execute the configured panic hook. We expect that unsafe code has been written which relies on code not unwinding which may have UB checks inserted. Ergo, an unwinding panic could easily turn works-as-intended UB into a much bigger problem. Calling the panic hook theoretically has the same implications, but we expect that the standard library panic hook will be stateless enough to be always called, and that if a user has configured a panic hook that the hook may be very helpful to debugging the detected UB.

unsound-mir-opts


The -Zunsound-mir-opts compiler flag enables MIR optimization passes which can cause unsound behavior. This flag should only be used by MIR optimization tests in the rustc test suite.

virtual-function-elimination

This option controls whether LLVM runs the Virtual Function Elimination (VFE) optimization. This optimization in only available with LTO, so this flag can only be passed if -Clto is also passed.

VFE makes it possible to remove functions from vtables that are never dynamically called by the rest of the code. Without this flag, LLVM makes the really conservative assumption, that if any function in a vtable is called, no function that is referenced by this vtable can be removed. With this flag additional information are given to LLVM, so that it can determine which functions are actually called and remove the unused functions.

Limitations

At the time of writing this flag may remove vtable functions too eagerly. One such example is in this code:

#![allow(unused)]
fn main() {
trait Foo { fn foo(&self) { println!("foo") } }

impl Foo for usize {}

pub struct FooBox(Box<dyn Foo>);

pub fn make_foo() -> FooBox { FooBox(Box::new(0)) }

#[inline]
pub fn f(a: FooBox) { a.0.foo() }
}

In the above code the Foo trait is private, so an assumption is made that its functions can only be seen/called from the current crate and can therefore get optimized out, if unused. However, with make_foo you can produce a wrapped dyn Foo type outside of the current crate, which can then be used in f. Due to inlining of f, Foo::foo can then be called from a foreign crate. This can lead to miscompilations.

Language features

aarch64_ver_target_feature

The tracking issue for this feature is: #44839


abi_avr_interrupt

The tracking issue for this feature is: #69664


abi_c_cmse_nonsecure_call

The tracking issue for this feature is: #81391


The TrustZone-M feature is available for targets with the Armv8-M architecture profile (thumbv8m in their target name). LLVM, the Rust compiler and the linker are providing support for the TrustZone-M feature.

One of the things provided, with this unstable feature, is the C-cmse-nonsecure-call function ABI. This ABI is used on function pointers to non-secure code to mark a non-secure function call (see section 5.5 for details).

With this ABI, the compiler will do the following to perform the call:

  • save registers needed after the call to Secure memory
  • clear all registers that might contain confidential information
  • clear the Least Significant Bit of the function address
  • branches using the BLXNS instruction

To avoid using the non-secure stack, the compiler will constrain the number and type of parameters/return value.

The extern "C-cmse-nonsecure-call" ABI is otherwise equivalent to the extern "C" ABI.

#![no_std]
#![feature(abi_c_cmse_nonsecure_call)]

#[no_mangle]
pub fn call_nonsecure_function(addr: usize) -> u32 {
    let non_secure_function =
        unsafe { core::mem::transmute::<usize, extern "C-cmse-nonsecure-call" fn() -> u32>(addr) };
    non_secure_function()
}
$ rustc --emit asm --crate-type lib --target thumbv8m.main-none-eabi function.rs

call_nonsecure_function:
        .fnstart
        .save   {r7, lr}
        push    {r7, lr}
        .setfp  r7, sp
        mov     r7, sp
        .pad    #16
        sub     sp, #16
        str     r0, [sp, #12]
        ldr     r0, [sp, #12]
        str     r0, [sp, #8]
        b       .LBB0_1
.LBB0_1:
        ldr     r0, [sp, #8]
        push.w  {r4, r5, r6, r7, r8, r9, r10, r11}
        bic     r0, r0, #1
        mov     r1, r0
        mov     r2, r0
        mov     r3, r0
        mov     r4, r0
        mov     r5, r0
        mov     r6, r0
        mov     r7, r0
        mov     r8, r0
        mov     r9, r0
        mov     r10, r0
        mov     r11, r0
        mov     r12, r0
        msr     apsr_nzcvq, r0
        blxns   r0
        pop.w   {r4, r5, r6, r7, r8, r9, r10, r11}
        str     r0, [sp, #4]
        b       .LBB0_2
.LBB0_2:
        ldr     r0, [sp, #4]
        add     sp, #16
        pop     {r7, pc}

abi_msp430_interrupt

The tracking issue for this feature is: #38487


In the MSP430 architecture, interrupt handlers have a special calling convention. You can use the "msp430-interrupt" ABI to make the compiler apply the right calling convention to the interrupt handlers you define.

#![feature(abi_msp430_interrupt)]
#![no_std]

// Place the interrupt handler at the appropriate memory address
// (Alternatively, you can use `#[used]` and remove `pub` and `#[no_mangle]`)
#[link_section = "__interrupt_vector_10"]
#[no_mangle]
pub static TIM0_VECTOR: extern "msp430-interrupt" fn() = tim0;

// The interrupt handler
extern "msp430-interrupt" fn tim0() {
    // ..
}
$ msp430-elf-objdump -CD ./target/msp430/release/app
Disassembly of section __interrupt_vector_10:

0000fff2 <TIM0_VECTOR>:
    fff2:       00 c0           interrupt service routine at 0xc000

Disassembly of section .text:

0000c000 <int::tim0>:
    c000:       00 13           reti

abi_ptx

The tracking issue for this feature is: #38788


When emitting PTX code, all vanilla Rust functions (fn) get translated to "device" functions. These functions are not callable from the host via the CUDA API so a crate with only device functions is not too useful!

OTOH, "global" functions can be called by the host; you can think of them as the real public API of your crate. To produce a global function use the "ptx-kernel" ABI.

#![feature(abi_ptx)]
#![no_std]

pub unsafe extern "ptx-kernel" fn global_function() {
    device_function();
}

pub fn device_function() {
    // ..
}
$ xargo rustc --target nvptx64-nvidia-cuda --release -- --emit=asm

$ cat $(find -name '*.s')
//
// Generated by LLVM NVPTX Back-End
//

.version 3.2
.target sm_20
.address_size 64

        // .globl       _ZN6kernel15global_function17h46111ebe6516b382E

.visible .entry _ZN6kernel15global_function17h46111ebe6516b382E()
{


        ret;
}

        // .globl       _ZN6kernel15device_function17hd6a0e4993bbf3f78E
.visible .func _ZN6kernel15device_function17hd6a0e4993bbf3f78E()
{


        ret;
}

abi_riscv_interrupt

The tracking issue for this feature is: #111889


abi_unadjusted

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


abi_vectorcall

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


abi_x86_interrupt

The tracking issue for this feature is: #40180


adt_const_params

The tracking issue for this feature is: #95174


Allows for using more complex types for const parameters, such as structs or enums.

#![allow(unused)]
#![feature(adt_const_params)]
#![allow(incomplete_features)]

fn main() {
use std::marker::ConstParamTy;

#[derive(ConstParamTy, PartialEq, Eq)]
enum Foo {
    A,
    B,
    C,
}

#[derive(ConstParamTy, PartialEq, Eq)]
struct Bar {
    flag: bool,
}

fn is_foo_a_and_bar_true<const F: Foo, const B: Bar>() -> bool {
    match (F, B.flag) {
        (Foo::A, true) => true,
        _ => false,
    }
}
}

alloc_error_handler

The tracking issue for this feature is: #51540


allocator_internals

This feature does not have a tracking issue, it is an unstable implementation detail of the global_allocator feature not intended for use outside the compiler.


allow_internal_unsafe

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


allow_internal_unstable

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


anonymous_lifetime_in_impl_trait

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


arbitrary_self_types

The tracking issue for this feature is: #44874


arm_target_feature

The tracking issue for this feature is: #44839


asm_const

The tracking issue for this feature is: #93332


This feature adds a const <expr> operand type to asm! and global_asm!.

  • <expr> must be an integer constant expression.
  • The value of the expression is formatted as a string and substituted directly into the asm template string.

asm_experimental_arch

The tracking issue for this feature is: #93335


This feature tracks asm! and global_asm! support for the following architectures:

  • NVPTX
  • PowerPC
  • Hexagon
  • MIPS32r2 and MIPS64r2
  • wasm32
  • BPF
  • SPIR-V
  • AVR
  • MSP430
  • M68k
  • CSKY
  • s390x
  • Arm64EC

Register classes

ArchitectureRegister classRegistersLLVM constraint code
MIPSreg$[2-25]r
MIPSfreg$f[0-31]f
NVPTXreg16None*h
NVPTXreg32None*r
NVPTXreg64None*l
Hexagonregr[0-28]r
PowerPCregr[0-31]r
PowerPCreg_nonzeror[1-31]b
PowerPCfregf[0-31]f
PowerPCcrcr[0-7], crOnly clobbers
PowerPCxerxerOnly clobbers
wasm32localNone*r
BPFregr[0-10]r
BPFwregw[0-10]w
AVRregr[2-25], XH, XL, ZH, ZLr
AVRreg_upperr[16-25], XH, XL, ZH, ZLd
AVRreg_pairr3r2 .. r25r24, X, Zr
AVRreg_iwr25r24, X, Zw
AVRreg_ptrX, Ze
MSP430regr[0-15]r
M68kregd[0-7], a[0-7]r
M68kreg_datad[0-7]d
M68kreg_addra[0-3]a
CSKYregr[0-31]r
CSKYfregf[0-31]f
s390xregr[0-10], r[12-14]r
s390xfregf[0-15]f
Arm64ECregx[0-12], x[15-22], x[25-27], x30r
Arm64ECvregv[0-15]w
Arm64ECvreg_low16v[0-15]x

Notes:

  • NVPTX doesn't have a fixed register set, so named registers are not supported.

  • WebAssembly doesn't have registers, so named registers are not supported.

Register class supported types

ArchitectureRegister classTarget featureAllowed types
MIPS32regNonei8, i16, i32, f32
MIPS32fregNonef32, f64
MIPS64regNonei8, i16, i32, i64, f32, f64
MIPS64fregNonef32, f64
NVPTXreg16Nonei8, i16
NVPTXreg32Nonei8, i16, i32, f32
NVPTXreg64Nonei8, i16, i32, f32, i64, f64
HexagonregNonei8, i16, i32, f32
PowerPCregNonei8, i16, i32
PowerPCreg_nonzeroNonei8, i16, i32
PowerPCfregNonef32, f64
PowerPCcrN/AOnly clobbers
PowerPCxerN/AOnly clobbers
wasm32localNonei8 i16 i32 i64 f32 f64
BPFregNonei8 i16 i32 i64
BPFwregalu32i8 i16 i32
AVRreg, reg_upperNonei8
AVRreg_pair, reg_iw, reg_ptrNonei16
MSP430regNonei8, i16
M68kreg, reg_addrNonei16, i32
M68kreg_dataNonei8, i16, i32
CSKYregNonei8, i16, i32
CSKYfregNonef32,
s390xreg, reg_addrNonei8, i16, i32, i64
s390xfregNonef32, f64
Arm64ECregNonei8, i16, i32, f32, i64, f64
Arm64ECvregNonei8, i16, i32, f32, i64, f64,
i8x8, i16x4, i32x2, i64x1, f32x2, f64x1,
i8x16, i16x8, i32x4, i64x2, f32x4, f64x2

Register aliases

ArchitectureBase registerAliases
Hexagonr29sp
Hexagonr30fr
Hexagonr31lr
BPFr[0-10]w[0-10]
AVRXHr27
AVRXLr26
AVRZHr31
AVRZLr30
MSP430r0pc
MSP430r1sp
MSP430r2sr
MSP430r3cg
MSP430r4fp
M68ka5bp
M68ka6fp
M68ka7sp, usp, ssp, isp
CSKYr[0-3]a[0-3]
CSKYr[4-11]l[0-7]
CSKYr[12-13]t[0-1]
CSKYr14sp
CSKYr15lr
CSKYr[16-17]l[8-9]
CSKYr[18-25]t[2-9]
CSKYr28rgb
CSKYr29rtb
CSKYr30svbr
CSKYr31tls
Arm64ECx[0-30]w[0-30]
Arm64ECx29fp
Arm64ECx30lr
Arm64ECspwsp
Arm64ECxzrwzr
Arm64ECv[0-15]b[0-15], h[0-15], s[0-15], d[0-15], q[0-15]

Notes:

  • TI does not mandate a frame pointer for MSP430, but toolchains are allowed to use one; LLVM uses r4.

Unsupported registers

ArchitectureUnsupported registerReason
Allsp, r15 (s390x)The stack pointer must be restored to its original value at the end of an asm code block.
Allfr (Hexagon), $fp (MIPS), Y (AVR), r4 (MSP430), a6 (M68k), r11 (s390x), x29 (Arm64EC)The frame pointer cannot be used as an input or output.
Allr19 (Hexagon), x19 (Arm64EC)This is used internally by LLVM as a "base pointer" for functions with complex stack frames.
MIPS$0 or $zeroThis is a constant zero register which can't be modified.
MIPS$1 or $atReserved for assembler.
MIPS$26/$k0, $27/$k1OS-reserved registers.
MIPS$28/$gpGlobal pointer cannot be used as inputs or outputs.
MIPS$raReturn address cannot be used as inputs or outputs.
HexagonlrThis is the link register which cannot be used as an input or output.
AVRr0, r1, r1r0Due to an issue in LLVM, the r0 and r1 registers cannot be used as inputs or outputs. If modified, they must be restored to their original values before the end of the block.
MSP430r0, r2, r3These are the program counter, status register, and constant generator respectively. Neither the status register nor constant generator can be written to.
M68ka4, a5Used internally by LLVM for the base pointer and global base pointer.
CSKYr7, r28Used internally by LLVM for the base pointer and global base pointer.
CSKYr8Used internally by LLVM for the frame pointer.
CSKYr14Used internally by LLVM for the stack pointer.
CSKYr15This is the link register.
CSKYr[26-30]Reserved by its ABI.
CSKYr31This is the TLS register.
Arm64ECxzrThis is a constant zero register which can't be modified.
Arm64ECx18This is an OS-reserved register.
Arm64ECx13, x14, x23, x24, x28, v[16-31]These are AArch64 registers that are not supported for Arm64EC.

Template modifiers

ArchitectureRegister classModifierExample outputLLVM modifier
MIPSregNone$2None
MIPSfregNone$f0None
NVPTXreg16Noners0None
NVPTXreg32Noner0None
NVPTXreg64Nonerd0None
HexagonregNoner0None
PowerPCregNone0None
PowerPCreg_nonzeroNone3None
PowerPCfregNone0None
s390xregNone%r0None
s390xreg_addrNone%r1None
s390xfregNone%f0None
CSKYregNoner0None
CSKYfregNonef0None
Arm64ECregNonex0x
Arm64ECregww0w
Arm64ECregxx0x
Arm64ECvregNonev0None
Arm64ECvregvv0None
Arm64ECvregbb0b
Arm64ECvreghh0h
Arm64ECvregss0s
Arm64ECvregdd0d
Arm64ECvregqq0q

Flags covered by preserves_flags

These flags registers must be restored upon exiting the asm block if the preserves_flags option is set:

  • AVR
    • The status register SREG.
  • MSP430
    • The status register r2.
  • M68k
    • The condition code register ccr.
  • s390x
    • The condition code register cc.
  • Arm64EC
    • Condition flags (NZCV register).
    • Floating-point status (FPSR register).

asm_goto

The tracking issue for this feature is: #119364


This feature adds a label <block> operand type to asm!.

Example:


unsafe {
    asm!(
        "jmp {}",
        label {
            println!("Jumped from asm!");
        }
    );
}

The block must have unit type or diverge.

When label <block> is used together with noreturn option, it means that the assembly will not fallthrough. It's allowed to jump to a label within the assembly. In this case, the entire asm! expression will have an unit type as opposed to diverging, if not all label blocks diverge. The asm! expression still diverges if noreturn option is used and all label blocks diverge.

asm_unwind

The tracking issue for this feature is: #93334


This feature adds a may_unwind option to asm! which allows an asm block to unwind stack and be part of the stack unwinding process. This option is only supported by the LLVM backend right now.

associated_const_equality

The tracking issue for this feature is: #92827


associated_type_defaults

The tracking issue for this feature is: #29661


async_closure

The tracking issue for this feature is: #62290


async_fn_track_caller

The tracking issue for this feature is: #110011


async_for_loop

The tracking issue for this feature is: #118898


auto_traits

The tracking issue for this feature is #13231


The auto_traits feature gate allows you to define auto traits.

Auto traits, like Send or Sync in the standard library, are marker traits that are automatically implemented for every type, unless the type, or a type it contains, has explicitly opted out via a negative impl. (Negative impls are separately controlled by the negative_impls feature.)

impl !Trait for Type {}

Example:

#![feature(negative_impls)]
#![feature(auto_traits)]

auto trait Valid {}

struct True;
struct False;

impl !Valid for False {}

struct MaybeValid<T>(T);

fn must_be_valid<T: Valid>(_t: T) { }

fn main() {
    // works
    must_be_valid( MaybeValid(True) );

    // compiler error - trait bound not satisfied
    // must_be_valid( MaybeValid(False) );
}

Automatic trait implementations

When a type is declared as an auto trait, we will automatically create impls for every struct/enum/union, unless an explicit impl is provided. These automatic impls contain a where clause for each field of the form T: AutoTrait, where T is the type of the field and AutoTrait is the auto trait in question. As an example, consider the struct List and the auto trait Send:

#![allow(unused)]
fn main() {
struct List<T> {
  data: T,
  next: Option<Box<List<T>>>,
}
}

Presuming that there is no explicit impl of Send for List, the compiler will supply an automatic impl of the form:

#![allow(unused)]
fn main() {
struct List<T> {
  data: T,
  next: Option<Box<List<T>>>,
}

unsafe impl<T> Send for List<T>
where
  T: Send, // from the field `data`
  Option<Box<List<T>>>: Send, // from the field `next`
{ }
}

Explicit impls may be either positive or negative. They take the form:

impl<...> AutoTrait for StructName<..> { }
impl<...> !AutoTrait for StructName<..> { }

Coinduction: Auto traits permit cyclic matching

Unlike ordinary trait matching, auto traits are coinductive. This means, in short, that cycles which occur in trait matching are considered ok. As an example, consider the recursive struct List introduced in the previous section. In attempting to determine whether List: Send, we would wind up in a cycle: to apply the impl, we must show that Option<Box<List>>: Send, which will in turn require Box<List>: Send and then finally List: Send again. Under ordinary trait matching, this cycle would be an error, but for an auto trait it is considered a successful match.

Items

Auto traits cannot have any trait items, such as methods or associated types. This ensures that we can generate default implementations.

Supertraits

Auto traits cannot have supertraits. This is for soundness reasons, as the interaction of coinduction with implied bounds is difficult to reconcile.

avx512_target_feature

The tracking issue for this feature is: #44839


box_patterns

The tracking issue for this feature is: #29641


Box patterns let you match on Box<T>s:

#![feature(box_patterns)]

fn main() {
    let b = Some(Box::new(5));
    match b {
        Some(box n) if n < 0 => {
            println!("Box contains negative number {n}");
        },
        Some(box n) if n >= 0 => {
            println!("Box contains non-negative number {n}");
        },
        None => {
            println!("No box");
        },
        _ => unreachable!()
    }
}

bpf_target_feature

The tracking issue for this feature is: #44839


builtin_syntax

The tracking issue for this feature is: #110680


c_unwind

The tracking issue for this feature is: #74990


Introduces new ABI strings:

  • "C-unwind"
  • "cdecl-unwind"
  • "stdcall-unwind"
  • "fastcall-unwind"
  • "vectorcall-unwind"
  • "thiscall-unwind"
  • "aapcs-unwind"
  • "win64-unwind"
  • "sysv64-unwind"
  • "system-unwind"

These enable unwinding from other languages (such as C++) into Rust frames and from Rust into other languages.

See RFC 2945 for more information.

c_variadic

The tracking issue for this feature is: #44930


The c_variadic language feature enables C-variadic functions to be defined in Rust. They may be called both from within Rust and via FFI.

Examples

#![allow(unused)]
#![feature(c_variadic)]

fn main() {
pub unsafe extern "C" fn add(n: usize, mut args: ...) -> usize {
    let mut sum = 0;
    for _ in 0..n {
        sum += args.arg::<usize>();
    }
    sum
}
}

cfg_overflow_checks

The tracking issue for this feature is: #111466


cfg_relocation_model

The tracking issue for this feature is: #114929


cfg_sanitize

The tracking issue for this feature is: #39699


The cfg_sanitize feature makes it possible to execute different code depending on whether a particular sanitizer is enabled or not.

Examples

#![allow(unused)]
#![feature(cfg_sanitize)]

fn main() {
#[cfg(sanitize = "thread")]
fn a() {
    // ...
}

#[cfg(not(sanitize = "thread"))]
fn a() {
    // ...
}

fn b() {
    if cfg!(sanitize = "leak") {
        // ...
    } else {
        // ...
    }
}
}

cfg_sanitizer_cfi

The tracking issue for this feature is: #89653


cfg_target_compact

The tracking issue for this feature is: #96901


cfg_target_has_atomic

The tracking issue for this feature is: #94039


cfg_target_has_atomic_equal_alignment

The tracking issue for this feature is: #93822


cfg_target_thread_local

The tracking issue for this feature is: #29594


cfg_ub_checks

The tracking issue for this feature is: #123499


cfg_version

The tracking issue for this feature is: #64796


The cfg_version feature makes it possible to execute different code depending on the compiler version. It will return true if the compiler version is greater than or equal to the specified version.

Examples

#![allow(unused)]
#![feature(cfg_version)]

fn main() {
#[cfg(version("1.42"))] // 1.42 and above
fn a() {
    // ...
}

#[cfg(not(version("1.42")))] // 1.41 and below
fn a() {
    // ...
}

fn b() {
    if cfg!(version("1.42")) {
        // ...
    } else {
        // ...
    }
}
}

cfi_encoding

The tracking issue for this feature is: #89653


The cfi_encoding feature allows the user to define a CFI encoding for a type. It allows the user to use a different names for types that otherwise would be required to have the same name as used in externally defined C functions.

Examples

#![allow(unused)]
#![feature(cfi_encoding, extern_types)]

fn main() {
#[cfi_encoding = "3Foo"]
pub struct Type1(i32);

extern {
    #[cfi_encoding = "3Bar"]
    type Type2;
}
}

closure_lifetime_binder

The tracking issue for this feature is: #97362


closure_track_caller

The tracking issue for this feature is: #87417


Allows using the #[track_caller] attribute on closures and coroutines. Calls made to the closure or coroutine will have caller information available through std::panic::Location::caller(), just like using #[track_caller] on a function.

cmse_nonsecure_entry

The tracking issue for this feature is: #75835


The TrustZone-M feature is available for targets with the Armv8-M architecture profile (thumbv8m in their target name). LLVM, the Rust compiler and the linker are providing support for the TrustZone-M feature.

One of the things provided, with this unstable feature, is the cmse_nonsecure_entry attribute. This attribute marks a Secure function as an entry function (see section 5.4 for details). With this attribute, the compiler will do the following:

  • add a special symbol on the function which is the __acle_se_ prefix and the standard function name
  • constrain the number of parameters to avoid using the Non-Secure stack
  • before returning from the function, clear registers that might contain Secure information
  • use the BXNS instruction to return

Because the stack can not be used to pass parameters, there will be compilation errors if:

  • the total size of all parameters is too big (for example more than four 32 bits integers)
  • the entry function is not using a C ABI

The special symbol __acle_se_ will be used by the linker to generate a secure gateway veneer.

#![feature(cmse_nonsecure_entry)]

#[no_mangle]
#[cmse_nonsecure_entry]
pub extern "C" fn entry_function(input: u32) -> u32 {
    input + 6
}
$ rustc --emit obj --crate-type lib --target thumbv8m.main-none-eabi function.rs
$ arm-none-eabi-objdump -D function.o

00000000 <entry_function>:
   0:   b580            push    {r7, lr}
   2:   466f            mov     r7, sp
   4:   b082            sub     sp, #8
   6:   9001            str     r0, [sp, #4]
   8:   1d81            adds    r1, r0, #6
   a:   460a            mov     r2, r1
   c:   4281            cmp     r1, r0
   e:   9200            str     r2, [sp, #0]
  10:   d30b            bcc.n   2a <entry_function+0x2a>
  12:   e7ff            b.n     14 <entry_function+0x14>
  14:   9800            ldr     r0, [sp, #0]
  16:   b002            add     sp, #8
  18:   e8bd 4080       ldmia.w sp!, {r7, lr}
  1c:   4671            mov     r1, lr
  1e:   4672            mov     r2, lr
  20:   4673            mov     r3, lr
  22:   46f4            mov     ip, lr
  24:   f38e 8800       msr     CPSR_f, lr
  28:   4774            bxns    lr
  2a:   f240 0000       movw    r0, #0
  2e:   f2c0 0000       movt    r0, #0
  32:   f240 0200       movw    r2, #0
  36:   f2c0 0200       movt    r2, #0
  3a:   211c            movs    r1, #28
  3c:   f7ff fffe       bl      0 <_ZN4core9panicking5panic17h5c028258ca2fb3f5E>
  40:   defe            udf     #254    ; 0xfe

collapse_debuginfo

The tracking issue for this feature is: #100758


compiler_builtins

This feature is internal to the Rust compiler and is not intended for general use.


const_async_blocks

The tracking issue for this feature is: #85368


const_closures

The tracking issue for this feature is: #106003


const_extern_fn

The tracking issue for this feature is: #64926


const_fn_floating_point_arithmetic

The tracking issue for this feature is: #57241


const_for

The tracking issue for this feature is: #87575


const_mut_refs

The tracking issue for this feature is: #57349


const_precise_live_drops

The tracking issue for this feature is: #73255


const_refs_to_cell

The tracking issue for this feature is: #80384


const_refs_to_static

The tracking issue for this feature is: #119618


const_trait_impl

The tracking issue for this feature is: #67792


const_try

The tracking issue for this feature is: #74935


coroutine_clone

The tracking issue for this feature is: #95360


coroutines

The tracking issue for this feature is: #43122


The coroutines feature gate in Rust allows you to define coroutine or coroutine literals. A coroutine is a "resumable function" that syntactically resembles a closure but compiles to much different semantics in the compiler itself. The primary feature of a coroutine is that it can be suspended during execution to be resumed at a later date. Coroutines use the yield keyword to "return", and then the caller can resume a coroutine to resume execution just after the yield keyword.

Coroutines are an extra-unstable feature in the compiler right now. Added in RFC 2033 they're mostly intended right now as a information/constraint gathering phase. The intent is that experimentation can happen on the nightly compiler before actual stabilization. A further RFC will be required to stabilize coroutines and will likely contain at least a few small tweaks to the overall design.

A syntactical example of a coroutine is:

#![feature(coroutines, coroutine_trait)]

use std::ops::{Coroutine, CoroutineState};
use std::pin::Pin;

fn main() {
    let mut coroutine = || {
        yield 1;
        return "foo"
    };

    match Pin::new(&mut coroutine).resume(()) {
        CoroutineState::Yielded(1) => {}
        _ => panic!("unexpected value from resume"),
    }
    match Pin::new(&mut coroutine).resume(()) {
        CoroutineState::Complete("foo") => {}
        _ => panic!("unexpected value from resume"),
    }
}

Coroutines are closure-like literals which can contain a yield statement. The yield statement takes an optional expression of a value to yield out of the coroutine. All coroutine literals implement the Coroutine trait in the std::ops module. The Coroutine trait has one main method, resume, which resumes execution of the coroutine at the previous suspension point.

An example of the control flow of coroutines is that the following example prints all numbers in order:

#![feature(coroutines, coroutine_trait)]

use std::ops::Coroutine;
use std::pin::Pin;

fn main() {
    let mut coroutine = || {
        println!("2");
        yield;
        println!("4");
    };

    println!("1");
    Pin::new(&mut coroutine).resume(());
    println!("3");
    Pin::new(&mut coroutine).resume(());
    println!("5");
}

At this time the main intended use case of coroutines is an implementation primitive for async/await syntax, but coroutines will likely be extended to ergonomic implementations of iterators and other primitives in the future. Feedback on the design and usage is always appreciated!

The Coroutine trait

The Coroutine trait in std::ops currently looks like:

#![allow(unused)]
fn main() {
#![feature(arbitrary_self_types, coroutine_trait)]
use std::ops::CoroutineState;
use std::pin::Pin;

pub trait Coroutine<R = ()> {
    type Yield;
    type Return;
    fn resume(self: Pin<&mut Self>, resume: R) -> CoroutineState<Self::Yield, Self::Return>;
}
}

The Coroutine::Yield type is the type of values that can be yielded with the yield statement. The Coroutine::Return type is the returned type of the coroutine. This is typically the last expression in a coroutine's definition or any value passed to return in a coroutine. The resume function is the entry point for executing the Coroutine itself.

The return value of resume, CoroutineState, looks like:

#![allow(unused)]
fn main() {
pub enum CoroutineState<Y, R> {
    Yielded(Y),
    Complete(R),
}
}

The Yielded variant indicates that the coroutine can later be resumed. This corresponds to a yield point in a coroutine. The Complete variant indicates that the coroutine is complete and cannot be resumed again. Calling resume after a coroutine has returned Complete will likely result in a panic of the program.

Closure-like semantics

The closure-like syntax for coroutines alludes to the fact that they also have closure-like semantics. Namely:

  • When created, a coroutine executes no code. A closure literal does not actually execute any of the closure's code on construction, and similarly a coroutine literal does not execute any code inside the coroutine when constructed.

  • Coroutines can capture outer variables by reference or by move, and this can be tweaked with the move keyword at the beginning of the closure. Like closures all coroutines will have an implicit environment which is inferred by the compiler. Outer variables can be moved into a coroutine for use as the coroutine progresses.

  • Coroutine literals produce a value with a unique type which implements the std::ops::Coroutine trait. This allows actual execution of the coroutine through the Coroutine::resume method as well as also naming it in return types and such.

  • Traits like Send and Sync are automatically implemented for a Coroutine depending on the captured variables of the environment. Unlike closures, coroutines also depend on variables live across suspension points. This means that although the ambient environment may be Send or Sync, the coroutine itself may not be due to internal variables live across yield points being not-Send or not-Sync. Note that coroutines do not implement traits like Copy or Clone automatically.

  • Whenever a coroutine is dropped it will drop all captured environment variables.

Coroutines as state machines

In the compiler, coroutines are currently compiled as state machines. Each yield expression will correspond to a different state that stores all live variables over that suspension point. Resumption of a coroutine will dispatch on the current state and then execute internally until a yield is reached, at which point all state is saved off in the coroutine and a value is returned.

Let's take a look at an example to see what's going on here:

#![feature(coroutines, coroutine_trait)]

use std::ops::Coroutine;
use std::pin::Pin;

fn main() {
    let ret = "foo";
    let mut coroutine = move || {
        yield 1;
        return ret
    };

    Pin::new(&mut coroutine).resume(());
    Pin::new(&mut coroutine).resume(());
}

This coroutine literal will compile down to something similar to:

#![feature(arbitrary_self_types, coroutines, coroutine_trait)]

use std::ops::{Coroutine, CoroutineState};
use std::pin::Pin;

fn main() {
    let ret = "foo";
    let mut coroutine = {
        enum __Coroutine {
            Start(&'static str),
            Yield1(&'static str),
            Done,
        }

        impl Coroutine for __Coroutine {
            type Yield = i32;
            type Return = &'static str;

            fn resume(mut self: Pin<&mut Self>, resume: ()) -> CoroutineState<i32, &'static str> {
                use std::mem;
                match mem::replace(&mut *self, __Coroutine::Done) {
                    __Coroutine::Start(s) => {
                        *self = __Coroutine::Yield1(s);
                        CoroutineState::Yielded(1)
                    }

                    __Coroutine::Yield1(s) => {
                        *self = __Coroutine::Done;
                        CoroutineState::Complete(s)
                    }

                    __Coroutine::Done => {
                        panic!("coroutine resumed after completion")
                    }
                }
            }
        }

        __Coroutine::Start(ret)
    };

    Pin::new(&mut coroutine).resume(());
    Pin::new(&mut coroutine).resume(());
}

Notably here we can see that the compiler is generating a fresh type, __Coroutine in this case. This type has a number of states (represented here as an enum) corresponding to each of the conceptual states of the coroutine. At the beginning we're closing over our outer variable foo and then that variable is also live over the yield point, so it's stored in both states.

When the coroutine starts it'll immediately yield 1, but it saves off its state just before it does so indicating that it has reached the yield point. Upon resuming again we'll execute the return ret which returns the Complete state.

Here we can also note that the Done state, if resumed, panics immediately as it's invalid to resume a completed coroutine. It's also worth noting that this is just a rough desugaring, not a normative specification for what the compiler does.

coverage_attribute

The tracking issue for this feature is: #84605


The coverage attribute can be used to selectively disable coverage instrumentation in an annotated function. This might be useful to:

  • Avoid instrumentation overhead in a performance critical function
  • Avoid generating coverage for a function that is not meant to be executed, but still target 100% coverage for the rest of the program.

Example

#![allow(unused)]
#![feature(coverage_attribute)]

fn main() {
// `foo()` will get coverage instrumentation (by default)
fn foo() {
  // ...
}

#[coverage(off)]
fn bar() {
  // ...
}
}

csky_target_feature

The tracking issue for this feature is: #44839


custom_code_classes_in_docs

The tracking issue for this feature is: #79483


custom_inner_attributes

The tracking issue for this feature is: #54726


custom_mir

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


custom_test_frameworks

The tracking issue for this feature is: #50297


The custom_test_frameworks feature allows the use of #[test_case] and #![test_runner]. Any function, const, or static can be annotated with #[test_case] causing it to be aggregated (like #[test]) and be passed to the test runner determined by the #![test_runner] crate attribute.

#![allow(unused)]
#![feature(custom_test_frameworks)]
#![test_runner(my_runner)]

fn main() {
fn my_runner(tests: &[&i32]) {
    for t in tests {
        if **t == 0 {
            println!("PASSED");
        } else {
            println!("FAILED");
        }
    }
}

#[test_case]
const WILL_PASS: i32 = 0;

#[test_case]
const WILL_FAIL: i32 = 4;
}

decl_macro

The tracking issue for this feature is: #39412


default_type_parameter_fallback

The tracking issue for this feature is: #27336


deprecated_safe

The tracking issue for this feature is: #94978


deprecated_suggestion

The tracking issue for this feature is: #94785


deref_patterns

The tracking issue for this feature is: #87121


do_not_recommend

The tracking issue for this feature is: #51992


doc_auto_cfg

The tracking issue for this feature is: #43781


doc_cfg

The tracking issue for this feature is: #43781


The doc_cfg feature allows an API be documented as only available in some specific platforms. This attribute has two effects:

  1. In the annotated item's documentation, there will be a message saying "Available on (platform) only".

  2. The item's doc-tests will only run on the specific platform.

In addition to allowing the use of the #[doc(cfg)] attribute, this feature enables the use of a special conditional compilation flag, #[cfg(doc)], set whenever building documentation on your crate.

This feature was introduced as part of PR #43348 to allow the platform-specific parts of the standard library be documented.

#![allow(unused)]
#![feature(doc_cfg)]

fn main() {
#[cfg(any(windows, doc))]
#[doc(cfg(windows))]
/// The application's icon in the notification area (a.k.a. system tray).
///
/// # Examples
///
/// ```no_run
/// extern crate my_awesome_ui_library;
/// use my_awesome_ui_library::current_app;
/// use my_awesome_ui_library::windows::notification;
///
/// let icon = current_app().get::<notification::Icon>();
/// icon.show();
/// icon.show_message("Hello");
/// ```
pub struct Icon {
    // ...
}
}

doc_cfg_hide

The tracking issue for this feature is: #43781


doc_masked

The tracking issue for this feature is: #44027


The doc_masked feature allows a crate to exclude types from a given crate from appearing in lists of trait implementations. The specifics of the feature are as follows:

  1. When rustdoc encounters an extern crate statement annotated with a #[doc(masked)] attribute, it marks the crate as being masked.

  2. When listing traits a given type implements, rustdoc ensures that traits from masked crates are not emitted into the documentation.

  3. When listing types that implement a given trait, rustdoc ensures that types from masked crates are not emitted into the documentation.

This feature was introduced in PR #44026 to ensure that compiler-internal and implementation-specific types and traits were not included in the standard library's documentation. Such types would introduce broken links into the documentation.

doc_notable_trait

The tracking issue for this feature is: #45040

The doc_notable_trait feature allows the use of the #[doc(notable_trait)] attribute, which will display the trait in a "Notable traits" dialog for functions returning types that implement the trait. For example, this attribute is applied to the Iterator, Future, io::Read, and io::Write traits in the standard library.

You can do this on your own traits like so:

#![feature(doc_notable_trait)]

#[doc(notable_trait)]
pub trait MyTrait {}

pub struct MyStruct;
impl MyTrait for MyStruct {}

/// The docs for this function will have a button that displays a dialog about
/// `MyStruct` implementing `MyTrait`.
pub fn my_fn() -> MyStruct { MyStruct }

This feature was originally implemented in PR #45039.

See also its documentation in the rustdoc book.

dropck_eyepatch

The tracking issue for this feature is: #34761


dyn_star

The tracking issue for this feature is: #102425


effects

The tracking issue for this feature is: #102090


ermsb_target_feature

The tracking issue for this feature is: #44839


exclusive_range_pattern

The tracking issue for this feature is: #37854.


The exclusive_range_pattern feature allows non-inclusive range patterns (0..10) to be used in appropriate pattern matching contexts. It also can be combined with #![feature(half_open_range_patterns] to be able to use RangeTo patterns (..10).

It also enabled RangeFrom patterns but that has since been stabilized.

#![allow(unused)]
#![feature(exclusive_range_pattern)]
fn main() {
    let x = 5;
    match x {
        0..10 => println!("single digit"),
        10 => println!("ten isn't part of the above range"),
        _ => println!("nor is everything else.")
    }
}

exhaustive_patterns

The tracking issue for this feature is: #51085


explicit_tail_calls

The tracking issue for this feature is: #112788


extended_varargs_abi_support

The tracking issue for this feature is: #100189


This feature adds the possibility of using sysv64, win64 or efiapi calling conventions on functions with varargs.

extern_types

The tracking issue for this feature is: #43467


f128

The tracking issue for this feature is: #116909


Enable the f128 type for IEEE 128-bit floating numbers (quad precision).

f16

The tracking issue for this feature is: #116909


Enable the f16 type for IEEE 16-bit floating numbers (half precision).

ffi_const

The tracking issue for this feature is: #58328


The #[ffi_const] attribute applies clang's const attribute to foreign functions declarations.

That is, #[ffi_const] functions shall have no effects except for its return value, which can only depend on the values of the function parameters, and is not affected by changes to the observable state of the program.

Applying the #[ffi_const] attribute to a function that violates these requirements is undefined behaviour.

This attribute enables Rust to perform common optimizations, like sub-expression elimination, and it can avoid emitting some calls in repeated invocations of the function with the same argument values regardless of other operations being performed in between these functions calls (as opposed to #[ffi_pure] functions).

Pitfalls

A #[ffi_const] function can only read global memory that would not affect its return value for the whole execution of the program (e.g. immutable global memory). #[ffi_const] functions are referentially-transparent and therefore more strict than #[ffi_pure] functions.

A common pitfall involves applying the #[ffi_const] attribute to a function that reads memory through pointer arguments which do not necessarily point to immutable global memory.

A #[ffi_const] function that returns unit has no effect on the abstract machine's state, and a #[ffi_const] function cannot be #[ffi_pure].

A #[ffi_const] function must not diverge, neither via a side effect (e.g. a call to abort) nor by infinite loops.

When translating C headers to Rust FFI, it is worth verifying for which targets the const attribute is enabled in those headers, and using the appropriate cfg macros in the Rust side to match those definitions. While the semantics of const are implemented identically by many C and C++ compilers, e.g., clang, GCC, ARM C/C++ compiler, IBM ILE C/C++, etc. they are not necessarily implemented in this way on all of them. It is therefore also worth verifying that the semantics of the C toolchain used to compile the binary being linked against are compatible with those of the #[ffi_const].

ffi_pure

The tracking issue for this feature is: #58329


The #[ffi_pure] attribute applies clang's pure attribute to foreign functions declarations.

That is, #[ffi_pure] functions shall have no effects except for its return value, which shall not change across two consecutive function calls with the same parameters.

Applying the #[ffi_pure] attribute to a function that violates these requirements is undefined behavior.

This attribute enables Rust to perform common optimizations, like sub-expression elimination and loop optimizations. Some common examples of pure functions are strlen or memcmp.

These optimizations are only applicable when the compiler can prove that no program state observable by the #[ffi_pure] function has changed between calls of the function, which could alter the result. See also the #[ffi_const] attribute, which provides stronger guarantees regarding the allowable behavior of a function, enabling further optimization.

Pitfalls

A #[ffi_pure] function can read global memory through the function parameters (e.g. pointers), globals, etc. #[ffi_pure] functions are not referentially-transparent, and are therefore more relaxed than #[ffi_const] functions.

However, accessing global memory through volatile or atomic reads can violate the requirement that two consecutive function calls shall return the same value.

A pure function that returns unit has no effect on the abstract machine's state.

A #[ffi_pure] function must not diverge, neither via a side effect (e.g. a call to abort) nor by infinite loops.

When translating C headers to Rust FFI, it is worth verifying for which targets the pure attribute is enabled in those headers, and using the appropriate cfg macros in the Rust side to match those definitions. While the semantics of pure are implemented identically by many C and C++ compilers, e.g., clang, GCC, ARM C/C++ compiler, IBM ILE C/C++, etc. they are not necessarily implemented in this way on all of them. It is therefore also worth verifying that the semantics of the C toolchain used to compile the binary being linked against are compatible with those of the #[ffi_pure].

fn_align

The tracking issue for this feature is: #82232


fn_delegation

The tracking issue for this feature is: #118212


freeze_impls

The tracking issue for this feature is: #121675


fundamental

The tracking issue for this feature is: #29635


gen_blocks

The tracking issue for this feature is: #117078


generic_arg_infer

The tracking issue for this feature is: #85077


generic_assert

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


generic_associated_types_extended

The tracking issue for this feature is: #95451


generic_const_exprs

The tracking issue for this feature is: #76560


generic_const_items

The tracking issue for this feature is: #113521


half_open_range_patterns_in_slices

The tracking issue for this feature is: #67264 It is part of the exclusive_range_pattern feature, tracked at #37854.


This feature allow using top-level half-open range patterns in slices.

#![feature(half_open_range_patterns_in_slices)]
#![feature(exclusive_range_pattern)]

fn main() {
    let xs = [13, 1, 5, 2, 3, 1, 21, 8];
    let [a @ 3.., b @ ..3, c @ 4..6, ..] = xs else { return; };
}

Note that this feature is not required if the patterns are wrapped between parenthesis.

fn main() {
    let xs = [13, 1];
    let [(a @ 3..), c] = xs else { return; };
}

hexagon_target_feature

The tracking issue for this feature is: #44839


if_let_guard

The tracking issue for this feature is: #51114


impl_trait_in_assoc_type

The tracking issue for this feature is: #63063


impl_trait_in_fn_trait_return

The tracking issue for this feature is: #99697


inherent_associated_types

The tracking issue for this feature is: #8995


inline_const

The tracking issue for this feature is: #76001

See also inline_const_pat


This feature allows you to use inline constant expressions. For example, you can turn this code:

fn add_one(x: i32) -> i32 { x + 1 }
const MY_COMPUTATION: i32 = 1 + 2 * 3 / 4;

fn main() {
    let x = add_one(MY_COMPUTATION);
}

into this code:

#![feature(inline_const)]

fn add_one(x: i32) -> i32 { x + 1 }
fn main() {
    let x = add_one(const { 1 + 2 * 3 / 4 });
}

inline_const_pat

The tracking issue for this feature is: #76001

See also inline_const


This feature allows you to use inline constant expressions in pattern position:

#![allow(unused)]
#![feature(inline_const_pat)]

fn main() {
const fn one() -> i32 { 1 }

let some_int = 3;
match some_int {
    const { 1 + 2 } => println!("Matched 1 + 2"),
    const { one() } => println!("Matched const fn returning 1"),
    _ => println!("Didn't match anything :("),
}
}

intra-doc-pointers

The tracking issue for this feature is: #80896


Rustdoc does not currently allow disambiguating between *const and *mut, and raw pointers in intra-doc links are unstable until it does.

#![allow(unused)]
#![feature(intra_doc_pointers)]
fn main() {
//! [pointer::add]
}

intrinsics

The tracking issue for this feature is: None.

Intrinsics are rarely intended to be stable directly, but are usually exported in some sort of stable manner. Prefer using the stable interfaces to the intrinsic directly when you can.


Intrinsics with fallback logic

Many intrinsics can be written in pure rust, albeit inefficiently or without supporting some features that only exist on some backends. Backends can simply not implement those intrinsics without causing any code miscompilations or failures to compile. All intrinsic fallback bodies are automatically made cross-crate inlineable (like #[inline]) by the codegen backend, but not the MIR inliner.

#![allow(unused)]
#![feature(rustc_attrs, effects)]
#![allow(internal_features)]

fn main() {
#[rustc_intrinsic]
const unsafe fn const_deallocate(_ptr: *mut u8, _size: usize, _align: usize) {}
}

Since these are just regular functions, it is perfectly ok to create the intrinsic twice:

#![allow(unused)]
#![feature(rustc_attrs, effects)]
#![allow(internal_features)]

fn main() {
#[rustc_intrinsic]
const unsafe fn const_deallocate(_ptr: *mut u8, _size: usize, _align: usize) {}

mod foo {
    #[rustc_intrinsic]
    const unsafe fn const_deallocate(_ptr: *mut u8, _size: usize, _align: usize) {
        panic!("noisy const dealloc")
    }
}

}

The behaviour on backends that override the intrinsic is exactly the same. On other backends, the intrinsic behaviour depends on which implementation is called, just like with any regular function.

Intrinsics lowered to MIR instructions

Various intrinsics have native MIR operations that they correspond to. Instead of requiring backends to implement both the intrinsic and the MIR operation, the lower_intrinsics pass will convert the calls to the MIR operation. Backends do not need to know about these intrinsics at all. These intrinsics only make sense without a body, and can either be declared as a "rust-intrinsic" or as a #[rustc_intrinsic]. The body is never used, as calls to the intrinsic do not exist anymore after MIR analyses.

Intrinsics without fallback logic

These must be implemented by all backends.

#[rustc_intrinsic] declarations

These are written like intrinsics with fallback bodies, but the body is irrelevant. Use loop {} for the body or call the intrinsic recursively and add #[rustc_intrinsic_must_be_overridden] to the function to ensure that backends don't invoke the body.

Legacy extern ABI based intrinsics

These are imported as if they were FFI functions, with the special rust-intrinsic ABI. For example, if one was in a freestanding context, but wished to be able to transmute between types, and perform efficient pointer arithmetic, one would import those functions via a declaration like

#![feature(intrinsics)]
#![allow(internal_features)]
fn main() {}

extern "rust-intrinsic" {
    fn transmute<T, U>(x: T) -> U;

    fn arith_offset<T>(dst: *const T, offset: isize) -> *const T;
}

As with any other FFI functions, these are by default always unsafe to call. You can add #[rustc_safe_intrinsic] to the intrinsic to make it safe to call.

lahfsahf_target_feature

The tracking issue for this feature is: #44839


lang_items

The tracking issue for this feature is: None.


The rustc compiler has certain pluggable operations, that is, functionality that isn't hard-coded into the language, but is implemented in libraries, with a special marker to tell the compiler it exists. The marker is the attribute #[lang = "..."] and there are various different values of ..., i.e. various different 'lang items'. Most of them can only be defined once.

Lang items are loaded lazily by the compiler; e.g. if one never uses Box then there is no need to define a function for exchange_malloc. rustc will emit an error when an item is needed but not found in the current crate or any that it depends on.

Some features provided by lang items:

  • overloadable operators via traits: the traits corresponding to the ==, <, dereferencing (*) and + (etc.) operators are all marked with lang items; those specific four are eq, partial_ord, deref/deref_mut, and add respectively.
  • panicking: the panic and panic_impl lang items, among others.
  • stack unwinding: the lang item eh_personality is a function used by the failure mechanisms of the compiler. This is often mapped to GCC's personality function (see the std implementation for more information), but programs which don't trigger a panic can be assured that this function is never called. Additionally, a eh_catch_typeinfo static is needed for certain targets which implement Rust panics on top of C++ exceptions.
  • the traits in core::marker used to indicate types of various kinds; e.g. lang items sized, sync and copy.
  • memory allocation, see below.

Most lang items are defined by core, but if you're trying to build an executable without the std crate, you might run into the need for lang item definitions.

Example: Implementing a Box

Box pointers require two lang items: one for the type itself and one for allocation. A freestanding program that uses the Box sugar for dynamic allocations via malloc and free:

#![feature(lang_items, start, core_intrinsics, rustc_private, panic_unwind, rustc_attrs)]
#![allow(internal_features)]
#![no_std]

extern crate libc;
extern crate unwind;

use core::ffi::c_void;
use core::intrinsics;
use core::panic::PanicInfo;
use core::ptr::NonNull;

pub struct Global; // the global allocator
struct Unique<T>(NonNull<T>);

#[lang = "owned_box"]
pub struct Box<T, A = Global>(Unique<T>, A);

impl<T> Box<T> {
    pub fn new(x: T) -> Self {
        #[rustc_box]
        Box::new(x)
    }
}

impl<T, A> Drop for Box<T, A> {
    fn drop(&mut self) {
        unsafe {
            libc::free(self.0.0.as_ptr() as *mut c_void);
        }
    }
}

#[lang = "exchange_malloc"]
unsafe fn allocate(size: usize, _align: usize) -> *mut u8 {
    let p = libc::malloc(size) as *mut u8;

    // Check if `malloc` failed:
    if p.is_null() {
        intrinsics::abort();
    }

    p
}

#[start]
fn main(_argc: isize, _argv: *const *const u8) -> isize {
    let _x = Box::new(1);

    0
}

#[lang = "eh_personality"]
fn rust_eh_personality() {}

#[panic_handler]
fn panic_handler(_info: &PanicInfo) -> ! { intrinsics::abort() }

Note the use of abort: the exchange_malloc lang item is assumed to return a valid pointer, and so needs to do the check internally.

List of all language items

An up-to-date list of all language items can be found here in the compiler code.

large_assignments

The tracking issue for this feature is: #83518


lazy_type_alias

The tracking issue for this feature is: #112792


let_chains

The tracking issue for this feature is: #53667


lifetime_capture_rules_2024

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


link_arg_attribute

The tracking issue for this feature is: #99427


The link_arg_attribute feature allows passing arguments into the linker from inside of the source code. Order is preserved for link attributes as they were defined on a single extern block:

#![allow(unused)]
#![feature(link_arg_attribute)]

fn main() {
#[link(kind = "link-arg", name = "--start-group")]
#[link(kind = "static", name = "c")]
#[link(kind = "static", name = "gcc")]
#[link(kind = "link-arg", name = "--end-group")]
extern "C" {}
}

link_cfg

This feature is internal to the Rust compiler and is not intended for general use.


link_llvm_intrinsics

The tracking issue for this feature is: #29602


linkage

The tracking issue for this feature is: #29603


lint_reasons

The tracking issue for this feature is: #54503


loongarch_target_feature

The tracking issue for this feature is: #44839


macro_metavar_expr

The tracking issue for this feature is: #83527


marker_trait_attr

The tracking issue for this feature is: #29864


Normally, Rust keeps you from adding trait implementations that could overlap with each other, as it would be ambiguous which to use. This feature, however, carves out an exception to that rule: a trait can opt-in to having overlapping implementations, at the cost that those implementations are not allowed to override anything (and thus the trait itself cannot have any associated items, as they're pointless when they'd need to do the same thing for every type anyway).

#![allow(unused)]
#![feature(marker_trait_attr)]

fn main() {
#[marker] trait CheapToClone: Clone {}

impl<T: Copy> CheapToClone for T {}

// These could potentially overlap with the blanket implementation above,
// so are only allowed because CheapToClone is a marker trait.
impl<T: CheapToClone, U: CheapToClone> CheapToClone for (T, U) {}
impl<T: CheapToClone> CheapToClone for std::ops::Range<T> {}

fn cheap_clone<T: CheapToClone>(t: T) -> T {
    t.clone()
}
}

This is expected to replace the unstable overlapping_marker_traits feature, which applied to all empty traits (without needing an opt-in).

min_exhaustive_patterns

The tracking issue for this feature is: #119612


min_specialization

The tracking issue for this feature is: #31844


mips_target_feature

The tracking issue for this feature is: #44839


more_qualified_paths

The more_qualified_paths feature can be used in order to enable the use of qualified paths in patterns.

Example

#![feature(more_qualified_paths)]

fn main() {
    // destructure through a qualified path
    let <Foo as A>::Assoc { br } = StructStruct { br: 2 };
}

struct StructStruct {
    br: i8,
}

struct Foo;

trait A {
    type Assoc;
}

impl A for Foo {
    type Assoc = StructStruct;
}

multiple_supertrait_upcastable

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


must_not_suspend

The tracking issue for this feature is: #83310


mut_preserve_binding_mode_2024

The tracking issue for this feature is: #123076


mut_ref

The tracking issue for this feature is: #123076


naked_functions

The tracking issue for this feature is: #90957


native_link_modifiers_as_needed

The tracking issue for this feature is: #81490


The native_link_modifiers_as_needed feature allows you to use the as-needed modifier.

as-needed is only compatible with the dynamic and framework linking kinds. Using any other kind will result in a compiler error.

+as-needed means that the library will be actually linked only if it satisfies some undefined symbols at the point at which it is specified on the command line, making it similar to static libraries in this regard.

This modifier translates to --as-needed for ld-like linkers, and to -dead_strip_dylibs / -needed_library / -needed_framework for ld64. The modifier does nothing for linkers that don't support it (e.g. link.exe).

The default for this modifier is unclear, some targets currently specify it as +as-needed, some do not. We may want to try making +as-needed a default for all targets.

needs_panic_runtime

The tracking issue for this feature is: #32837


negative_bounds

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


negative_impls

The tracking issue for this feature is #68318.


With the feature gate negative_impls, you can write negative impls as well as positive ones:

#![allow(unused)]
#![feature(negative_impls)]
fn main() {
trait DerefMut { }
impl<T: ?Sized> !DerefMut for &T { }
}

Negative impls indicate a semver guarantee that the given trait will not be implemented for the given types. Negative impls play an additional purpose for auto traits, described below.

Negative impls have the following characteristics:

  • They do not have any items.
  • They must obey the orphan rules as if they were a positive impl.
  • They cannot "overlap" with any positive impls.

Semver interaction

It is a breaking change to remove a negative impl. Negative impls are a commitment not to implement the given trait for the named types.

Orphan and overlap rules

Negative impls must obey the same orphan rules as a positive impl. This implies you cannot add a negative impl for types defined in upstream crates and so forth.

Similarly, negative impls cannot overlap with positive impls, again using the same "overlap" check that we ordinarily use to determine if two impls overlap. (Note that positive impls typically cannot overlap with one another either, except as permitted by specialization.)

Interaction with auto traits

Declaring a negative impl impl !SomeAutoTrait for SomeType for an auto-trait serves two purposes:

  • as with any trait, it declares that SomeType will never implement SomeAutoTrait;
  • it disables the automatic SomeType: SomeAutoTrait impl that would otherwise have been generated.

Note that, at present, there is no way to indicate that a given type does not implement an auto trait but that it may do so in the future. For ordinary types, this is done by simply not declaring any impl at all, but that is not an option for auto traits. A workaround is that one could embed a marker type as one of the fields, where the marker type is !AutoTrait.

Immediate uses

Negative impls are used to declare that &T: !DerefMut and &mut T: !Clone, as required to fix the soundness of Pin described in #66544.

This serves two purposes:

  • For proving the correctness of unsafe code, we can use that impl as evidence that no DerefMut or Clone impl exists.
  • It prevents downstream crates from creating such impls.

never_patterns

The tracking issue for this feature is: #118155


never_type

The tracking issue for this feature is: #35121


never_type_fallback

The tracking issue for this feature is: #65992


no_core

The tracking issue for this feature is: #29639


no_sanitize

The tracking issue for this feature is: #39699


The no_sanitize attribute can be used to selectively disable sanitizer instrumentation in an annotated function. This might be useful to: avoid instrumentation overhead in a performance critical function, or avoid instrumenting code that contains constructs unsupported by given sanitizer.

The precise effect of this annotation depends on particular sanitizer in use. For example, with no_sanitize(thread), the thread sanitizer will no longer instrument non-atomic store / load operations, but it will instrument atomic operations to avoid reporting false positives and provide meaning full stack traces.

Examples

#![allow(unused)]
#![feature(no_sanitize)]

fn main() {
#[no_sanitize(address)]
fn foo() {
  // ...
}
}

non_exhaustive_omitted_patterns_lint

The tracking issue for this feature is: #89554


non_lifetime_binders

The tracking issue for this feature is: #108185


object_safe_for_dispatch

The tracking issue for this feature is: #43561


offset_of_enum

The tracking issue for this feature is: #120141


offset_of_nested

The tracking issue for this feature is: #120140


omit_gdb_pretty_printer_section

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


optimize_attribute

The tracking issue for this feature is: #54882


panic_runtime

The tracking issue for this feature is: #32837


pattern_complexity

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


pattern_types

The tracking issue for this feature is: #123646


postfix-match

postfix-match adds the feature for matching upon values postfix the expressions that generate the values.

#![allow(unused)]
#![feature(postfix_match)]

fn main() {
enum Foo {
    Bar,
    Baz
}

fn get_foo() -> Foo {
    Foo::Bar
}

get_foo().match {
    Foo::Bar => {},
    Foo::Baz => panic!(),
}
}

powerpc_target_feature

The tracking issue for this feature is: #44839


precise_capturing

The tracking issue for this feature is: #123432


prelude_import

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


prfchw_target_feature

The tracking issue for this feature is: #44839


proc_macro_hygiene

The tracking issue for this feature is: #54727


profiler_runtime

The tracking issue for this feature is: #42524.


raw_ref_op

The tracking issue for this feature is: #64490


ref_pat_eat_one_layer_2024

The tracking issue for this feature is: #123076


ref_pat_everywhere

The tracking issue for this feature is: #123076


register_tool

The tracking issue for this feature is: #66079


repr_simd

The tracking issue for this feature is: #27731


repr128

The tracking issue for this feature is: #56071


The repr128 feature adds support for #[repr(u128)] on enums.

#![allow(unused)]
#![feature(repr128)]

fn main() {
#[repr(u128)]
enum Foo {
    Bar(u64),
}
}

return_type_notation

The tracking issue for this feature is: #109417


riscv_target_feature

The tracking issue for this feature is: #44839


rtm_target_feature

The tracking issue for this feature is: #44839


rust_cold_cc

The tracking issue for this feature is: #97544


rustc_allow_const_fn_unstable

The tracking issue for this feature is: #69399


rustc_attrs

This feature has no tracking issue, and is therefore internal to the compiler, not being intended for general use.

Note: rustc_attrs enables many rustc-internal attributes and this page only discuss a few of them.


The rustc_attrs feature allows debugging rustc type layouts by using #[rustc_layout(...)] to debug layout at compile time (it even works with cargo check) as an alternative to rustc -Z print-type-sizes that is way more verbose.

Options provided by #[rustc_layout(...)] are debug, size, align, abi. Note that it only works on sized types without generics.

Examples

#![allow(unused)]
#![feature(rustc_attrs)]

fn main() {
#[rustc_layout(abi, size)]
pub enum X {
    Y(u8, u8, u8),
    Z(isize),
}
}

When that is compiled, the compiler will error with something like

error: abi: Aggregate { sized: true }
 --> src/lib.rs:4:1
  |
4 | / pub enum T {
5 | |     Y(u8, u8, u8),
6 | |     Z(isize),
7 | | }
  | |_^

error: size: Size { raw: 16 }
 --> src/lib.rs:4:1
  |
4 | / pub enum T {
5 | |     Y(u8, u8, u8),
6 | |     Z(isize),
7 | | }
  | |_^

error: aborting due to 2 previous errors

rustc_private

The tracking issue for this feature is: #27812


rustdoc_internals

The tracking issue for this feature is: #90418


rustdoc_missing_doc_code_examples

The tracking issue for this feature is: #101730


simd_ffi

The tracking issue for this feature is: #27731


specialization

The tracking issue for this feature is: #31844


sse4a_target_feature

The tracking issue for this feature is: #44839


staged_api

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


start

The tracking issue for this feature is: #29633


Allows you to mark a function as the entry point of the executable, which is necessary in #![no_std] environments.

The function marked #[start] is passed the command line parameters in the same format as the C main function (aside from the integer types being used). It has to be non-generic and have the following signature:

let _:
fn(isize, *const *const u8) -> isize
;

This feature should not be confused with the start lang item which is defined by the std crate and is written #[lang = "start"].

Usage together with the std crate

#[start] can be used in combination with the std crate, in which case the normal main function (which would get called from the std crate) won't be used as an entry point. The initialization code in std will be skipped this way.

Example:

#![allow(unused)]
#![feature(start)]

fn main() {
#[start]
fn start(_argc: isize, _argv: *const *const u8) -> isize {
    0
}
}

Unwinding the stack past the #[start] function is currently considered Undefined Behavior (for any unwinding implementation):

#![feature(start)]

#[start]
fn start(_argc: isize, _argv: *const *const u8) -> isize {
    std::panic::catch_unwind(|| {
        panic!(); // panic safely gets caught or safely aborts execution
    });

    panic!(); // UB!

    0
}

stmt_expr_attributes

The tracking issue for this feature is: #15701


strict_provenance

The tracking issue for this feature is: #95228


The strict_provenance feature allows to enable the fuzzy_provenance_casts and lossy_provenance_casts lints. These lint on casts between integers and pointers, that are recommended against or invalid in the strict provenance model. The same feature gate is also used for the experimental strict provenance API in std (actually core).

Example

#![feature(strict_provenance)]
#![warn(fuzzy_provenance_casts)]

fn main() {
    let _dangling = 16_usize as *const u8;
    //~^ WARNING: strict provenance disallows casting integer `usize` to pointer `*const u8`
}

string_deref_patterns

The tracking issue for this feature is: #87121


This feature permits pattern matching String to &str through its Deref implementation.

#![allow(unused)]
#![feature(string_deref_patterns)]

fn main() {
pub enum Value {
    String(String),
    Number(u32),
}

pub fn is_it_the_answer(value: Value) -> bool {
    match value {
        Value::String("42") => true,
        Value::Number(42) => true,
        _ => false,
    }
}
}

Without this feature other constructs such as match guards have to be used.

#![allow(unused)]
fn main() {
pub enum Value {
   String(String),
   Number(u32),
}

pub fn is_it_the_answer(value: Value) -> bool {
    match value {
        Value::String(s) if s == "42" => true,
        Value::Number(42) => true,
        _ => false,
    }
}
}

structural_match

The tracking issue for this feature is: #31434


target_feature_11

The tracking issue for this feature is: #69098


tbm_target_feature

The tracking issue for this feature is: #44839


test_unstable_lint

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


thread_local

The tracking issue for this feature is: #29594


trait_alias

The tracking issue for this feature is: #41517


The trait_alias feature adds support for trait aliases. These allow aliases to be created for one or more traits (currently just a single regular trait plus any number of auto-traits), and used wherever traits would normally be used as either bounds or trait objects.

#![feature(trait_alias)]

trait Foo = std::fmt::Debug + Send;
trait Bar = Foo + Sync;

// Use trait alias as bound on type parameter.
fn foo<T: Foo>(v: &T) {
    println!("{:?}", v);
}

pub fn main() {
    foo(&1);

    // Use trait alias for trait objects.
    let a: &Bar = &123;
    println!("{:?}", a);
    let b = Box::new(456) as Box<dyn Foo>;
    println!("{:?}", b);
}

trait_upcasting

The tracking issue for this feature is: #65991


The trait_upcasting feature adds support for trait upcasting coercion. This allows a trait object of type dyn Bar to be cast to a trait object of type dyn Foo so long as Bar: Foo.

#![allow(unused)]
#![feature(trait_upcasting)]
#![allow(incomplete_features)]

fn main() {
trait Foo {}

trait Bar: Foo {}

impl Foo for i32 {}

impl<T: Foo + ?Sized> Bar for T {}

let bar: &dyn Bar = &123;
let foo: &dyn Foo = bar;
}

transmute_generic_consts

The tracking issue for this feature is: #109929


transparent_unions

The tracking issue for this feature is #60405


The transparent_unions feature allows you mark unions as #[repr(transparent)]. A union may be #[repr(transparent)] in exactly the same conditions in which a struct may be #[repr(transparent)] (generally, this means the union must have exactly one non-zero-sized field). Some concrete illustrations follow.

#![allow(unused)]
#![feature(transparent_unions)]

fn main() {
// This union has the same representation as `f32`.
#[repr(transparent)]
union SingleFieldUnion {
    field: f32,
}

// This union has the same representation as `usize`.
#[repr(transparent)]
union MultiFieldUnion {
    field: usize,
    nothing: (),
}
}

For consistency with transparent structs, unions must have exactly one non-zero-sized field. If all fields are zero-sized, the union must not be #[repr(transparent)]:

#![allow(unused)]
#![feature(transparent_unions)]

fn main() {
// This (non-transparent) union is already valid in stable Rust:
pub union GoodUnion {
    pub nothing: (),
}

// Error: transparent union needs exactly one non-zero-sized field, but has 0
// #[repr(transparent)]
// pub union BadUnion {
//     pub nothing: (),
// }
}

The one exception is if the union is generic over T and has a field of type T, it may be #[repr(transparent)] even if T is a zero-sized type:

#![allow(unused)]
#![feature(transparent_unions)]

fn main() {
// This union has the same representation as `T`.
#[repr(transparent)]
pub union GenericUnion<T: Copy> { // Unions with non-`Copy` fields are unstable.
    pub field: T,
    pub nothing: (),
}

// This is okay even though `()` is a zero-sized type.
pub const THIS_IS_OKAY: GenericUnion<()> = GenericUnion { field: () };
}

Like transparent structs, a transparent union of type U has the same layout, size, and ABI as its single non-ZST field. If it is generic over a type T, and all its fields are ZSTs except for exactly one field of type T, then it has the same layout and ABI as T (even if T is a ZST when monomorphized).

Like transparent structs, transparent unions are FFI-safe if and only if their underlying representation type is also FFI-safe.

A union may not be eligible for the same nonnull-style optimizations that a struct or enum (with the same fields) are eligible for. Adding #[repr(transparent)] to union does not change this. To give a more concrete example, it is unspecified whether size_of::<T>() is equal to size_of::<Option<T>>(), where T is a union (regardless of whether or not it is transparent). The Rust compiler is free to perform this optimization if possible, but is not required to, and different compiler versions may differ in their application of these optimizations.

trivial_bounds

The tracking issue for this feature is: #48214


try_blocks

The tracking issue for this feature is: #31436


The try_blocks feature adds support for try blocks. A try block creates a new scope one can use the ? operator in.

#![allow(unused)]
#![feature(try_blocks)]

fn main() {
use std::num::ParseIntError;

let result: Result<i32, ParseIntError> = try {
    "1".parse::<i32>()?
        + "2".parse::<i32>()?
        + "3".parse::<i32>()?
};
assert_eq!(result, Ok(6));

let result: Result<i32, ParseIntError> = try {
    "1".parse::<i32>()?
        + "foo".parse::<i32>()?
        + "3".parse::<i32>()?
};
assert!(result.is_err());
}

type_alias_impl_trait

The tracking issue for this feature is: #63063


type_ascription

The tracking issue for this feature is: #23416


type_changing_struct_update

The tracking issue for this feature is: #86555


This implements RFC2528. When turned on, you can create instances of the same struct that have different generic type or lifetime parameters.

#![allow(unused_variables, dead_code)]
#![feature(type_changing_struct_update)]

fn main () {
    struct Foo<T, U> {
        field1: T,
        field2: U,
    }

    let base: Foo<String, i32> = Foo {
        field1: String::from("hello"),
        field2: 1234,
    };
    let updated: Foo<f64, i32> = Foo {
        field1: 3.14,
        ..base
    };
}

unboxed_closures

The tracking issue for this feature is #29625

See Also: fn_traits


The unboxed_closures feature allows you to write functions using the "rust-call" ABI, required for implementing the Fn* family of traits. "rust-call" functions must have exactly one (non self) argument, a tuple representing the argument list.

#![feature(unboxed_closures)]

extern "rust-call" fn add_args(args: (u32, u32)) -> u32 {
    args.0 + args.1
}

fn main() {}

unix_sigpipe

The tracking issue for this feature is: #97889


The #[unix_sigpipe = "..."] attribute on fn main() can be used to specify how libstd shall setup SIGPIPE on Unix platforms before invoking fn main(). This attribute is ignored on non-Unix targets. There are three variants:

  • #[unix_sigpipe = "inherit"]
  • #[unix_sigpipe = "sig_dfl"]
  • #[unix_sigpipe = "sig_ign"]

#[unix_sigpipe = "inherit"]

Leave SIGPIPE untouched before entering fn main(). Unless the parent process has changed the default SIGPIPE handler from SIG_DFL to something else, this will behave the same as #[unix_sigpipe = "sig_dfl"].

#[unix_sigpipe = "sig_dfl"]

Set the SIGPIPE handler to SIG_DFL. This will result in your program getting killed if it tries to write to a closed pipe. This is normally what you want if your program produces textual output.

Example

#![feature(unix_sigpipe)]
#[unix_sigpipe = "sig_dfl"]
fn main() { loop { println!("hello world"); } }
% ./main | head -n 1
hello world

#[unix_sigpipe = "sig_ign"]

Set the SIGPIPE handler to SIG_IGN before invoking fn main(). This will result in ErrorKind::BrokenPipe errors if you program tries to write to a closed pipe. This is normally what you want if you for example write socket servers, socket clients, or pipe peers.

This is what libstd has done by default since 2014. (However, see the note on child processes below.)

Example

#![feature(unix_sigpipe)]
#[unix_sigpipe = "sig_ign"]
fn main() { loop { println!("hello world"); } }
% ./main | head -n 1
hello world
thread 'main' panicked at 'failed printing to stdout: Broken pipe (os error 32)', library/std/src/io/stdio.rs:1016:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Note on child processes

When spawning child processes, the legacy Rust behavior if #[unix_sigpipe] is not specified is to reset SIGPIPE to SIG_DFL.

If #[unix_sigpipe = "..."] is specified, no matter what its value is, the signal disposition of SIGPIPE is no longer reset. This means that the child inherits the parent's SIGPIPE behavior.

unnamed_fields

The tracking issue for this feature is: #49804


unsized_fn_params

The tracking issue for this feature is: #48055


unsized_locals

The tracking issue for this feature is: #48055


This implements RFC1909. When turned on, you can have unsized arguments and locals:

#![allow(incomplete_features)]
#![feature(unsized_locals, unsized_fn_params)]

use std::any::Any;

fn main() {
    let x: Box<dyn Any> = Box::new(42);
    let x: dyn Any = *x;
    //  ^ unsized local variable
    //               ^^ unsized temporary
    foo(x);
}

fn foo(_: dyn Any) {}
//     ^^^^^^ unsized argument

The RFC still forbids the following unsized expressions:

#![feature(unsized_locals)]

use std::any::Any;

struct MyStruct<T: ?Sized> {
    content: T,
}

struct MyTupleStruct<T: ?Sized>(T);

fn answer() -> Box<dyn Any> {
    Box::new(42)
}

fn main() {
    // You CANNOT have unsized statics.
    static X: dyn Any = *answer();  // ERROR
    const Y: dyn Any = *answer();  // ERROR

    // You CANNOT have struct initialized unsized.
    MyStruct { content: *answer() };  // ERROR
    MyTupleStruct(*answer());  // ERROR
    (42, *answer());  // ERROR

    // You CANNOT have unsized return types.
    fn my_function() -> dyn Any { *answer() }  // ERROR

    // You CAN have unsized local variables...
    let mut x: dyn Any = *answer();  // OK
    // ...but you CANNOT reassign to them.
    x = *answer();  // ERROR

    // You CANNOT even initialize them separately.
    let y: dyn Any;  // OK
    y = *answer();  // ERROR

    // Not mentioned in the RFC, but by-move captured variables are also Sized.
    let x: dyn Any = *answer();
    (move || {  // ERROR
        let y = x;
    })();

    // You CAN create a closure with unsized arguments,
    // but you CANNOT call it.
    // This is an implementation detail and may be changed in the future.
    let f = |x: dyn Any| {};
    f(*answer());  // ERROR
}

By-value trait objects

With this feature, you can have by-value self arguments without Self: Sized bounds.

#![feature(unsized_fn_params)]

trait Foo {
    fn foo(self) {}
}

impl<T: ?Sized> Foo for T {}

fn main() {
    let slice: Box<[i32]> = Box::new([1, 2, 3]);
    <[i32] as Foo>::foo(*slice);
}

And Foo will also be object-safe.

#![feature(unsized_fn_params)]

trait Foo {
    fn foo(self) {}
}

impl<T: ?Sized> Foo for T {}

fn main () {
    let slice: Box<dyn Foo> = Box::new([1, 2, 3]);
    // doesn't compile yet
    <dyn Foo as Foo>::foo(*slice);
}

One of the objectives of this feature is to allow Box<dyn FnOnce>.

Variable length arrays

The RFC also describes an extension to the array literal syntax: [e; dyn n]. In the syntax, n isn't necessarily a constant expression. The array is dynamically allocated on the stack and has the type of [T], instead of [T; n].

#![feature(unsized_locals)]

fn mergesort<T: Ord>(a: &mut [T]) {
    let mut tmp = [T; dyn a.len()];
    // ...
}

fn main() {
    let mut a = [3, 1, 5, 6];
    mergesort(&mut a);
    assert_eq!(a, [1, 3, 5, 6]);
}

VLAs are not implemented yet. The syntax isn't final, either. We may need an alternative syntax for Rust 2015 because, in Rust 2015, expressions like [e; dyn(1)] would be ambiguous. One possible alternative proposed in the RFC is [e; n]: if n captures one or more local variables, then it is considered as [e; dyn n].

Advisory on stack usage

It's advised not to casually use the #![feature(unsized_locals)] feature. Typical use-cases are:

  • When you need a by-value trait objects.
  • When you really need a fast allocation of small temporary arrays.

Another pitfall is repetitive allocation and temporaries. Currently the compiler simply extends the stack frame every time it encounters an unsized assignment. So for example, the code

#![feature(unsized_locals)]

fn main() {
    let x: Box<[i32]> = Box::new([1, 2, 3, 4, 5]);
    let _x = {{{{{{{{{{*x}}}}}}}}}};
}

and the code

#![feature(unsized_locals)]

fn main() {
    for _ in 0..10 {
        let x: Box<[i32]> = Box::new([1, 2, 3, 4, 5]);
        let _x = *x;
    }
}

will unnecessarily extend the stack frame.

unsized_tuple_coercion

The tracking issue for this feature is: #42877


This is a part of RFC0401. According to the RFC, there should be an implementation like this:

impl<..., T, U: ?Sized> Unsized<(..., U)> for (..., T) where T: Unsized<U> {}

This implementation is currently gated behind #[feature(unsized_tuple_coercion)] to avoid insta-stability. Therefore you can use it like this:

#![feature(unsized_tuple_coercion)]

fn main() {
    let x : ([i32; 3], [i32; 3]) = ([1, 2, 3], [4, 5, 6]);
    let y : &([i32; 3], [i32]) = &x;
    assert_eq!(y.1[0], 4);
}

used_with_arg

The tracking issue for this feature is: #93798


wasm_abi

The tracking issue for this feature is: #83788


wasm_target_feature

The tracking issue for this feature is: #44839


with_negative_coherence

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


yeet_expr

The tracking issue for this feature is: #96373


The yeet_expr feature adds support for do yeet expressions, which can be used to early-exit from a function or try block.

These are highly experimental, thus the placeholder syntax.

#![allow(unused)]
#![feature(yeet_expr)]

fn main() {
fn foo() -> Result<String, i32> {
    do yeet 4;
}
assert_eq!(foo(), Err(4));

fn bar() -> Option<String> {
    do yeet;
}
assert_eq!(bar(), None);
}

Library Features

absolute_path

The tracking issue for this feature is: #92750


acceptfilter

The tracking issue for this feature is: #121891


addr_parse_ascii

The tracking issue for this feature is: #101035


alloc_error_hook

The tracking issue for this feature is: #51245


alloc_internals

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


alloc_layout_extra

The tracking issue for this feature is: #55724


allocator_api

The tracking issue for this feature is #32838


Sometimes you want the memory for one collection to use a different allocator than the memory for another collection. In this case, replacing the global allocator is not a workable option. Instead, you need to pass in an instance of an AllocRef to each collection for which you want a custom allocator.

TBD

array_chunks

The tracking issue for this feature is: #74985


array_into_iter_constructors

The tracking issue for this feature is: #91583


array_ptr_get

The tracking issue for this feature is: #119834


array_try_from_fn

The tracking issue for this feature is: #89379


array_try_map

The tracking issue for this feature is: #79711


array_windows

The tracking issue for this feature is: #75027


as_array_of_cells

The tracking issue for this feature is: #88248


ascii_char

The tracking issue for this feature is: #110998


ascii_char_variants

The tracking issue for this feature is: #110998


assert_matches

The tracking issue for this feature is: #82775


async_fn_traits

See Also: fn_traits


The async_fn_traits feature allows for implementation of the AsyncFn* traits for creating custom closure-like types that return futures.

The main difference to the Fn* family of traits is that AsyncFn can return a future that borrows from itself (FnOnce::Output has no lifetime parameters, while AsyncFnMut::CallRefFuture does).

async_gen_internals

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


async_iter_from_iter

The tracking issue for this feature is: #81798


async_iterator

The tracking issue for this feature is: #79024


atomic_bool_fetch_not

The tracking issue for this feature is: #98485


atomic_from_mut

The tracking issue for this feature is: #76314


backtrace_frames

The tracking issue for this feature is: #79676


bigint_helper_methods

The tracking issue for this feature is: #85532


binary_heap_as_slice

The tracking issue for this feature is: #83659


binary_heap_drain_sorted

The tracking issue for this feature is: #59278


binary_heap_into_iter_sorted

The tracking issue for this feature is: #59278


bound_as_ref

The tracking issue for this feature is: #80996


box_into_boxed_slice

The tracking issue for this feature is: #71582


box_into_inner

The tracking issue for this feature is: #80437


btree_cursors

The tracking issue for this feature is: #107540


btree_extract_if

The tracking issue for this feature is: #70530


btreemap_alloc

The tracking issue for this feature is: #32838


buf_read_has_data_left

The tracking issue for this feature is: #86423


bufread_skip_until

The tracking issue for this feature is: #111735


build_hasher_default_const_new

The tracking issue for this feature is: #123197


byte_slice_trim_ascii

The tracking issue for this feature is: #94035


c_size_t

The tracking issue for this feature is: #88345


c_str_module

The tracking issue for this feature is: #112134


c_void_variant

This feature is internal to the Rust compiler and is not intended for general use.


can_vector

The tracking issue for this feature is: #69941


cell_leak

The tracking issue for this feature is: #69099


cell_update

The tracking issue for this feature is: #50186


cfg_accessible

The tracking issue for this feature is: #64797


cfg_eval

The tracking issue for this feature is: #82679


cfg_match

The tracking issue for this feature is: #115585


char_indices_offset

The tracking issue for this feature is: #83871


char_internals

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


char_min

The tracking issue for this feature is: #114298


cmp_minmax

The tracking issue for this feature is: #115939


coerce_unsized

The tracking issue for this feature is: #18598


concat_bytes

The tracking issue for this feature is: #87555


concat_idents

The tracking issue for this feature is: #29599


The concat_idents feature adds a macro for concatenating multiple identifiers into one identifier.

Examples

#![feature(concat_idents)]

fn main() {
    fn foobar() -> u32 { 23 }
    let f = concat_idents!(foo, bar);
    assert_eq!(f(), 23);
}

const_align_of_val

The tracking issue for this feature is: #46571


const_align_of_val_raw

The tracking issue for this feature is: #46571


const_align_offset

The tracking issue for this feature is: #90962


const_alloc_error

The tracking issue for this feature is: #92523


const_alloc_layout

The tracking issue for this feature is: #67521


const_arguments_as_str

The tracking issue for this feature is: #103900


const_array_from_ref

The tracking issue for this feature is: #90206


const_array_into_iter_constructors

The tracking issue for this feature is: #91583


const_atomic_from_ptr

The tracking issue for this feature is: #108652


const_bigint_helper_methods

The tracking issue for this feature is: #85532


const_binary_heap_constructor

The tracking issue for this feature is: #112353


const_black_box

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


const_box

The tracking issue for this feature is: #92521


const_btree_len

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


const_cell_into_inner

The tracking issue for this feature is: #78729


const_char_from_u32_unchecked

The tracking issue for this feature is: #89259


const_cmp

The tracking issue for this feature is: #92391


const_collections_with_hasher

The tracking issue for this feature is: #102575


const_cow_is_borrowed

The tracking issue for this feature is: #65143


const_cstr_from_ptr

The tracking issue for this feature is: #113219


const_eval_select

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


const_exact_div

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


const_float_bits_conv

The tracking issue for this feature is: #72447


const_float_classify

The tracking issue for this feature is: #72505


const_fmt_arguments_new

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


const_format_args

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


const_hash

The tracking issue for this feature is: #104061


const_heap

The tracking issue for this feature is: #79597


const_hint_assert_unchecked

The tracking issue for this feature is: #119131


const_index_range_slice_index

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


const_int_from_str

The tracking issue for this feature is: #59133


const_intoiterator_identity

The tracking issue for this feature is: #90603


const_intrinsic_compare_bytes

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


const_intrinsic_copy

The tracking issue for this feature is: #80697


const_intrinsic_forget

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


const_intrinsic_raw_eq

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


const_ip

The tracking issue for this feature is: #76205


const_ipv4

The tracking issue for this feature is: #76205


const_ipv6

The tracking issue for this feature is: #76205


const_likely

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


const_maybe_uninit_array_assume_init

The tracking issue for this feature is: #96097


const_maybe_uninit_as_mut_ptr

The tracking issue for this feature is: #75251


const_maybe_uninit_assume_init

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


const_maybe_uninit_uninit_array

The tracking issue for this feature is: #96097


const_maybe_uninit_write

The tracking issue for this feature is: #63567


const_nonnull_new

The tracking issue for this feature is: #93235


const_num_midpoint

The tracking issue for this feature is: #110840


const_ops

The tracking issue for this feature is: #90080


const_option

The tracking issue for this feature is: #67441


const_option_ext

The tracking issue for this feature is: #91930


const_pin

The tracking issue for this feature is: #76654


const_pointer_is_aligned

The tracking issue for this feature is: #104203


const_pref_align_of

The tracking issue for this feature is: #91971


const_ptr_as_ref

The tracking issue for this feature is: #91822


const_ptr_is_null

The tracking issue for this feature is: #74939


const_ptr_sub_ptr

The tracking issue for this feature is: #95892


const_ptr_write

The tracking issue for this feature is: #86302


const_range_bounds

The tracking issue for this feature is: #108082


const_raw_ptr_comparison

The tracking issue for this feature is: #53020


const_replace

The tracking issue for this feature is: #83164


const_result

The tracking issue for this feature is: #82814


const_size_of_val

The tracking issue for this feature is: #46571


const_size_of_val_raw

The tracking issue for this feature is: #46571


const_slice_first_last

The tracking issue for this feature is: #83570


const_slice_first_last_chunk

The tracking issue for this feature is: #111774


const_slice_from_mut_ptr_range

The tracking issue for this feature is: #89792


const_slice_from_ptr_range

The tracking issue for this feature is: #89792


const_slice_from_raw_parts_mut

The tracking issue for this feature is: #67456


const_slice_from_ref

The tracking issue for this feature is: #90206


const_slice_index

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


const_slice_split_at_mut

The tracking issue for this feature is: #101804


const_str_from_raw_parts_mut

The tracking issue for this feature is: #119206


const_str_from_utf8

The tracking issue for this feature is: #91006


const_str_from_utf8_unchecked_mut

The tracking issue for this feature is: #91005


const_strict_overflow_ops

The tracking issue for this feature is: #118260


const_swap

The tracking issue for this feature is: #83163


const_three_way_compare

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


const_type_id

The tracking issue for this feature is: #77125


const_type_name

The tracking issue for this feature is: #63084


const_typed_swap

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


const_ub_checks

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


const_unicode_case_lookup

The tracking issue for this feature is: #101400


const_unsafecell_get_mut

The tracking issue for this feature is: #88836


const_waker

The tracking issue for this feature is: #102012


container_error_extra

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


context_ext

The tracking issue for this feature is: #123392


control_flow_enum

The tracking issue for this feature is: #75744


convert_float_to_int

The tracking issue for this feature is: #67057


core_intrinsics

This feature is internal to the Rust compiler and is not intended for general use.


core_io_borrowed_buf

The tracking issue for this feature is: #117693


core_pattern_type

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


core_pattern_types

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


core_private_bignum

This feature is internal to the Rust compiler and is not intended for general use.


core_private_diy_float

This feature is internal to the Rust compiler and is not intended for general use.


coroutine_trait

The tracking issue for this feature is: #43122


cow_is_borrowed

The tracking issue for this feature is: #65143


cstr_bytes

The tracking issue for this feature is: #112115


cursor_remaining

The tracking issue for this feature is: #86369


deadline_api

The tracking issue for this feature is: #46316


debug_closure_helpers

The tracking issue for this feature is: #117729


dec2flt

This feature is internal to the Rust compiler and is not intended for general use.


deref_pure_trait

The tracking issue for this feature is: #87121


derive_clone_copy

This feature is internal to the Rust compiler and is not intended for general use.


derive_const

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


derive_eq

This feature is internal to the Rust compiler and is not intended for general use.


dir_entry_ext2

The tracking issue for this feature is: #85573


discriminant_kind

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


dispatch_from_dyn

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


div_duration

The tracking issue for this feature is: #63139


downcast_unchecked

The tracking issue for this feature is: #90850


drain_keep_rest

The tracking issue for this feature is: #101122


duration_abs_diff

The tracking issue for this feature is: #117618


duration_constants

The tracking issue for this feature is: #57391


duration_constructors

The tracking issue for this feature is: #120301


Add the methods from_mins, from_hours and from_days to Duration.

duration_consts_float

The tracking issue for this feature is: #72440


duration_millis_float

The tracking issue for this feature is: #122451


duration_units

The tracking issue for this feature is: #120301


edition_panic

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


entry_insert

The tracking issue for this feature is: #65225


error_generic_member_access

The tracking issue for this feature is: #99301


error_in_core

The tracking issue for this feature is: #103765


error_iter

The tracking issue for this feature is: #58520


error_reporter

The tracking issue for this feature is: #90172


error_type_id

The tracking issue for this feature is: #60784


exact_size_is_empty

The tracking issue for this feature is: #35428


exclusive_wrapper

The tracking issue for this feature is: #98407


exit_status_error

The tracking issue for this feature is: #84908


exitcode_exit_method

The tracking issue for this feature is: #97100


exposed_provenance

The tracking issue for this feature is: #95228


extend_one

The tracking issue for this feature is: #72631


extract_if

The tracking issue for this feature is: #43244


fd

This feature is internal to the Rust compiler and is not intended for general use.


fd_read

This feature is internal to the Rust compiler and is not intended for general use.


float_gamma

The tracking issue for this feature is: #99842


float_minimum_maximum

The tracking issue for this feature is: #91079


float_next_up_down

The tracking issue for this feature is: #91399


flt2dec

This feature is internal to the Rust compiler and is not intended for general use.


fmt_helpers_for_derive

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


fmt_internals

This feature is internal to the Rust compiler and is not intended for general use.


fn_ptr_trait

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


fn_traits

The tracking issue for this feature is #29625

See Also: unboxed_closures


The fn_traits feature allows for implementation of the Fn* traits for creating custom closure-like types.

#![feature(unboxed_closures)]
#![feature(fn_traits)]

struct Adder {
    a: u32
}

impl FnOnce<(u32, )> for Adder {
    type Output = u32;
    extern "rust-call" fn call_once(self, b: (u32, )) -> Self::Output {
        self.a + b.0
    }
}

fn main() {
    let adder = Adder { a: 3 };
    assert_eq!(adder(2), 5);
}

forget_unsized

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


format_args_nl

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


freeze

The tracking issue for this feature is: #121675


fs_try_exists

The tracking issue for this feature is: #83186


future_join

The tracking issue for this feature is: #91642


gen_future

The tracking issue for this feature is: #50547


generic_assert_internals

The tracking issue for this feature is: #44838


generic_nonzero

The tracking issue for this feature is: #120257


get_many_mut

The tracking issue for this feature is: #104642


get_mut_unchecked

The tracking issue for this feature is: #63292


hash_extract_if

The tracking issue for this feature is: #59618


hash_raw_entry

The tracking issue for this feature is: #56167


hash_set_entry

The tracking issue for this feature is: #60896


hasher_prefixfree_extras

The tracking issue for this feature is: #96762


hashmap_internals

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


hint_assert_unchecked

The tracking issue for this feature is: #119131


hint_must_use

The tracking issue for this feature is: #94745


inplace_iteration

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


int_roundings

The tracking issue for this feature is: #88581


integer_atomics

The tracking issue for this feature is: #99069


internal_impls_macro

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


internal_output_capture

This feature is internal to the Rust compiler and is not intended for general use.


io_error_downcast

The tracking issue for this feature is: #99262


io_error_more

The tracking issue for this feature is: #86442


io_error_uncategorized

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


io_slice_advance

The tracking issue for this feature is: #62726


ip

The tracking issue for this feature is: #27709


ip_bits

The tracking issue for this feature is: #113744


is_ascii_octdigit

The tracking issue for this feature is: #101288


is_riscv_feature_detected

The tracking issue for this feature is: #111192


is_sorted

The tracking issue for this feature is: #53485


Add the methods is_sorted, is_sorted_by and is_sorted_by_key to [T]; add the methods is_sorted, is_sorted_by and is_sorted_by_key to Iterator.

is_val_statically_known

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


isqrt

The tracking issue for this feature is: #116226


iter_advance_by

The tracking issue for this feature is: #77404


iter_array_chunks

The tracking issue for this feature is: #100450


iter_collect_into

The tracking issue for this feature is: #94780


iter_from_coroutine

The tracking issue for this feature is: #43122


iter_intersperse

The tracking issue for this feature is: #79524


iter_is_partitioned

The tracking issue for this feature is: #62544


iter_map_windows

The tracking issue for this feature is: #87155


iter_next_chunk

The tracking issue for this feature is: #98326


iter_order_by

The tracking issue for this feature is: #64295


iter_partition_in_place

The tracking issue for this feature is: #62543


iter_repeat_n

The tracking issue for this feature is: #104434


iterator_try_collect

The tracking issue for this feature is: #94047


iterator_try_reduce

The tracking issue for this feature is: #87053


junction_point

The tracking issue for this feature is: #121709


layout_for_ptr

The tracking issue for this feature is: #69835


lazy_cell

The tracking issue for this feature is: #109736


lazy_cell_consume

The tracking issue for this feature is: #109736


liballoc_internals

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


libstd_sys_internals

This feature is internal to the Rust compiler and is not intended for general use.


linked_list_cursors

The tracking issue for this feature is: #58533


linked_list_remove

The tracking issue for this feature is: #69210


linked_list_retain

The tracking issue for this feature is: #114135


linux_pidfd

The tracking issue for this feature is: #82971


local_waker

The tracking issue for this feature is: #118959


log_syntax

The tracking issue for this feature is: #29598


map_entry_replace

The tracking issue for this feature is: #44286


map_many_mut

The tracking issue for this feature is: #97601


map_try_insert

The tracking issue for this feature is: #82766


mapped_lock_guards

The tracking issue for this feature is: #117108


maybe_uninit_array_assume_init

The tracking issue for this feature is: #96097


maybe_uninit_as_bytes

The tracking issue for this feature is: #93092


maybe_uninit_fill

The tracking issue for this feature is: #117428


maybe_uninit_slice

The tracking issue for this feature is: #63569


maybe_uninit_uninit_array

The tracking issue for this feature is: #96097


maybe_uninit_uninit_array_transpose

The tracking issue for this feature is: #96097


maybe_uninit_write_slice

The tracking issue for this feature is: #79995


mem_copy_fn

The tracking issue for this feature is: #98262


more_float_constants

The tracking issue for this feature is: #103883


new_uninit

The tracking issue for this feature is: #63291


non_null_convenience

The tracking issue for this feature is: #117691


non_zero_count_ones

The tracking issue for this feature is: #120287


nonzero_from_mut

The tracking issue for this feature is: #106290


nonzero_internals

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


nonzero_ops

The tracking issue for this feature is: #84186


noop_waker

The tracking issue for this feature is: #98286


num_midpoint

The tracking issue for this feature is: #110840


numfmt

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


once_cell_get_mut

The tracking issue for this feature is: #121641


once_cell_try

The tracking issue for this feature is: #109737


once_cell_try_insert

The tracking issue for this feature is: #116693


one_sided_range

The tracking issue for this feature is: #69780


option_get_or_insert_default

The tracking issue for this feature is: #82901


option_take_if

The tracking issue for this feature is: #98934


option_zip

The tracking issue for this feature is: #70086


os_str_display

The tracking issue for this feature is: #120048


os_str_slice

The tracking issue for this feature is: #118485


panic_abort

The tracking issue for this feature is: #32837


panic_always_abort

The tracking issue for this feature is: #84438


panic_backtrace_config

The tracking issue for this feature is: #93346


panic_can_unwind

The tracking issue for this feature is: #92988


panic_info_message

The tracking issue for this feature is: #66745


panic_internals

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


panic_unwind

The tracking issue for this feature is: #32837


panic_update_hook

The tracking issue for this feature is: #92649


path_file_prefix

The tracking issue for this feature is: #86319


pattern

The tracking issue for this feature is: #27721


peer_credentials_unix_socket

The tracking issue for this feature is: #42839


pin_deref_mut

The tracking issue for this feature is: #86918


pointer_is_aligned_to

The tracking issue for this feature is: #96284


pointer_like_trait

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


portable_simd

The tracking issue for this feature is: #86656


prelude_2024

The tracking issue for this feature is: #121042


print_internals

This feature is internal to the Rust compiler and is not intended for general use.


proc_macro_def_site

The tracking issue for this feature is: #54724


proc_macro_diagnostic

The tracking issue for this feature is: #54140


proc_macro_expand

The tracking issue for this feature is: #90765


proc_macro_internals

The tracking issue for this feature is: #27812


proc_macro_quote

The tracking issue for this feature is: #54722


proc_macro_span

The tracking issue for this feature is: #54725


proc_macro_tracked_env

The tracking issue for this feature is: #99515


process_exitcode_internals

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


process_internals

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


profiler_runtime_lib

This feature is internal to the Rust compiler and is not intended for general use.


ptr_alignment_type

The tracking issue for this feature is: #102070


ptr_as_uninit

The tracking issue for this feature is: #75402


ptr_internals

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


ptr_mask

The tracking issue for this feature is: #98290


ptr_metadata

The tracking issue for this feature is: #81513


ptr_sub_ptr

The tracking issue for this feature is: #95892


ptr_to_from_bits

The tracking issue for this feature is: #91126


pub_crate_should_not_need_unstable_attr

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


raw_os_error_ty

The tracking issue for this feature is: #107792


raw_slice_split

The tracking issue for this feature is: #95595


raw_vec_internals

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


read_buf

The tracking issue for this feature is: #78485


ready_into_inner

The tracking issue for this feature is: #101196


receiver_trait

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


reentrant_lock

The tracking issue for this feature is: #121440


restricted_std

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


result_flattening

The tracking issue for this feature is: #70142


round_char_boundary

The tracking issue for this feature is: #93743


rt

This feature is internal to the Rust compiler and is not intended for general use.


rustc_encodable_decodable

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


sealed

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


seek_seek_relative

The tracking issue for this feature is: #117374


seek_stream_len

The tracking issue for this feature is: #59359


set_ptr_value

The tracking issue for this feature is: #75091


setgroups

The tracking issue for this feature is: #90747


sgx_platform

The tracking issue for this feature is: #56975


sized_type_properties

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


slice_as_chunks

The tracking issue for this feature is: #74985


slice_concat_ext

The tracking issue for this feature is: #27747


slice_concat_trait

The tracking issue for this feature is: #27747


slice_flatten

The tracking issue for this feature is: #95629


slice_from_ptr_range

The tracking issue for this feature is: #89792


slice_index_methods

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


slice_internals

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


slice_iter_mut_as_mut_slice

The tracking issue for this feature is: #93079


slice_partition_dedup

The tracking issue for this feature is: #54279


slice_pattern

The tracking issue for this feature is: #56345


slice_ptr_get

The tracking issue for this feature is: #74265


slice_range

The tracking issue for this feature is: #76393


slice_split_once

The tracking issue for this feature is: #112811


slice_swap_unchecked

The tracking issue for this feature is: #88539


slice_take

The tracking issue for this feature is: #62280


solid_ext

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


sort_floats

The tracking issue for this feature is: #93396


sort_internals

This feature is internal to the Rust compiler and is not intended for general use.


split_array

The tracking issue for this feature is: #90091


split_as_slice

The tracking issue for this feature is: #96137


split_at_checked

The tracking issue for this feature is: #119128


std_internals

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


stdarch_arm_feature_detection

The tracking issue for this feature is: #111190


stdarch_mips_feature_detection

The tracking issue for this feature is: #111188


stdarch_powerpc_feature_detection

The tracking issue for this feature is: #111191


stdio_makes_pipe

The tracking issue for this feature is: #98288


step_trait

The tracking issue for this feature is: #42168


str_from_raw_parts

The tracking issue for this feature is: #119206


str_from_utf16_endian

The tracking issue for this feature is: #116258


str_internals

This feature is internal to the Rust compiler and is not intended for general use.


str_lines_remainder

The tracking issue for this feature is: #77998


str_split_inclusive_remainder

The tracking issue for this feature is: #77998


str_split_remainder

The tracking issue for this feature is: #77998


str_split_whitespace_remainder

The tracking issue for this feature is: #77998


strict_overflow_ops

The tracking issue for this feature is: #118260


strict_provenance_atomic_ptr

The tracking issue for this feature is: #99108


string_extend_from_within

The tracking issue for this feature is: #103806


string_remove_matches

The tracking issue for this feature is: #72826


sync_unsafe_cell

The tracking issue for this feature is: #95439


tcp_deferaccept

The tracking issue for this feature is: #119639


tcp_linger

The tracking issue for this feature is: #88494


tcp_quickack

The tracking issue for this feature is: #96256


tcplistener_into_incoming

The tracking issue for this feature is: #88339


test

The tracking issue for this feature is: None.


The internals of the test crate are unstable, behind the test flag. The most widely used part of the test crate are benchmark tests, which can test the performance of your code. Let's make our src/lib.rs look like this (comments elided):

#![allow(unused)]
#![feature(test)]

fn main() {
extern crate test;

pub fn add_two(a: i32) -> i32 {
    a + 2
}

#[cfg(test)]
mod tests {
    use super::*;
    use test::Bencher;

    #[test]
    fn it_works() {
        assert_eq!(4, add_two(2));
    }

    #[bench]
    fn bench_add_two(b: &mut Bencher) {
        b.iter(|| add_two(2));
    }
}
}

Note the test feature gate, which enables this unstable feature.

We've imported the test crate, which contains our benchmarking support. We have a new function as well, with the bench attribute. Unlike regular tests, which take no arguments, benchmark tests take a &mut Bencher. This Bencher provides an iter method, which takes a closure. This closure contains the code we'd like to benchmark.

We can run benchmark tests with cargo bench:

$ cargo bench
   Compiling adder v0.0.1 (file:///home/steve/tmp/adder)
     Running target/release/adder-91b3e234d4ed382a

running 2 tests
test tests::it_works ... ignored
test tests::bench_add_two ... bench:         1 ns/iter (+/- 0)

test result: ok. 0 passed; 0 failed; 1 ignored; 1 measured

Our non-benchmark test was ignored. You may have noticed that cargo bench takes a bit longer than cargo test. This is because Rust runs our benchmark a number of times, and then takes the average. Because we're doing so little work in this example, we have a 1 ns/iter (+/- 0), but this would show the variance if there was one.

Advice on writing benchmarks:

  • Move setup code outside the iter loop; only put the part you want to measure inside
  • Make the code do "the same thing" on each iteration; do not accumulate or change state
  • Make the outer function idempotent too; the benchmark runner is likely to run it many times
  • Make the inner iter loop short and fast so benchmark runs are fast and the calibrator can adjust the run-length at fine resolution
  • Make the code in the iter loop do something simple, to assist in pinpointing performance improvements (or regressions)

Gotcha: optimizations

There's another tricky part to writing benchmarks: benchmarks compiled with optimizations activated can be dramatically changed by the optimizer so that the benchmark is no longer benchmarking what one expects. For example, the compiler might recognize that some calculation has no external effects and remove it entirely.

#![allow(unused)]
#![feature(test)]

fn main() {
extern crate test;
use test::Bencher;

#[bench]
fn bench_xor_1000_ints(b: &mut Bencher) {
    b.iter(|| {
        (0..1000).fold(0, |old, new| old ^ new);
    });
}
}

gives the following results

running 1 test
test bench_xor_1000_ints ... bench:         0 ns/iter (+/- 0)

test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured

The benchmarking runner offers two ways to avoid this. Either, the closure that the iter method receives can return an arbitrary value which forces the optimizer to consider the result used and ensures it cannot remove the computation entirely. This could be done for the example above by adjusting the b.iter call to

#![allow(unused)]
fn main() {
struct X;
impl X { fn iter<T, F>(&self, _: F) where F: FnMut() -> T {} } let b = X;
b.iter(|| {
    // Note lack of `;` (could also use an explicit `return`).
    (0..1000).fold(0, |old, new| old ^ new)
});
}

Or, the other option is to call the generic test::black_box function, which is an opaque "black box" to the optimizer and so forces it to consider any argument as used.

#![feature(test)]

extern crate test;

fn main() {
struct X;
impl X { fn iter<T, F>(&self, _: F) where F: FnMut() -> T {} } let b = X;
b.iter(|| {
    let n = test::black_box(1000);

    (0..n).fold(0, |a, b| a ^ b)
})
}

Neither of these read or modify the value, and are very cheap for small values. Larger values can be passed indirectly to reduce overhead (e.g. black_box(&huge_struct)).

Performing either of the above changes gives the following benchmarking results

running 1 test
test bench_xor_1000_ints ... bench:       131 ns/iter (+/- 3)

test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured

However, the optimizer can still modify a testcase in an undesirable manner even when using either of the above.

thin_box

The tracking issue for this feature is: #92791


thread_id_value

The tracking issue for this feature is: #67939


thread_local_internals

This feature is internal to the Rust compiler and is not intended for general use.


thread_sleep_until

The tracking issue for this feature is: #113752


thread_spawn_unchecked

The tracking issue for this feature is: #55132


trace_macros

The tracking issue for this feature is #29598.


With trace_macros you can trace the expansion of macros in your code.

Examples

#![feature(trace_macros)]

fn main() {
    trace_macros!(true);
    println!("Hello, Rust!");
    trace_macros!(false);
}

The cargo build output:

note: trace_macro
 --> src/main.rs:5:5
  |
5 |     println!("Hello, Rust!");
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: expanding `println! { "Hello, Rust!" }`
  = note: to `print ! ( concat ! ( "Hello, Rust!" , "\n" ) )`
  = note: expanding `print! { concat ! ( "Hello, Rust!" , "\n" ) }`
  = note: to `$crate :: io :: _print ( format_args ! ( concat ! ( "Hello, Rust!" , "\n" ) )
          )`

    Finished dev [unoptimized + debuginfo] target(s) in 0.60 secs

track_path

The tracking issue for this feature is: #99515


transmutability

The tracking issue for this feature is: #99571


trusted_fused

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


trusted_len

The tracking issue for this feature is: #37572


trusted_len_next_unchecked

The tracking issue for this feature is: #37572


trusted_random_access

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


trusted_step

The tracking issue for this feature is: #85731


try_find

The tracking issue for this feature is: #63178


try_reserve_kind

The tracking issue for this feature is: #48043


try_trait_v2

The tracking issue for this feature is: #84277


try_trait_v2_residual

The tracking issue for this feature is: #91285


try_trait_v2_yeet

The tracking issue for this feature is: #96374


try_with_capacity

The tracking issue for this feature is: #91913


tuple_trait

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


uefi_std

The tracking issue for this feature is: #100499


unchecked_neg

The tracking issue for this feature is: #85122


unchecked_shifts

The tracking issue for this feature is: #85122


unicode_internals

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


unique_rc_arc

The tracking issue for this feature is: #112566


unix_file_vectored_at

The tracking issue for this feature is: #89517


unix_set_mark

The tracking issue for this feature is: #96467


unix_socket_ancillary_data

The tracking issue for this feature is: #76915


unix_socket_peek

The tracking issue for this feature is: #76923


unsafe_cell_from_mut

The tracking issue for this feature is: #111645


unsafe_pin_internals

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.


unsize

The tracking issue for this feature is: #18598


unwrap_infallible

The tracking issue for this feature is: #61695


update_panic_count

This feature is internal to the Rust compiler and is not intended for general use.


utf16_extra

The tracking issue for this feature is: #94919


utf16_extra_const

The tracking issue for this feature is: #94919


utf8_chunks

The tracking issue for this feature is: #99543


variant_count

The tracking issue for this feature is: #73662


vec_into_raw_parts

The tracking issue for this feature is: #65816


vec_pop_if

The tracking issue for this feature is: #122741


vec_push_within_capacity

The tracking issue for this feature is: #100486


vec_split_at_spare

The tracking issue for this feature is: #81944


waker_getters

The tracking issue for this feature is: #96992


wasi_ext

The tracking issue for this feature is: #71213


windows_by_handle

The tracking issue for this feature is: #63010


windows_c

This feature is internal to the Rust compiler and is not intended for general use.


windows_handle

This feature is internal to the Rust compiler and is not intended for general use.


windows_net

This feature is internal to the Rust compiler and is not intended for general use.


windows_process_exit_code_from

The tracking issue for this feature is: #111688


windows_process_extensions_async_pipes

The tracking issue for this feature is: #98289


windows_process_extensions_force_quotes

The tracking issue for this feature is: #82227


windows_process_extensions_main_thread_handle

The tracking issue for this feature is: #96723


windows_process_extensions_raw_attribute

The tracking issue for this feature is: #114854


windows_stdio

This feature is internal to the Rust compiler and is not intended for general use.


wrapping_int_impl

The tracking issue for this feature is: #32463


wrapping_next_power_of_two

The tracking issue for this feature is: #32463


write_all_vectored

The tracking issue for this feature is: #70436


yeet_desugar_details

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.