1. 1. Introduction
  2. 2. Getting Started
  3. 3. Tutorial: Guessing Game
  4. 4. Syntax and Semantics
    1. 4.1. Variable Bindings
    2. 4.2. Functions
    3. 4.3. Primitive Types
    4. 4.4. Comments
    5. 4.5. if
    6. 4.6. Loops
    7. 4.7. Vectors
    8. 4.8. Ownership
    9. 4.9. References and Borrowing
    10. 4.10. Lifetimes
    11. 4.11. Mutability
    12. 4.12. Structs
    13. 4.13. Enums
    14. 4.14. Match
    15. 4.15. Patterns
    16. 4.16. Method Syntax
    17. 4.17. Strings
    18. 4.18. Generics
    19. 4.19. Traits
    20. 4.20. Drop
    21. 4.21. if let
    22. 4.22. Trait Objects
    23. 4.23. Closures
    24. 4.24. Universal Function Call Syntax
    25. 4.25. Crates and Modules
    26. 4.26. `const` and `static`
    27. 4.27. Attributes
    28. 4.28. `type` aliases
    29. 4.29. Casting between types
    30. 4.30. Associated Types
    31. 4.31. Unsized Types
    32. 4.32. Operators and Overloading
    33. 4.33. Deref coercions
    34. 4.34. Macros
    35. 4.35. Raw Pointers
    36. 4.36. `unsafe`
  5. 5. Effective Rust
    1. 5.1. The Stack and the Heap
    2. 5.2. Testing
    3. 5.3. Conditional Compilation
    4. 5.4. Documentation
    5. 5.5. Iterators
    6. 5.6. Concurrency
    7. 5.7. Error Handling
    8. 5.8. Choosing your Guarantees
    9. 5.9. FFI
    10. 5.10. Borrow and AsRef
    11. 5.11. Release Channels
    12. 5.12. Using Rust without the standard library
  6. 6. Nightly Rust
    1. 6.1. Compiler Plugins
    2. 6.2. Inline Assembly
    3. 6.3. No stdlib
    4. 6.4. Intrinsics
    5. 6.5. Lang items
    6. 6.6. Advanced linking
    7. 6.7. Benchmark Tests
    8. 6.8. Box Syntax and Patterns
    9. 6.9. Slice Patterns
    10. 6.10. Associated Constants
    11. 6.11. Custom Allocators
  7. 7. Glossary
  8. 8. Syntax Index
  9. 9. Bibliography

No stdlib

Rust’s standard library provides a lot of useful functionality, but assumes support for various features of its host system: threads, networking, heap allocation, and others. There are systems that do not have these features, however, and Rust can work with those too! To do so, we tell Rust that we don’t want to use the standard library via an attribute: #![no_std].

Note: This feature is technically stable, but there are some caveats. For one, you can build a #![no_std] library on stable, but not a binary. For details on libraries without the standard library, see the chapter on #![no_std]

Obviously there's more to life than just libraries: one can use #[no_std] with an executable.

Using libc

In order to build a #[no_std] executable we will need libc as a dependency. We can specify this using our Cargo.toml file:

[dependencies]
libc = { version = "0.2.14", default-features = false }

Note that the default features have been disabled. This is a critical step - the default features of libc include the standard library and so must be disabled.

Writing an executable without stdlib

Controlling the entry point is possible in two ways: the #[start] attribute, or overriding the default shim for the C main function with your own.

The function marked #[start] is passed the command line parameters in the same format as C:

#![feature(lang_items)]
#![feature(start)]
#![no_std]

// Pull in the system libc library for what crt0.o likely requires
extern crate libc;

// Entry point for this program
#[start]
fn start(_argc: isize, _argv: *const *const u8) -> isize {
    0
}

// These functions are used by the compiler, but not
// for a bare-bones hello world. These are normally
// provided by libstd.
#[lang = "eh_personality"]
#[no_mangle]
pub extern fn rust_eh_personality() {
}

// This function may be needed based on the compilation target.
#[lang = "eh_unwind_resume"]
#[no_mangle]
pub extern fn rust_eh_unwind_resume() {
}

#[lang = "panic_fmt"]
#[no_mangle]
pub extern fn rust_begin_panic(_msg: core::fmt::Arguments,
                               _file: &'static str,
                               _line: u32) -> ! {
    loop {}
}Run

To override the compiler-inserted main shim, one has to disable it with #![no_main] and then create the appropriate symbol with the correct ABI and the correct name, which requires overriding the compiler's name mangling too:

#![feature(lang_items)]
#![feature(start)]
#![no_std]
#![no_main]

// Pull in the system libc library for what crt0.o likely requires
extern crate libc;

// Entry point for this program
#[no_mangle] // ensure that this symbol is called `main` in the output
pub extern fn main(_argc: i32, _argv: *const *const u8) -> i32 {
    0
}

// These functions are used by the compiler, but not
// for a bare-bones hello world. These are normally
// provided by libstd.
#[lang = "eh_personality"]
#[no_mangle]
pub extern fn rust_eh_personality() {
}

// This function may be needed based on the compilation target.
#[lang = "eh_unwind_resume"]
#[no_mangle]
pub extern fn rust_eh_unwind_resume() {
}

#[lang = "panic_fmt"]
#[no_mangle]
pub extern fn rust_begin_panic(_msg: core::fmt::Arguments,
                               _file: &'static str,
                               _line: u32) -> ! {
    loop {}
}Run

More about the language items

The compiler currently makes a few assumptions about symbols which are available in the executable to call. Normally these functions are provided by the standard library, but without it you must define your own. These symbols are called "language items", and they each have an internal name, and then a signature that an implementation must conform to.

The first of these functions, rust_eh_personality, is used by the failure mechanisms of the compiler. This is often mapped to GCC's personality function (see the libstd implementation for more information), but crates which do not trigger a panic can be assured that this function is never called. The language item's name is eh_personality.

The second function, rust_begin_panic, is also used by the failure mechanisms of the compiler. When a panic happens, this controls the message that's displayed on the screen. While the language item's name is panic_fmt, the symbol name is rust_begin_panic.

A third function, rust_eh_unwind_resume, is also needed if the custom_unwind_resume flag is set in the options of the compilation target. It allows customizing the process of resuming unwind at the end of the landing pads. The language item's name is eh_unwind_resume.