Crate core

1.6.0 · source ·
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§The Rust Core Library

The Rust Core Library is the dependency-free1 foundation of The Rust Standard Library. It is the portable glue between the language and its libraries, defining the intrinsic and primitive building blocks of all Rust code. It links to no upstream libraries, no system libraries, and no libc.

The core library is minimal: it isn’t even aware of heap allocation, nor does it provide concurrency or I/O. These things require platform integration, and this library is platform-agnostic.

§How to use the core library

Please note that all of these details are currently not considered stable.

This library is built on the assumption of a few existing symbols:

  • memcpy, memmove, memset, memcmp, bcmp, strlen - These are core memory routines which are generated by Rust codegen backends. Additionally, this library can make explicit calls to strlen. Their signatures are the same as found in C, but there are extra assumptions about their semantics: For memcpy, memmove, memset, memcmp, and bcmp, if the n parameter is 0, the function is assumed to not be UB, even if the pointers are NULL or dangling. (Note that making extra assumptions about these functions is common among compilers: clang and GCC do the same.) These functions are often provided by the system libc, but can also be provided by the compiler-builtins crate. Note that the library does not guarantee that it will always make these assumptions, so Rust user code directly calling the C functions should follow the C specification! The advice for Rust user code is to call the functions provided by this library instead (such as ptr::copy).

  • Panic handler - This function takes one argument, a &panic::PanicInfo. It is up to consumers of this core library to define this panic function; it is only required to never return. You should mark your implementation using #[panic_handler].

  • rust_eh_personality - is used by the failure mechanisms of the compiler. This is often mapped to GCC’s personality function, but crates which do not trigger a panic can be assured that this function is never called. The lang attribute is called eh_personality.

  1. Strictly speaking, there are some symbols which are needed but they aren’t always necessary. 

Primitive Types§

  • A fixed-size array, denoted [T; N], for the element type, T, and the non-negative compile-time constant size, N.
  • The boolean type.
  • A character type.
  • A 32-bit floating point type (specifically, the “binary32” type defined in IEEE 754-2008).
  • A 64-bit floating point type (specifically, the “binary64” type defined in IEEE 754-2008).
  • Function pointers, like fn(usize) -> bool.
  • The 8-bit signed integer type.
  • The 16-bit signed integer type.
  • The 32-bit signed integer type.
  • The 64-bit signed integer type.
  • The 128-bit signed integer type.
  • The pointer-sized signed integer type.
  • Raw, unsafe pointers, *const T, and *mut T.
  • References, &T and &mut T.
  • A dynamically-sized view into a contiguous sequence, [T]. Contiguous here means that elements are laid out so that every element is the same distance from its neighbors.
  • String slices.
  • A finite heterogeneous sequence, (T, U, ..).
  • The 8-bit unsigned integer type.
  • The 16-bit unsigned integer type.
  • The 32-bit unsigned integer type.
  • The 64-bit unsigned integer type.
  • The 128-bit unsigned integer type.
  • The () type, also called “unit”.
  • The pointer-sized unsigned integer type.
  • f16Experimental
    A 16-bit floating point type (specifically, the “binary16” type defined in IEEE 754-2008).
  • f128Experimental
    A 128-bit floating point type (specifically, the “binary128” type defined in IEEE 754-2008).
  • neverExperimental
    The ! type, also called “never”.


  • Memory allocation APIs
  • Utilities for dynamic typing or type reflection.
  • SIMD and vendor intrinsics module.
  • Utilities for the array primitive type.
  • Operations on ASCII strings and characters.
  • Utilities for working with borrowed data.
  • Shareable mutable containers.
  • Utilities for the char primitive type.
  • The Clone trait for types that cannot be ‘implicitly copied’.
  • Utilities for comparing and ordering values.
  • Traits for conversions between types.
  • The Default trait for types with a default value.
  • Interfaces for working with Errors.
  • Constants for the f32 single-precision floating point type.
  • Constants for the f64 double-precision floating point type.
  • Platform-specific types, as defined by C.
  • Utilities for formatting and printing strings.
  • Asynchronous basic functionality.
  • Generic hashing support.
  • Hints to compiler that affects how code should be emitted or optimized. Hints may be compile time or runtime.
  • i8Deprecation planned
    Redundant constants module for the i8 primitive type.
  • i16Deprecation planned
    Redundant constants module for the i16 primitive type.
  • i32Deprecation planned
    Redundant constants module for the i32 primitive type.
  • i64Deprecation planned
    Redundant constants module for the i64 primitive type.
  • i128Deprecation planned
    Redundant constants module for the i128 primitive type.
  • isizeDeprecation planned
    Redundant constants module for the isize primitive type.
  • Composable external iteration.
  • Primitive traits and types representing basic properties of types.
  • Basic functions for dealing with memory.
  • Networking primitives for IP communication.
  • Numeric traits and functions for the built-in numeric types.
  • Overloadable operators.
  • Optional values.
  • Panic support in the standard library.
  • Types that pin data to a location in memory.
  • The core prelude
  • This module reexports the primitive types to allow usage that is not possibly shadowed by other declared types.
  • Manually manage memory through raw pointers.
  • Error handling with the Result type.
  • Slice management and manipulation.
  • String manipulation.
  • Synchronization primitives
  • Types and Traits for working with asynchronous tasks.
  • Temporal quantification.
  • u8Deprecation planned
    Redundant constants module for the u8 primitive type.
  • u16Deprecation planned
    Redundant constants module for the u16 primitive type.
  • u32Deprecation planned
    Redundant constants module for the u32 primitive type.
  • u64Deprecation planned
    Redundant constants module for the u64 primitive type.
  • u128Deprecation planned
    Redundant constants module for the u128 primitive type.
  • usizeDeprecation planned
    Redundant constants module for the usize primitive type.
  • assert_matchesExperimental
    Unstable module containing the unstable assert_matches macro.
  • async_iterExperimental
    Composable asynchronous iteration.
  • f16Experimental
    Constants for the f16 half-precision floating point type.
  • f128Experimental
    Constants for the f128 quadruple-precision floating point type.
  • intrinsicsExperimental
    Compiler intrinsics.
  • ioExperimental
    Traits, helpers, and type definitions for core I/O functionality.
  • panickingExperimental
    Panic support for core
  • patExperimental
    Helper module for exporting the pattern_type macro
  • rangeExperimental
    Experimental replacement range types
  • simdExperimental
    Portable SIMD module.
  • ub_checksExperimental
    Provides the assert_unsafe_precondition macro as well as some utility functions that cover common preconditions.
  • unicodeExperimental


  • Asserts that a boolean expression is true at runtime.
  • Asserts that two expressions are equal to each other (using PartialEq).
  • Asserts that two expressions are not equal to each other (using PartialEq).
  • Evaluates boolean combinations of configuration flags at compile-time.
  • Expands to the column number at which it was invoked.
  • Causes compilation to fail with the given error message when encountered.
  • Concatenates literals into a static string slice.
  • Asserts that a boolean expression is true at runtime.
  • Asserts that two expressions are equal to each other.
  • Asserts that two expressions are not equal to each other.
  • Inspects an environment variable at compile time.
  • Expands to the file name in which it was invoked.
  • Constructs parameters for the other string-formatting macros.
  • Parses a file as an expression or an item according to the context.
  • Includes a file as a reference to a byte array.
  • Includes a UTF-8 encoded file as a string.
  • Expands to the line number on which it was invoked.
  • Returns whether the given expression matches the provided pattern.
  • Expands to a string that represents the current module path.
  • Optionally inspects an environment variable at compile time.
  • Panics the current thread.
  • Stringifies its arguments.
  • Indicates unfinished code.
  • tryDeprecated
    Unwraps a result or propagates its error.
  • Indicates unimplemented code by panicking with a message of “not implemented”.
  • Indicates unreachable code.
  • Writes formatted data into a buffer.
  • Write formatted data into a buffer, with a newline appended.
  • Check that the preconditions of an unsafe function are followed. The check is enabled at runtime if debug assertions are enabled when the caller is monomorphized. In const-eval/Miri checks implemented with this macro for language UB are always ignored.
  • cfg_matchExperimental
    A macro for defining #[cfg] match-like statements.
  • concat_bytesExperimental
    Concatenates literals into a byte slice.
  • concat_identsExperimental
    Concatenates identifiers into one identifier.
  • const_format_argsExperimental
    Same as format_args, but can be used in some const contexts.
  • format_args_nlExperimental
    Same as format_args, but adds a newline in the end.
  • log_syntaxExperimental
    Prints passed tokens into the standard output.
  • pattern_typeExperimental
    Creates a pattern type.
  • trace_macrosExperimental
    Enables or disables tracing functionality used for debugging other macros.