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, memcmp, memset, strlen - These are core memory routines which are often generated by LLVM. Additionally, this library can make explicit calls to these functions. Their signatures are the same as found in C. These functions are often provided by the system libc, but can also be provided by the compiler-builtins crate.

  • rust_begin_panic - This function takes four arguments, a fmt::Arguments, a &'static str, and two u32’s. These four arguments dictate the panic message, the file at which panic was invoked, and the line and column inside the file. It is up to consumers of this core library to define this panic function; it is only required to never return. This requires a lang attribute named panic_impl.

  • 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


The ! type, also called “never”.

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, both shared and mutable.

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.



Unstable module containing the unstable assert_matches macro.


Composable asynchronous iteration.


Compiler intrinsics.


Lazy values and one-time initialization of static data.


Panic support for libcore


Portable SIMD module.


Memory allocation APIs

This module implements the Any trait, which enables dynamic typing of any 'static type through runtime reflection.

SIMD and vendor intrinsics module.

Helper functions and types for fixed-length arrays.

Operations on ASCII strings and characters.

A module for working with borrowed data.

Shareable mutable containers.

A character type.

The Clone trait for types that cannot be ‘implicitly copied’.

Functionality for ordering and comparison.

Traits for conversions between types.

The Default trait for types which may have meaningful default values.

Constants specific to the f32 single-precision floating point type.

Constants specific to 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

Constants for the 8-bit signed integer type.

i16Deprecation planned

Constants for the 16-bit signed integer type.

i32Deprecation planned

Constants for the 32-bit signed integer type.

i64Deprecation planned

Constants for the 64-bit signed integer type.

i128Deprecation planned

Constants for the 128-bit signed integer type.

isizeDeprecation planned

Constants for the pointer-sized signed integer type.

Composable external iteration.

Primitive traits and types representing basic properties of types.

Basic functions for dealing with memory.

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 its location in memory.

The libcore 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

Constants for the 8-bit unsigned integer type.

u16Deprecation planned

Constants for the 16-bit unsigned integer type.

u32Deprecation planned

Constants for the 32-bit unsigned integer type.

u64Deprecation planned

Constants for the 64-bit unsigned integer type.

u128Deprecation planned

Constants for the 128-bit unsigned integer type.

usizeDeprecation planned

Constants for the pointer-sized unsigned integer type.



Concatenates literals into a byte slice.


Concatenates identifiers into one identifier.


Same as format_args, but can be used in some const contexts.


Same as format_args, but adds a newline in the end.


Prints passed tokens into the standard output.


Enables or disables tracing functionality used for debugging other macros.

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 any of the given patterns.

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.


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.