Crate std

The Rust Standard Library

The Rust Standard Library is the foundation of portable Rust software, a set of minimal and battle-tested shared abstractions for the broader Rust ecosystem. It offers core types, like Vec<T> and Option<T>, library-defined operations on language primitives, standard macros, I/O and multithreading, among many other things.

std is available to all Rust crates by default. Therefore, the standard library can be accessed in use statements through the path std, as in use std::env.

How to read this documentation

If you already know the name of what you are looking for, the fastest way to find it is to use the search bar at the top of the page.

Otherwise, you may want to jump to one of these useful sections:

• std::* modules
• Primitive types
• Standard macros
• The Rust Prelude

If this is your first time, the documentation for the standard library is written to be casually perused. Clicking on interesting things should generally lead you to interesting places. Still, there are important bits you don’t want to miss, so read on for a tour of the standard library and its documentation!

Once you are familiar with the contents of the standard library you may begin to find the verbosity of the prose distracting. At this stage in your development you may want to press the “ Summary” button near the top of the page to collapse it into a more skimmable view.

While you are looking at the top of the page, also notice the “Source” link. Rust’s API documentation comes with the source code and you are encouraged to read it. The standard library source is generally high quality and a peek behind the curtains is often enlightening.

What is in the standard library documentation?

First of all, The Rust Standard Library is divided into a number of focused modules, all listed further down this page. These modules are the bedrock upon which all of Rust is forged, and they have mighty names like std::slice and std::cmp. Modules’ documentation typically includes an overview of the module along with examples, and are a smart place to start familiarizing yourself with the library.

Second, implicit methods on primitive types are documented here. This can be a source of confusion for two reasons:

1. While primitives are implemented by the compiler, the standard library implements methods directly on the primitive types (and it is the only library that does so), which are documented in the section on primitives.
2. The standard library exports many modules with the same name as primitive types. These define additional items related to the primitive type, but not the all-important methods.

So for example there is a page for the primitive type i32 that lists all the methods that can be called on 32-bit integers (very useful), and there is a page for the module std::i32 that documents the constant values MIN and MAX (rarely useful).

Note the documentation for the primitives str and [T] (also called ‘slice’). Many method calls on String and Vec<T> are actually calls to methods on str and [T] respectively, via deref coercions.

Third, the standard library defines The Rust Prelude, a small collection of items - mostly traits - that are imported into every module of every crate. The traits in the prelude are pervasive, making the prelude documentation a good entry point to learning about the library.

And finally, the standard library exports a number of standard macros, and lists them on this page (technically, not all of the standard macros are defined by the standard library - some are defined by the compiler - but they are documented here the same). Like the prelude, the standard macros are imported by default into all crates.

Contributing changes to the documentation

Check out the Rust contribution guidelines here. The source for this documentation can be found on GitHub in the ‘library/std/’ directory. To contribute changes, make sure you read the guidelines first, then submit pull-requests for your suggested changes.

Contributions are appreciated! If you see a part of the docs that can be improved, submit a PR, or chat with us first on Discord #docs.

A Tour of The Rust Standard Library

The rest of this crate documentation is dedicated to pointing out notable features of The Rust Standard Library.

Containers and collections

The option and result modules define optional and error-handling types, Option<T> and Result<T, E>. The iter module defines Rust’s iterator trait, Iterator, which works with the for loop to access collections.

The standard library exposes three common ways to deal with contiguous regions of memory:

• Vec<T> - A heap-allocated vector that is resizable at runtime.
• [T; N] - An inline array with a fixed size at compile time.
• [T] - A dynamically sized slice into any other kind of contiguous storage, whether heap-allocated or not.

Slices can only be handled through some kind of pointer, and as such come in many flavors such as:

• &[T] - shared slice
• &mut [T] - mutable slice
• Box<[T]> - owned slice

str, a UTF-8 string slice, is a primitive type, and the standard library defines many methods for it. Rust strs are typically accessed as immutable references: &str. Use the owned String for building and mutating strings.

For converting to strings use the format! macro, and for converting from strings use the FromStr trait.

Data may be shared by placing it in a reference-counted box or the Rc type, and if further contained in a Cell or RefCell, may be mutated as well as shared. Likewise, in a concurrent setting it is common to pair an atomically-reference-counted box, Arc, with a Mutex to get the same effect.

The collections module defines maps, sets, linked lists and other typical collection types, including the common HashMap<K, V>.

Platform abstractions and I/O

Besides basic data types, the standard library is largely concerned with abstracting over differences in common platforms, most notably Windows and Unix derivatives.

Common types of I/O, including files, TCP, and UDP, are defined in the io, fs, and net modules.

The thread module contains Rust’s threading abstractions. sync contains further primitive shared memory types, including atomic, mpmc and mpsc, which contains the channel types for message passing.

Use before and after main()

Many parts of the standard library are expected to work before and after main(); but this is not guaranteed or ensured by tests. It is recommended that you write your own tests and run them on each platform you wish to support. This means that use of std before/after main, especially of features that interact with the OS or global state, is exempted from stability and portability guarantees and instead only provided on a best-effort basis. Nevertheless bug reports are appreciated.

On the other hand core and alloc are most likely to work in such environments with the caveat that any hookable behavior such as panics, oom handling or allocators will also depend on the compatibility of the hooks.

Some features may also behave differently outside main, e.g. stdio could become unbuffered, some panics might turn into aborts, backtraces might not get symbolicated or similar.

Non-exhaustive list of known limitations:

• after-main use of thread-locals, which also affects additional features:
  • thread::current()
• before-main stdio file descriptors are not guaranteed to be open on unix platforms

Modules

alloc

Memory allocation APIs.

arch

SIMD and vendor intrinsics module.

ascii

Operations on ASCII strings and characters.

backtrace

Support for capturing a stack backtrace of an OS thread

collections

Collection types.

env

Inspection and manipulation of the process’s environment.

error

Interfaces for working with Errors.

f32

Constants for the f32 single-precision floating point type.

f64

Constants for the f64 double-precision floating point type.

ffi

Utilities related to FFI bindings.

fs

Filesystem manipulation operations.

hash

Generic hashing support.

io

Traits, helpers, and type definitions for core I/O functionality.

net

Networking primitives for TCP/UDP communication.

num

Additional functionality for numerics.

os

OS-specific functionality.

panic

Panic support in the standard library.

path

Cross-platform path manipulation.

prelude

The Rust Prelude

process

A module for working with processes.

sync

Useful synchronization primitives.

task

Types and Traits for working with asynchronous tasks.

thread

Native threads.

time

Temporal quantification.

autodiffExperimental

This module provides support for automatic differentiation.

bstrExperimental

The ByteStr and ByteString types and trait implementations.

f16Experimental

Constants for the f16 half-precision floating point type.

f128Experimental

Constants for the f128 quadruple-precision floating point type.

patExperimental

Helper module for exporting the pattern_type macro

randomExperimental

Random value generation.

simdExperimental

Portable SIMD module.

Macros

assert

Asserts that a boolean expression is true at runtime.

assert_eq

Asserts that two expressions are equal to each other (using PartialEq).

assert_ne

Asserts that two expressions are not equal to each other (using PartialEq).

cfg

Evaluates boolean combinations of configuration flags at compile-time.

column

Expands to the column number at which it was invoked.

compile_error

Causes compilation to fail with the given error message when encountered.

concat

Concatenates literals into a static string slice.

dbg

Prints and returns the value of a given expression for quick and dirty debugging.

debug_assert

Asserts that a boolean expression is true at runtime.

debug_assert_eq

Asserts that two expressions are equal to each other.

debug_assert_ne

Asserts that two expressions are not equal to each other.

env

Inspects an environment variable at compile time.

eprint

Prints to the standard error.

eprintln

Prints to the standard error, with a newline.

file

Expands to the file name in which it was invoked.

format

Creates a String using interpolation of runtime expressions.

format_args

Constructs parameters for the other string-formatting macros.

include

Parses a file as an expression or an item according to the context.

include_bytes

Includes a file as a reference to a byte array.

include_str

Includes a UTF-8 encoded file as a string.

is_x86_feature_detectedx86 or x86-64

A macro to test at runtime whether a CPU feature is available on x86/x86-64 platforms.

line

Expands to the line number on which it was invoked.

matches

Returns whether the given expression matches the provided pattern.

module_path

Expands to a string that represents the current module path.

option_env

Optionally inspects an environment variable at compile time.

panic

Panics the current thread.

print

Prints to the standard output.

println

Prints to the standard output, with a newline.

stringify

Stringifies its arguments.

thread_local

Declare a new thread local storage key of type std::thread::LocalKey.

todo

Indicates unfinished code.

tryDeprecated

Unwraps a result or propagates its error.

unimplemented

Indicates unimplemented code by panicking with a message of “not implemented”.

unreachable

Indicates unreachable code.

vecNon-no_global_oom_handling

Creates a Vec containing the arguments.

write

Writes formatted data into a buffer.

writeln

Writes formatted data into a buffer, with a newline appended.

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.

trace_macrosExperimental

Enables or disables tracing functionality used for debugging other macros.