# Crate std

1.0.0 · source ·
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## 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 `[-]` button near the top of the page to collapse it into a more skimmable view.

While you are looking at that `[-]` button 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. 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 `str`s 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` 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()`
  - `thread::scope()`
  - `sync::mpsc`
- before-main stdio file descriptors are not guaranteed to be open on unix platforms

## Primitive Types

- never Experimental
  The `!` type, also called “never”.
- array
  A fixed-size array, denoted `[T; N]`, for the element type, `T`, and the non-negative compile-time constant size, `N`.
- bool
  The boolean type.
- char
  A character type.
- f32
  A 32-bit floating point type (specifically, the “binary32” type defined in IEEE 754-2008).
- f64
  A 64-bit floating point type (specifically, the “binary64” type defined in IEEE 754-2008).
- fn
  Function pointers, like `fn(usize) -> bool`.
- i8
  The 8-bit signed integer type.
- i16
  The 16-bit signed integer type.
- i32
  The 32-bit signed integer type.
- i64
  The 64-bit signed integer type.
- i128
  The 128-bit signed integer type.
- isize
  The pointer-sized signed integer type.
- pointer
  Raw, unsafe pointers, `*const T`, and `*mut T`.
- reference
  References, `&T` and `&mut T`.
- slice
  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.
- str
  String slices.
- tuple
  A finite heterogeneous sequence, `(T, U, ..)`.
- u8
  The 8-bit unsigned integer type.
- u16
  The 16-bit unsigned integer type.
- u32
  The 32-bit unsigned integer type.
- u64
  The 64-bit unsigned integer type.
- u128
  The 128-bit unsigned integer type.
- unit
  The `()` type, also called “unit”.
- usize
  The pointer-sized unsigned integer type.

## Modules

- assert_matches Experimental
  Unstable module containing the unstable `assert_matches` macro.
- async_iter Experimental
  Composable asynchronous iteration.
- intrinsics Experimental
  Compiler intrinsics.
- simd Experimental
  Portable SIMD module.
- alloc
  Memory allocation APIs.
- any
  Utilities for dynamic typing or type reflection.
- arch
  SIMD and vendor intrinsics module.
- array
  Utilities for the array primitive type.
- ascii
  Operations on ASCII strings and characters.
- backtrace
  Support for capturing a stack backtrace of an OS thread
- borrow
  A module for working with borrowed data.
- boxed
  The `Box<T>` type for heap allocation.
- cell
  Shareable mutable containers.
- char
  Utilities for the `char` primitive type.
- clone
  The `Clone` trait for types that cannot be ‘implicitly copied’.
- cmp
  Utilities for comparing and ordering values.
- collections
  Collection types.
- convert
  Traits for conversions between types.
- default
  The `Default` trait for types with a default value.
- 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.
- fmt
  Utilities for formatting and printing `String`s.
- fs
  Filesystem manipulation operations.
- future
  Asynchronous basic functionality.
- hash
  Generic hashing support.
- hint
  Hints to compiler that affects how code should be emitted or optimized. Hints may be compile time or runtime.
- i8 Deprecation planned
  Redundant constants module for the `i8` primitive type.
- i16 Deprecation planned
  Redundant constants module for the `i16` primitive type.
- i32 Deprecation planned
  Redundant constants module for the `i32` primitive type.
- i64 Deprecation planned
  Redundant constants module for the `i64` primitive type.
- i128 Deprecation planned
  Redundant constants module for the `i128` primitive type.
- io
  Traits, helpers, and type definitions for core I/O functionality.
- isize Deprecation planned
  Redundant constants module for the `isize` primitive type.
- iter
  Composable external iteration.
- marker
  Primitive traits and types representing basic properties of types.
- mem
  Basic functions for dealing with memory.
- net
  Networking primitives for TCP/UDP communication.
- num
  Additional functionality for numerics.
- ops
  Overloadable operators.
- option
  Optional values.
- os
  OS-specific functionality.
- panic
  Panic support in the standard library.
- path
  Cross-platform path manipulation.
- pin
  Types that pin data to its location in memory.
- prelude
  The Rust Prelude
- primitive
  This module reexports the primitive types to allow usage that is not possibly shadowed by other declared types.
- process
  A module for working with processes.
- ptr
  Manually manage memory through raw pointers.
- rc
  Single-threaded reference-counting pointers. ‘Rc’ stands for ‘Reference Counted’.
- result
  Error handling with the `Result` type.
- slice
  Utilities for the slice primitive type.
- str
  Utilities for the `str` primitive type.
- string
  A UTF-8–encoded, growable string.
- sync
  Useful synchronization primitives.
- task
  Types and Traits for working with asynchronous tasks.
- thread
  Native threads.
- time
  Temporal quantification.
- u8 Deprecation planned
  Redundant constants module for the `u8` primitive type.
- u16 Deprecation planned
  Redundant constants module for the `u16` primitive type.
- u32 Deprecation planned
  Redundant constants module for the `u32` primitive type.
- u64 Deprecation planned
  Redundant constants module for the `u64` primitive type.
- u128 Deprecation planned
  Redundant constants module for the `u128` primitive type.
- usize Deprecation planned
  Redundant constants module for the `usize` primitive type.
- vec

## Macros

- cfg_match Experimental
  A macro for defining `#[cfg]` match-like statements.
- concat_bytes Experimental
  Concatenates literals into a byte slice.
- concat_idents Experimental
  Concatenates identifiers into one identifier.
- const_format_args Experimental
  Same as `format_args`, but can be used in some const contexts.
- format_args_nl Experimental
  Same as `format_args`, but adds a newline in the end.
- log_syntax Experimental
  Prints passed tokens into the standard output.
- trace_macros Experimental
  Enables or disables tracing functionality used for debugging other 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_detected x86 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 any of the given patterns.
- 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.
- try Deprecated
  Unwraps a result or propagates its error.
- unimplemented
  Indicates unimplemented code by panicking with a message of “not implemented”.
- unreachable
  Indicates unreachable code.
- vec
  Creates a `Vec` containing the arguments.
- write
  Writes formatted data into a buffer.
- writeln
  Write formatted data into a buffer, with a newline appended.

## Keywords

- SelfTy
  The implementing type within a `trait` or `impl` block, or the current type within a type definition.
- as
  Cast between types, or rename an import.
- async
  Return a `Future` instead of blocking the current thread.
- await
  Suspend execution until the result of a `Future` is ready.
- break
  Exit early from a loop.
- const
  Compile-time constants, compile-time evaluable functions, and raw pointers.
- continue
  Skip to the next iteration of a loop.
- crate
  A Rust binary or library.
- dyn
  `dyn` is a prefix of a trait object’s type.
- else
  What expression to evaluate when an `if` condition evaluates to `false`.
- enum
  A type that can be any one of several variants.
- extern
  Link to or import external code.
- false
  A value of type `bool` representing logical false.
- fn
  A function or function pointer.
- for
  Iteration with `in`, trait implementation with `impl`, or higher-ranked trait bounds (`for<'a>`).
- if
  Evaluate a block if a condition holds.
- impl
  Implement some functionality for a type.
- in
  Iterate over a series of values with `for`.
- let
  Bind a value to a variable.
- loop
  Loop indefinitely.
- match
  Control flow based on pattern matching.
- mod
  Organize code into modules.
- move
  Capture a closure’s environment by value.
- mut
  A mutable variable, reference, or pointer.
- pub
  Make an item visible to others.
- ref
  Bind by reference during pattern matching.
- return
  Return a value from a function.
- self
  The receiver of a method, or the current module.
- static
  A static item is a value which is valid for the entire duration of your program (a `'static` lifetime).
- struct
  A type that is composed of other types.
- super
  The parent of the current module.
- trait
  A common interface for a group of types.
- true
  A value of type `bool` representing logical true.
- type
  Define an alias for an existing type.
- union
  The Rust equivalent of a C-style union.
- unsafe
  Code or interfaces whose memory safety cannot be verified by the type system.
- use
  Import or rename items from other crates or modules.
- where
  Add constraints that must be upheld to use an item.
- while
  Loop while a condition is upheld.