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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:

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 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 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:

Primitive Types§

  • 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.
  • 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”.

Modules§

  • 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 Strings.
  • 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.
  • 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.
  • io
    Traits, helpers, and type definitions for core I/O functionality.
  • isizeDeprecation 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 a 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.
  • 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.
  • vec
    A contiguous growable array type with heap-allocated contents, written Vec<T>.
  • assert_matchesExperimental
    Unstable module containing the unstable assert_matches macro.
  • async_iterExperimental
    Composable asynchronous iteration.
  • f16Experimental
    Constants for the f16 double-precision floating point type.
  • f128Experimental
    Constants for the f128 double-precision floating point type.
  • intrinsicsExperimental
    Compiler intrinsics.
  • patExperimental
    Helper module for exporting the pattern_type macro
  • pipeExperimental
    Module for anonymous pipe
  • 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_detected
    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.
  • vec
    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.

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
    Returns 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 or labelled block.
  • 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
    Returns 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.