Crate std

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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§

  • 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.
  • Support for capturing a stack backtrace of an OS thread
  • A module for working with borrowed data.
  • The Box<T> type for heap allocation.
  • 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.
  • Collection types.
  • Traits for conversions between types.
  • The Default trait for types with a default value.
  • Inspection and manipulation of the process’s environment.
  • Interfaces for working with Errors.
  • Constants for the f32 single-precision floating point type.
  • Constants for the f64 double-precision floating point type.
  • Utilities related to FFI bindings.
  • Utilities for formatting and printing Strings.
  • Filesystem manipulation operations.
  • 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.
  • Traits, helpers, and type definitions for core I/O functionality.
  • 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 TCP/UDP communication.
  • Additional functionality for numerics.
  • Overloadable operators.
  • Optional values.
  • OS-specific functionality.
  • Panic support in the standard library.
  • Cross-platform path manipulation.
  • Types that pin data to a location in memory.
  • The Rust Prelude
  • This module reexports the primitive types to allow usage that is not possibly shadowed by other declared types.
  • A module for working with processes.
  • Manually manage memory through raw pointers.
  • Single-threaded reference-counting pointers. ‘Rc’ stands for ‘Reference Counted’.
  • Error handling with the Result type.
  • Utilities for the slice primitive type.
  • Utilities for the str primitive type.
  • A UTF-8–encoded, growable string.
  • Useful synchronization primitives.
  • Types and Traits for working with asynchronous tasks.
  • Native threads.
  • 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.
  • 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
  • simdExperimental
    Portable SIMD module.


  • 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.
  • Prints and returns the value of a given expression for quick and dirty debugging.
  • 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.
  • Prints to the standard error.
  • Prints to the standard error, with a newline.
  • Expands to the file name in which it was invoked.
  • Creates a String using interpolation of runtime expressions.
  • 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.
  • A macro to test at runtime whether a CPU feature is available on x86/x86-64 platforms.
  • 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.
  • Prints to the standard output.
  • Prints to the standard output, with a newline.
  • Stringifies its arguments.
  • Declare a new thread local storage key of type std::thread::LocalKey.
  • 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.
  • Creates a Vec containing the arguments.
  • Writes formatted data into a buffer.
  • Write 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.


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