std - Rust

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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](https://crates.io). It offers core types, like Vec<T> and Option<T>, library-defined [operations on language
primitives](#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](#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](https://doc.rust-lang.org/std/primitive.i32.html "primitive 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](https://doc.rust-lang.org/std/i32/index.html "mod 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](https://doc.rust-lang.org/book/ch15-02-deref.html#implicit-deref-coercions-with-functions-and-methods).

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 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, 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

arrayA fixed-size array, denoted [T; N], for the element type, T, and the non-negative compile-time constant size, N.boolThe boolean type.charA character type.f32A 32-bit floating-point type (specifically, the “binary32” type defined in IEEE 754-2008).f64A 64-bit floating-point type (specifically, the “binary64” type defined in IEEE 754-2008).fnFunction pointers, like fn(usize) -> bool.i8The 8-bit signed integer type.i16The 16-bit signed integer type.i32The 32-bit signed integer type.i64The 64-bit signed integer type.i128The 128-bit signed integer type.isizeThe pointer-sized signed integer type.pointerRaw, unsafe pointers, *const T, and *mut T.referenceReferences, &T and &mut T.sliceA dynamically-sized view into a contiguous sequence, [T].strString slices.tupleA finite heterogeneous sequence, (T, U, ..).u8The 8-bit unsigned integer type.u16The 16-bit unsigned integer type.u32The 32-bit unsigned integer type.u64The 64-bit unsigned integer type.u128The 128-bit unsigned integer type.unitThe () type, also called “unit”.usizeThe pointer-sized unsigned integer type.f16 ExperimentalA 16-bit floating-point type (specifically, the “binary16” type defined in IEEE 754-2008).f128 ExperimentalA 128-bit floating-point type (specifically, the “binary128” type defined in IEEE 754-2008).never ExperimentalThe ! type, also called “never”.allocMemory allocation APIs.anyUtilities for dynamic typing or type reflection.archSIMD and vendor intrinsics module.arrayUtilities for the array primitive type.asciiOperations on ASCII strings and characters.backtraceSupport for capturing a stack backtrace of an OS threadborrowA module for working with borrowed data.boxedThe Box<T> type for heap allocation.cellShareable mutable containers.charUtilities for the char primitive type.cloneThe Clone trait for types that cannot be ‘implicitly copied’.cmpUtilities for comparing and ordering values.collectionsCollection types.convertTraits for conversions between types.defaultThe Default trait for types with a default value.envInspection and manipulation of the process’s environment.errorInterfaces for working with Errors.f32Constants for the f32 single-precision floating point type.f64Constants for the f64 double-precision floating point type.ffiUtilities related to FFI bindings.fmtUtilities for formatting and printing String s.fsFilesystem manipulation operations.futureAsynchronous basic functionality.hashGeneric hashing support.hintHints to compiler that affects how code should be emitted or optimized.i8 Deprecation plannedRedundant constants module for the i8 primitive type.i16 Deprecation plannedRedundant constants module for the i16 primitive type.i32 Deprecation plannedRedundant constants module for the i32 primitive type.i64 Deprecation plannedRedundant constants module for the i64 primitive type.i128 Deprecation plannedRedundant constants module for the i128 primitive type.ioTraits, helpers, and type definitions for core I/O functionality.isize Deprecation plannedRedundant constants module for the isize primitive type.iterComposable external iteration.markerPrimitive traits and types representing basic properties of types.memBasic functions for dealing with memory.netNetworking primitives for TCP/UDP communication.numAdditional functionality for numerics.opsOverloadable operators.optionOptional values.osOS-specific functionality.panicPanic support in the standard library.pathCross-platform path manipulation.pinTypes that pin data to a location in memory.preludeThe Rust PreludeprimitiveThis module reexports the primitive types to allow usage that is not possibly shadowed by other declared types.processA module for working with processes.ptrManually manage memory through raw pointers.rcSingle-threaded reference-counting pointers. ‘Rc’ stands for ‘Reference Counted’.resultError handling with the Result type.sliceUtilities for the slice primitive type.strUtilities for the str primitive type.stringA UTF-8–encoded, growable string.syncUseful synchronization primitives.taskTypes and Traits for working with asynchronous tasks.threadNative threads.timeTemporal quantification.u8 Deprecation plannedRedundant constants module for the u8 primitive type.u16 Deprecation plannedRedundant constants module for the u16 primitive type.u32 Deprecation plannedRedundant constants module for the u32 primitive type.u64 Deprecation plannedRedundant constants module for the u64 primitive type.u128 Deprecation plannedRedundant constants module for the u128 primitive type.usize Deprecation plannedRedundant constants module for the usize primitive type.vecA contiguous growable array type with heap-allocated contents, written Vec<T>.assert_matches ExperimentalUnstable module containing the unstable assert_matches macro.async_iter ExperimentalComposable asynchronous iteration.autodiff ExperimentalThis module provides support for automatic differentiation.bstr ExperimentalThe ByteStr and ByteString types and trait implementations.f16 ExperimentalConstants for the f16 half-precision floating point type.f128 ExperimentalConstants for the f128 quadruple-precision floating point type.intrinsics ExperimentalCompiler intrinsics.pat ExperimentalHelper module for exporting the pattern_type macrorandom ExperimentalRandom value generation.range ExperimentalExperimental replacement range typessimd ExperimentalPortable SIMD module.unsafe_binder ExperimentalOperators used to turn types into unsafe binders and back.assertAsserts that a boolean expression is true at runtime.assert_eqAsserts that two expressions are equal to each other (using PartialEq).assert_neAsserts that two expressions are not equal to each other (using PartialEq).cfgEvaluates boolean combinations of configuration flags at compile-time.columnExpands to the column number at which it was invoked.compile_errorCauses compilation to fail with the given error message when encountered.concatConcatenates literals into a static string slice.dbgPrints and returns the value of a given expression for quick and dirty debugging.debug_assertAsserts that a boolean expression is true at runtime.debug_assert_eqAsserts that two expressions are equal to each other.debug_assert_neAsserts that two expressions are not equal to each other.envInspects an environment variable at compile time.eprintPrints to the standard error.eprintlnPrints to the standard error, with a newline.fileExpands to the file name in which it was invoked.formatCreates a String using interpolation of runtime expressions.format_argsConstructs parameters for the other string-formatting macros.includeParses a file as an expression or an item according to the context.include_bytesIncludes a file as a reference to a byte array.include_strIncludes a UTF-8 encoded file as a string.is_x86_feature_detectedA macro to test at runtime whether a CPU feature is available on x86/x86-64 platforms.lineExpands to the line number on which it was invoked.matchesReturns whether the given expression matches the provided pattern.module_pathExpands to a string that represents the current module path.option_envOptionally inspects an environment variable at compile time.panicPanics the current thread.printPrints to the standard output.printlnPrints to the standard output, with a newline.stringifyStringifies its arguments.thread_localDeclare a new thread local storage key of type std::thread::LocalKey.todoIndicates unfinished code.try DeprecatedUnwraps a result or propagates its error.unimplementedIndicates unimplemented code by panicking with a message of “not implemented”.unreachableIndicates unreachable code.vecCreates a Vec containing the arguments.writeWrites formatted data into a buffer.writelnWrites formatted data into a buffer, with a newline appended.cfg_match ExperimentalA macro for defining #[cfg] match-like statements.concat_bytes ExperimentalConcatenates literals into a byte slice.concat_idents ExperimentalConcatenates identifiers into one identifier.const_format_args ExperimentalSame as format_args, but can be used in some const contexts.format_args_nl ExperimentalSame as format_args, but adds a newline in the end.log_syntax ExperimentalPrints passed tokens into the standard output.trace_macros ExperimentalEnables or disables tracing functionality used for debugging other macros.SelfTyThe implementing type within a trait or impl block, or the current type within a type definition.asCast between types, or rename an import.asyncReturns a Future instead of blocking the current thread.awaitSuspend execution until the result of a Future is ready.breakExit early from a loop or labelled block.constCompile-time constants, compile-time evaluable functions, and raw pointers.continueSkip to the next iteration of a loop.crateA Rust binary or library.dyndyn is a prefix of a trait object’s type.elseWhat expression to evaluate when an if condition evaluates to false.enumA type that can be any one of several variants.externLink to or import external code.falseA value of type bool representing logical false.fnA function or function pointer.forIteration with in, trait implementation with impl, or higher-ranked trait bounds ( for<'a>).ifEvaluate a block if a condition holds.implImplementations of functionality for a type, or a type implementing some functionality.inIterate over a series of values with for.letBind a value to a variable.loopLoop indefinitely.matchControl flow based on pattern matching.modOrganize code into modules.moveCapture a closure’s environment by value.mutA mutable variable, reference, or pointer.pubMake an item visible to others.refBind by reference during pattern matching.returnReturns a value from a function.selfThe receiver of a method, or the current module.staticA static item is a value which is valid for the entire duration of your program (a 'static lifetime).structA type that is composed of other types.superThe parent of the current module.traitA common interface for a group of types.trueA value of type bool representing logical true.typeDefine an alias for an existing type.unionThe Rust equivalent of a C-style union.unsafeCode or interfaces whose memory safety cannot be verified by the type system.useImport or rename items from other crates or modules, or specify precise capturing with use<..>.whereAdd constraints that must be upheld to use an item.whileLoop while a condition is upheld.