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Rust 1.14.0

The Rust Reference

1 Introduction

This document is the primary reference for the Rust programming language. It provides three kinds of material:

This document does not serve as an introduction to the language. Background familiarity with the language is assumed. A separate book is available to help acquire such background familiarity.

This document also does not serve as a reference to the standard library included in the language distribution. Those libraries are documented separately by extracting documentation attributes from their source code. Many of the features that one might expect to be language features are library features in Rust, so what you're looking for may be there, not here.

You may also be interested in the grammar.

2 Notation

2.1 Unicode productions

A few productions in Rust's grammar permit Unicode code points outside the ASCII range. We define these productions in terms of character properties specified in the Unicode standard, rather than in terms of ASCII-range code points. The grammar has a Special Unicode Productions section that lists these productions.

2.2 String table productions

Some rules in the grammar — notably unary operators, binary operators, and keywords — are given in a simplified form: as a listing of a table of unquoted, printable whitespace-separated strings. These cases form a subset of the rules regarding the token rule, and are assumed to be the result of a lexical-analysis phase feeding the parser, driven by a DFA, operating over the disjunction of all such string table entries.

When such a string enclosed in double-quotes (") occurs inside the grammar, it is an implicit reference to a single member of such a string table production. See tokens for more information.

3 Lexical structure

3.1 Input format

Rust input is interpreted as a sequence of Unicode code points encoded in UTF-8. Most Rust grammar rules are defined in terms of printable ASCII-range code points, but a small number are defined in terms of Unicode properties or explicit code point lists. 1

3.2 Identifiers

An identifier is any nonempty Unicode2 string of the following form:



that does not occur in the set of keywords.

Note: XID_start and XID_continue as character properties cover the character ranges used to form the more familiar C and Java language-family identifiers.


Comments in Rust code follow the general C++ style of line (//) and block (/* ... */) comment forms. Nested block comments are supported.

Line comments beginning with exactly three slashes (///), and block comments (/** ... */), are interpreted as a special syntax for doc attributes. That is, they are equivalent to writing #[doc="..."] around the body of the comment, i.e., /// Foo turns into #[doc="Foo"].

Line comments beginning with //! and block comments /*! ... */ are doc comments that apply to the parent of the comment, rather than the item that follows. That is, they are equivalent to writing #![doc="..."] around the body of the comment. //! comments are usually used to document modules that occupy a source file.

Non-doc comments are interpreted as a form of whitespace.

3.4 Whitespace

Whitespace is any non-empty string containing only characters that have the Pattern_White_Space Unicode property, namely:

Rust is a "free-form" language, meaning that all forms of whitespace serve only to separate tokens in the grammar, and have no semantic significance.

A Rust program has identical meaning if each whitespace element is replaced with any other legal whitespace element, such as a single space character.

3.5 Tokens

Tokens are primitive productions in the grammar defined by regular (non-recursive) languages. "Simple" tokens are given in string table production form, and occur in the rest of the grammar as double-quoted strings. Other tokens have exact rules given.

3.5.1 Literals

A literal is an expression consisting of a single token, rather than a sequence of tokens, that immediately and directly denotes the value it evaluates to, rather than referring to it by name or some other evaluation rule. A literal is a form of constant expression, so is evaluated (primarily) at compile time. Examples Characters and strings
Example # sets Characters Escapes
Character 'H' N/A All Unicode Quote & Byte & Unicode
String "hello" N/A All Unicode Quote & Byte & Unicode
Raw r#"hello"# 0... All Unicode N/A
Byte b'H' N/A All ASCII Quote & Byte
Byte string b"hello" N/A All ASCII Quote & Byte
Raw byte string br#"hello"# 0... All ASCII N/A Byte escapes
\x7F 8-bit character code (exactly 2 digits)
\n Newline
\r Carriage return
\t Tab
\\ Backslash
\0 Null Unicode escapes
\u{7FFF} 24-bit Unicode character code (up to 6 digits) Quote escapes
\' Single quote
\" Double quote Numbers
Number literals* Example Exponentiation Suffixes
Decimal integer 98_222 N/A Integer suffixes
Hex integer 0xff N/A Integer suffixes
Octal integer 0o77 N/A Integer suffixes
Binary integer 0b1111_0000 N/A Integer suffixes
Floating-point 123.0E+77 Optional Floating-point suffixes

* All number literals allow _ as a visual separator: 1_234.0E+18f64 Suffixes
Integer Floating-point
u8, i8, u16, i16, u32, i32, u64, i64, isize, usize f32, f64 Character and string literals Character literals

A character literal is a single Unicode character enclosed within two U+0027 (single-quote) characters, with the exception of U+0027 itself, which must be escaped by a preceding U+005C character (\). String literals

A string literal is a sequence of any Unicode characters enclosed within two U+0022 (double-quote) characters, with the exception of U+0022 itself, which must be escaped by a preceding U+005C character (\).

Line-break characters are allowed in string literals. Normally they represent themselves (i.e. no translation), but as a special exception, when an unescaped U+005C character (\) occurs immediately before the newline (U+000A), the U+005C character, the newline, and all whitespace at the beginning of the next line are ignored. Thus a and b are equal:

let a = "foobar";
let b = "foo\

assert_eq!(a,b);Run Character escapes

Some additional escapes are available in either character or non-raw string literals. An escape starts with a U+005C (\) and continues with one of the following forms: Raw string literals

Raw string literals do not process any escapes. They start with the character U+0072 (r), followed by zero or more of the character U+0023 (#) and a U+0022 (double-quote) character. The raw string body can contain any sequence of Unicode characters and is terminated only by another U+0022 (double-quote) character, followed by the same number of U+0023 (#) characters that preceded the opening U+0022 (double-quote) character.

All Unicode characters contained in the raw string body represent themselves, the characters U+0022 (double-quote) (except when followed by at least as many U+0023 (#) characters as were used to start the raw string literal) or U+005C (\) do not have any special meaning.

Examples for string literals:

"foo"; r"foo";                     // foo
"\"foo\""; r#""foo""#;             // "foo"

"foo #\"# bar";
r##"foo #"# bar"##;                // foo #"# bar

"\x52"; "R"; r"R";                 // R
"\\x52"; r"\x52";                  // \x52Run Byte and byte string literals Byte literals

A byte literal is a single ASCII character (in the U+0000 to U+007F range) or a single escape preceded by the characters U+0062 (b) and U+0027 (single-quote), and followed by the character U+0027. If the character U+0027 is present within the literal, it must be escaped by a preceding U+005C (\) character. It is equivalent to a u8 unsigned 8-bit integer number literal. Byte string literals

A non-raw byte string literal is a sequence of ASCII characters and escapes, preceded by the characters U+0062 (b) and U+0022 (double-quote), and followed by the character U+0022. If the character U+0022 is present within the literal, it must be escaped by a preceding U+005C (\) character. Alternatively, a byte string literal can be a raw byte string literal, defined below. A byte string literal of length n is equivalent to a &'static [u8; n] borrowed fixed-sized array of unsigned 8-bit integers.

Some additional escapes are available in either byte or non-raw byte string literals. An escape starts with a U+005C (\) and continues with one of the following forms: Raw byte string literals

Raw byte string literals do not process any escapes. They start with the character U+0062 (b), followed by U+0072 (r), followed by zero or more of the character U+0023 (#), and a U+0022 (double-quote) character. The raw string body can contain any sequence of ASCII characters and is terminated only by another U+0022 (double-quote) character, followed by the same number of U+0023 (#) characters that preceded the opening U+0022 (double-quote) character. A raw byte string literal can not contain any non-ASCII byte.

All characters contained in the raw string body represent their ASCII encoding, the characters U+0022 (double-quote) (except when followed by at least as many U+0023 (#) characters as were used to start the raw string literal) or U+005C (\) do not have any special meaning.

Examples for byte string literals:

b"foo"; br"foo";                     // foo
b"\"foo\""; br#""foo""#;             // "foo"

b"foo #\"# bar";
br##"foo #"# bar"##;                 // foo #"# bar

b"\x52"; b"R"; br"R";                // R
b"\\x52"; br"\x52";                  // \x52Run Number literals

A number literal is either an integer literal or a floating-point literal. The grammar for recognizing the two kinds of literals is mixed. Integer literals

An integer literal has one of four forms:

Like any literal, an integer literal may be followed (immediately, without any spaces) by an integer suffix, which forcibly sets the type of the literal. The integer suffix must be the name of one of the integral types: u8, i8, u16, i16, u32, i32, u64, i64, isize, or usize.

The type of an unsuffixed integer literal is determined by type inference:

Examples of integer literals of various forms:

123i32;                            // type i32
123u32;                            // type u32
123_u32;                           // type u32
0xff_u8;                           // type u8
0o70_i16;                          // type i16
0b1111_1111_1001_0000_i32;         // type i32
0usize;                            // type usizeRun

Note that the Rust syntax considers -1i8 as an application of the unary minus operator to an integer literal 1i8, rather than a single integer literal. Floating-point literals

A floating-point literal has one of two forms:

Like integer literals, a floating-point literal may be followed by a suffix, so long as the pre-suffix part does not end with U+002E (.). The suffix forcibly sets the type of the literal. There are two valid floating-point suffixes, f32 and f64 (the 32-bit and 64-bit floating point types), which explicitly determine the type of the literal.

The type of an unsuffixed floating-point literal is determined by type inference:

Examples of floating-point literals of various forms:

123.0f64;        // type f64
0.1f64;          // type f64
0.1f32;          // type f32
12E+99_f64;      // type f64
let x: f64 = 2.; // type f64Run

This last example is different because it is not possible to use the suffix syntax with a floating point literal ending in a period. 2.f64 would attempt to call a method named f64 on 2.

The representation semantics of floating-point numbers are described in "Machine Types". Boolean literals

The two values of the boolean type are written true and false.

3.5.2 Symbols

Symbols are a general class of printable tokens that play structural roles in a variety of grammar productions. They are a set of remaining miscellaneous printable tokens that do not otherwise appear as unary operators, binary operators, or keywords. They are catalogued in the Symbols section of the Grammar document.

3.6 Paths

A path is a sequence of one or more path components logically separated by a namespace qualifier (::). If a path consists of only one component, it may refer to either an item or a variable in a local control scope. If a path has multiple components, it refers to an item.

Every item has a canonical path within its crate, but the path naming an item is only meaningful within a given crate. There is no global namespace across crates; an item's canonical path merely identifies it within the crate.

Two examples of simple paths consisting of only identifier components:


Path components are usually identifiers, but they may also include angle-bracket-enclosed lists of type arguments. In expression context, the type argument list is given after a :: namespace qualifier in order to disambiguate it from a relational expression involving the less-than symbol (<). In type expression context, the final namespace qualifier is omitted.

Two examples of paths with type arguments:

type T = HashMap<i32,String>; // Type arguments used in a type expression
let x  = id::<i32>(10);       // Type arguments used in a call expressionRun

Paths can be denoted with various leading qualifiers to change the meaning of how it is resolved:

mod a {
    pub fn foo() {}
mod b {
    pub fn foo() {
        ::a::foo(); // call a's foo function
mod a {
    pub fn foo() {}
mod b {
    pub fn foo() {
        super::a::foo(); // call a's foo function
fn foo() {}
fn bar() {

Additionally keyword super may be repeated several times after the first super or self to refer to ancestor modules.

mod a {
    fn foo() {}

    mod b {
        mod c {
            fn foo() {
                super::super::foo(); // call a's foo function
                self::super::super::foo(); // call a's foo function

4 Syntax extensions

A number of minor features of Rust are not central enough to have their own syntax, and yet are not implementable as functions. Instead, they are given names, and invoked through a consistent syntax: some_extension!(...).

Users of rustc can define new syntax extensions in two ways:

4.1 Macros

macro_rules allows users to define syntax extension in a declarative way. We call such extensions "macros by example" or simply "macros" — to be distinguished from the "procedural macros" defined in compiler plugins.

Currently, macros can expand to expressions, statements, items, or patterns.

(A sep_token is any token other than * and +. A non_special_token is any token other than a delimiter or $.)

The macro expander looks up macro invocations by name, and tries each macro rule in turn. It transcribes the first successful match. Matching and transcription are closely related to each other, and we will describe them together.

4.1.1 Macro By Example

The macro expander matches and transcribes every token that does not begin with a $ literally, including delimiters. For parsing reasons, delimiters must be balanced, but they are otherwise not special.

In the matcher, $ name : designator matches the nonterminal in the Rust syntax named by designator. Valid designators are:

In the transcriber, the designator is already known, and so only the name of a matched nonterminal comes after the dollar sign.

In both the matcher and transcriber, the Kleene star-like operator indicates repetition. The Kleene star operator consists of $ and parentheses, optionally followed by a separator token, followed by * or +. * means zero or more repetitions, + means at least one repetition. The parentheses are not matched or transcribed. On the matcher side, a name is bound to all of the names it matches, in a structure that mimics the structure of the repetition encountered on a successful match. The job of the transcriber is to sort that structure out.

The rules for transcription of these repetitions are called "Macro By Example". Essentially, one "layer" of repetition is discharged at a time, and all of them must be discharged by the time a name is transcribed. Therefore, ( $( $i:ident ),* ) => ( $i ) is an invalid macro, but ( $( $i:ident ),* ) => ( $( $i:ident ),* ) is acceptable (if trivial).

When Macro By Example encounters a repetition, it examines all of the $ name s that occur in its body. At the "current layer", they all must repeat the same number of times, so ( $( $i:ident ),* ; $( $j:ident ),* ) => ( $( ($i,$j) ),* ) is valid if given the argument (a,b,c ; d,e,f), but not (a,b,c ; d,e). The repetition walks through the choices at that layer in lockstep, so the former input transcribes to (a,d), (b,e), (c,f).

Nested repetitions are allowed.

4.1.2 Parsing limitations

The parser used by the macro system is reasonably powerful, but the parsing of Rust syntax is restricted in two ways:

  1. Macro definitions are required to include suitable separators after parsing expressions and other bits of the Rust grammar. This implies that a macro definition like $i:expr [ , ] is not legal, because [ could be part of an expression. A macro definition like $i:expr, or $i:expr; would be legal, however, because , and ; are legal separators. See RFC 550 for more information.
  2. The parser must have eliminated all ambiguity by the time it reaches a $ name : designator. This requirement most often affects name-designator pairs when they occur at the beginning of, or immediately after, a $(...)*; requiring a distinctive token in front can solve the problem.

5 Crates and source files

Although Rust, like any other language, can be implemented by an interpreter as well as a compiler, the only existing implementation is a compiler, and the language has always been designed to be compiled. For these reasons, this section assumes a compiler.

Rust's semantics obey a phase distinction between compile-time and run-time.3 Semantic rules that have a static interpretation govern the success or failure of compilation, while semantic rules that have a dynamic interpretation govern the behavior of the program at run-time.

The compilation model centers on artifacts called crates. Each compilation processes a single crate in source form, and if successful, produces a single crate in binary form: either an executable or some sort of library.4

A crate is a unit of compilation and linking, as well as versioning, distribution and runtime loading. A crate contains a tree of nested module scopes. The top level of this tree is a module that is anonymous (from the point of view of paths within the module) and any item within a crate has a canonical module path denoting its location within the crate's module tree.

The Rust compiler is always invoked with a single source file as input, and always produces a single output crate. The processing of that source file may result in other source files being loaded as modules. Source files have the extension .rs.

A Rust source file describes a module, the name and location of which — in the module tree of the current crate — are defined from outside the source file: either by an explicit mod_item in a referencing source file, or by the name of the crate itself. Every source file is a module, but not every module needs its own source file: module definitions can be nested within one file.

Each source file contains a sequence of zero or more item definitions, and may optionally begin with any number of attributes that apply to the containing module, most of which influence the behavior of the compiler. The anonymous crate module can have additional attributes that apply to the crate as a whole.

// Specify the crate name.
#![crate_name = "projx"]

// Specify the type of output artifact.
#![crate_type = "lib"]

// Turn on a warning.
// This can be done in any module, not just the anonymous crate module.

A crate that contains a main function can be compiled to an executable. If a main function is present, its return type must be () ("unit") and it must take no arguments.

6 Items and attributes

Crates contain items, each of which may have some number of attributes attached to it.

6.1 Items

An item is a component of a crate. Items are organized within a crate by a nested set of modules. Every crate has a single "outermost" anonymous module; all further items within the crate have paths within the module tree of the crate.

Items are entirely determined at compile-time, generally remain fixed during execution, and may reside in read-only memory.

There are several kinds of item:

Some items form an implicit scope for the declaration of sub-items. In other words, within a function or module, declarations of items can (in many cases) be mixed with the statements, control blocks, and similar artifacts that otherwise compose the item body. The meaning of these scoped items is the same as if the item was declared outside the scope — it is still a static item — except that the item's path name within the module namespace is qualified by the name of the enclosing item, or is private to the enclosing item (in the case of functions). The grammar specifies the exact locations in which sub-item declarations may appear.

6.1.1 Type Parameters

All items except modules, constants and statics may be parameterized by type. Type parameters are given as a comma-separated list of identifiers enclosed in angle brackets (<...>), after the name of the item and before its definition. The type parameters of an item are considered "part of the name", not part of the type of the item. A referencing path must (in principle) provide type arguments as a list of comma-separated types enclosed within angle brackets, in order to refer to the type-parameterized item. In practice, the type-inference system can usually infer such argument types from context. There are no general type-parametric types, only type-parametric items. That is, Rust has no notion of type abstraction: there are no higher-ranked (or "forall") types abstracted over other types, though higher-ranked types do exist for lifetimes.

6.1.2 Modules

A module is a container for zero or more items.

A module item is a module, surrounded in braces, named, and prefixed with the keyword mod. A module item introduces a new, named module into the tree of modules making up a crate. Modules can nest arbitrarily.

An example of a module:

mod math {
    type Complex = (f64, f64);
    fn sin(f: f64) -> f64 {
        /* ... */
    fn cos(f: f64) -> f64 {
        /* ... */
    fn tan(f: f64) -> f64 {
        /* ... */

Modules and types share the same namespace. Declaring a named type with the same name as a module in scope is forbidden: that is, a type definition, trait, struct, enumeration, or type parameter can't shadow the name of a module in scope, or vice versa.

A module without a body is loaded from an external file, by default with the same name as the module, plus the .rs extension. When a nested submodule is loaded from an external file, it is loaded from a subdirectory path that mirrors the module hierarchy.

// Load the `vec` module from ``
mod vec;

mod thread {
    // Load the `local_data` module from `thread/`
    // or `thread/local_data/`.
    mod local_data;

The directories and files used for loading external file modules can be influenced with the path attribute.

#[path = "thread_files"]
mod thread {
    // Load the `local_data` module from `thread_files/`
    #[path = ""]
    mod local_data;
}Run Extern crate declarations

An extern crate declaration specifies a dependency on an external crate. The external crate is then bound into the declaring scope as the ident provided in the extern_crate_decl.

The external crate is resolved to a specific soname at compile time, and a runtime linkage requirement to that soname is passed to the linker for loading at runtime. The soname is resolved at compile time by scanning the compiler's library path and matching the optional crateid provided against the crateid attributes that were declared on the external crate when it was compiled. If no crateid is provided, a default name attribute is assumed, equal to the ident given in the extern_crate_decl.

Three examples of extern crate declarations:

extern crate pcre;

extern crate std; // equivalent to: extern crate std as std;

extern crate std as ruststd; // linking to 'std' under another nameRun

When naming Rust crates, hyphens are disallowed. However, Cargo packages may make use of them. In such case, when Cargo.toml doesn't specify a crate name, Cargo will transparently replace - with _ (Refer to RFC 940 for more details).

Here is an example:

// Importing the Cargo package hello-world
extern crate hello_world; // hyphen replaced with an underscoreRun Use declarations

A use declaration creates one or more local name bindings synonymous with some other path. Usually a use declaration is used to shorten the path required to refer to a module item. These declarations may appear in modules and blocks, usually at the top.

Note: Unlike in many languages, use declarations in Rust do not declare linkage dependency with external crates. Rather, extern crate declarations declare linkage dependencies.

Use declarations support a number of convenient shortcuts:

An example of use declarations:

use std::option::Option::{Some, None};
use std::collections::hash_map::{self, HashMap};

fn foo<T>(_: T){}
fn bar(map1: HashMap<String, usize>, map2: hash_map::HashMap<String, usize>){}

fn main() {
    // Equivalent to 'foo(vec![std::option::Option::Some(1.0f64),
    // std::option::Option::None]);'
    foo(vec![Some(1.0f64), None]);

    // Both `hash_map` and `HashMap` are in scope.
    let map1 = HashMap::new();
    let map2 = hash_map::HashMap::new();
    bar(map1, map2);

Like items, use declarations are private to the containing module, by default. Also like items, a use declaration can be public, if qualified by the pub keyword. Such a use declaration serves to re-export a name. A public use declaration can therefore redirect some public name to a different target definition: even a definition with a private canonical path, inside a different module. If a sequence of such redirections form a cycle or cannot be resolved unambiguously, they represent a compile-time error.

An example of re-exporting:

mod quux {
    pub use quux::foo::{bar, baz};

    pub mod foo {
        pub fn bar() { }
        pub fn baz() { }

In this example, the module quux re-exports two public names defined in foo.

Also note that the paths contained in use items are relative to the crate root. So, in the previous example, the use refers to quux::foo::{bar, baz}, and not simply to foo::{bar, baz}. This also means that top-level module declarations should be at the crate root if direct usage of the declared modules within use items is desired. It is also possible to use self and super at the beginning of a use item to refer to the current and direct parent modules respectively. All rules regarding accessing declared modules in use declarations apply to both module declarations and extern crate declarations.

An example of what will and will not work for use items:

use foo::baz::foobaz;    // good: foo is at the root of the crate

mod foo {

    mod example {
        pub mod iter {}

    use foo::example::iter; // good: foo is at crate root
//  use example::iter;      // bad:  example is not at the crate root
    use self::baz::foobaz;  // good: self refers to module 'foo'
    use foo::bar::foobar;   // good: foo is at crate root

    pub mod bar {
        pub fn foobar() { }

    pub mod baz {
        use super::bar::foobar; // good: super refers to module 'foo'
        pub fn foobaz() { }

fn main() {}Run

6.1.3 Functions

A function item defines a sequence of statements and a final expression, along with a name and a set of parameters. Other than a name, all these are optional. Functions are declared with the keyword fn. Functions may declare a set of input variables as parameters, through which the caller passes arguments into the function, and the output type of the value the function will return to its caller on completion.

A function may also be copied into a first-class value, in which case the value has the corresponding function type, and can be used otherwise exactly as a function item (with a minor additional cost of calling the function indirectly).

Every control path in a function logically ends with a return expression or a diverging expression. If the outermost block of a function has a value-producing expression in its final-expression position, that expression is interpreted as an implicit return expression applied to the final-expression.

An example of a function:

fn add(x: i32, y: i32) -> i32 {
    x + y

As with let bindings, function arguments are irrefutable patterns, so any pattern that is valid in a let binding is also valid as an argument.

fn first((value, _): (i32, i32)) -> i32 { value }Run Generic functions

A generic function allows one or more parameterized types to appear in its signature. Each type parameter must be explicitly declared in an angle-bracket-enclosed and comma-separated list, following the function name.

// foo is generic over A and B

fn foo<A, B>(x: A, y: B) {Run

Inside the function signature and body, the name of the type parameter can be used as a type name. Trait bounds can be specified for type parameters to allow methods with that trait to be called on values of that type. This is specified using the where syntax:

fn foo<T>(x: T) where T: Debug {Run

When a generic function is referenced, its type is instantiated based on the context of the reference. For example, calling the foo function here:

use std::fmt::Debug;

fn foo<T>(x: &[T]) where T: Debug {
    // details elided

foo(&[1, 2]);Run

will instantiate type parameter T with i32.

The type parameters can also be explicitly supplied in a trailing path component after the function name. This might be necessary if there is not sufficient context to determine the type parameters. For example, mem::size_of::<u32>() == 4. Diverging functions

A special kind of function can be declared with a ! character where the output type would normally be. For example:

fn my_err(s: &str) -> ! {
    println!("{}", s);

We call such functions "diverging" because they never return a value to the caller. Every control path in a diverging function must end with a panic!() or a call to another diverging function on every control path. The ! annotation does not denote a type.

It might be necessary to declare a diverging function because as mentioned previously, the typechecker checks that every control path in a function ends with a return or diverging expression. So, if my_err were declared without the ! annotation, the following code would not typecheck:

fn f(i: i32) -> i32 {
    if i == 42 {
        return 42;
    else {
        my_err("Bad number!");

This will not compile without the ! annotation on my_err, since the else branch of the conditional in f does not return an i32, as required by the signature of f. Adding the ! annotation to my_err informs the typechecker that, should control ever enter my_err, no further type judgments about f need to hold, since control will never resume in any context that relies on those judgments. Thus the return type on f only needs to reflect the if branch of the conditional. Extern functions

Extern functions are part of Rust's foreign function interface, providing the opposite functionality to external blocks. Whereas external blocks allow Rust code to call foreign code, extern functions with bodies defined in Rust code can be called by foreign code. They are defined in the same way as any other Rust function, except that they have the extern modifier.

// Declares an extern fn, the ABI defaults to "C"
extern fn new_i32() -> i32 { 0 }

// Declares an extern fn with "stdcall" ABI
extern "stdcall" fn new_i32_stdcall() -> i32 { 0 }Run

Unlike normal functions, extern fns have type extern "ABI" fn(). This is the same type as the functions declared in an extern block.

let fptr: extern "C" fn() -> i32 = new_i32;Run

Extern functions may be called directly from Rust code as Rust uses large, contiguous stack segments like C.

6.1.4 Type aliases

A type alias defines a new name for an existing type. Type aliases are declared with the keyword type. Every value has a single, specific type, but may implement several different traits, or be compatible with several different type constraints.

For example, the following defines the type Point as a synonym for the type (u8, u8), the type of pairs of unsigned 8 bit integers:

type Point = (u8, u8);
let p: Point = (41, 68);Run

Currently a type alias to an enum type cannot be used to qualify the constructors:

enum E { A }
type F = E;
let _: F = E::A;  // OK
// let _: F = F::A;  // Doesn't workRun

6.1.5 Structs

A struct is a nominal struct type defined with the keyword struct.

An example of a struct item and its use:

struct Point {x: i32, y: i32}
let p = Point {x: 10, y: 11};
let px: i32 = p.x;Run

A tuple struct is a nominal tuple type, also defined with the keyword struct. For example:

struct Point(i32, i32);
let p = Point(10, 11);
let px: i32 = match p { Point(x, _) => x };Run

A unit-like struct is a struct without any fields, defined by leaving off the list of fields entirely. Such a struct implicitly defines a constant of its type with the same name. For example:

struct Cookie;
let c = [Cookie, Cookie {}, Cookie, Cookie {}];Run

is equivalent to

struct Cookie {}
const Cookie: Cookie = Cookie {};
let c = [Cookie, Cookie {}, Cookie, Cookie {}];Run

The precise memory layout of a struct is not specified. One can specify a particular layout using the repr attribute.

6.1.6 Enumerations

An enumeration is a simultaneous definition of a nominal enumerated type as well as a set of constructors, that can be used to create or pattern-match values of the corresponding enumerated type.

Enumerations are declared with the keyword enum.

An example of an enum item and its use:

enum Animal {

let mut a: Animal = Animal::Dog;
a = Animal::Cat;Run

Enumeration constructors can have either named or unnamed fields:

enum Animal {
    Dog (String, f64),
    Cat { name: String, weight: f64 },

let mut a: Animal = Animal::Dog("Cocoa".to_string(), 37.2);
a = Animal::Cat { name: "Spotty".to_string(), weight: 2.7 };Run

In this example, Cat is a struct-like enum variant, whereas Dog is simply called an enum variant.

Each enum value has a discriminant which is an integer associated to it. You can specify it explicitly:

enum Foo {
    Bar = 123,

The right hand side of the specification is interpreted as an isize value, but the compiler is allowed to use a smaller type in the actual memory layout. The repr attribute can be added in order to change the type of the right hand side and specify the memory layout.

If a discriminant isn't specified, they start at zero, and add one for each variant, in order.

You can cast an enum to get its discriminant:

let x = Foo::Bar as u32; // x is now 123u32Run

This only works as long as none of the variants have data attached. If it were Bar(i32), this is disallowed.

6.1.7 Constant items

A constant item is a named constant value which is not associated with a specific memory location in the program. Constants are essentially inlined wherever they are used, meaning that they are copied directly into the relevant context when used. References to the same constant are not necessarily guaranteed to refer to the same memory address.

Constant values must not have destructors, and otherwise permit most forms of data. Constants may refer to the address of other constants, in which case the address will have the static lifetime. The compiler is, however, still at liberty to translate the constant many times, so the address referred to may not be stable.

Constants must be explicitly typed. The type may be bool, char, a number, or a type derived from those primitive types. The derived types are references with the static lifetime, fixed-size arrays, tuples, enum variants, and structs.

const BIT1: u32 = 1 << 0;
const BIT2: u32 = 1 << 1;

const BITS: [u32; 2] = [BIT1, BIT2];
const STRING: &'static str = "bitstring";

struct BitsNStrings<'a> {
    mybits: [u32; 2],
    mystring: &'a str,

const BITS_N_STRINGS: BitsNStrings<'static> = BitsNStrings {
    mybits: BITS,
    mystring: STRING,

6.1.8 Static items

A static item is similar to a constant, except that it represents a precise memory location in the program. A static is never "inlined" at the usage site, and all references to it refer to the same memory location. Static items have the static lifetime, which outlives all other lifetimes in a Rust program. Static items may be placed in read-only memory if they do not contain any interior mutability.

Statics may contain interior mutability through the UnsafeCell language item. All access to a static is safe, but there are a number of restrictions on statics:

Constants should in general be preferred over statics, unless large amounts of data are being stored, or single-address and mutability properties are required. Mutable statics

If a static item is declared with the mut keyword, then it is allowed to be modified by the program. One of Rust's goals is to make concurrency bugs hard to run into, and this is obviously a very large source of race conditions or other bugs. For this reason, an unsafe block is required when either reading or writing a mutable static variable. Care should be taken to ensure that modifications to a mutable static are safe with respect to other threads running in the same process.

Mutable statics are still very useful, however. They can be used with C libraries and can also be bound from C libraries (in an extern block).

static mut LEVELS: u32 = 0;

// This violates the idea of no shared state, and this doesn't internally
// protect against races, so this function is `unsafe`
unsafe fn bump_levels_unsafe1() -> u32 {
    let ret = LEVELS;
    LEVELS += 1;
    return ret;

// Assuming that we have an atomic_add function which returns the old value,
// this function is "safe" but the meaning of the return value may not be what
// callers expect, so it's still marked as `unsafe`
unsafe fn bump_levels_unsafe2() -> u32 {
    return atomic_add(&mut LEVELS, 1);

Mutable statics have the same restrictions as normal statics, except that the type of the value is not required to ascribe to Sync.

6.1.9 Traits

A trait describes an abstract interface that types can implement. This interface consists of associated items, which come in three varieties:

Associated functions whose first parameter is named self are called methods and may be invoked using . notation (e.g.,

All traits define an implicit type parameter Self that refers to "the type that is implementing this interface". Traits may also contain additional type parameters. These type parameters (including Self) may be constrained by other traits and so forth as usual.

Trait bounds on Self are considered "supertraits". These are required to be acyclic. Supertraits are somewhat different from other constraints in that they affect what methods are available in the vtable when the trait is used as a trait object.

Traits are implemented for specific types through separate implementations.

Consider the following trait:

trait Shape {
    fn draw(&self, Surface);
    fn bounding_box(&self) -> BoundingBox;

This defines a trait with two methods. All values that have implementations of this trait in scope can have their draw and bounding_box methods called, using value.bounding_box() syntax.

Traits can include default implementations of methods, as in:

trait Foo {
    fn bar(&self);
    fn baz(&self) { println!("We called baz."); }

Here the baz method has a default implementation, so types that implement Foo need only implement bar. It is also possible for implementing types to override a method that has a default implementation.

Type parameters can be specified for a trait to make it generic. These appear after the trait name, using the same syntax used in generic functions.

trait Seq<T> {
    fn len(&self) -> u32;
    fn elt_at(&self, n: u32) -> T;
    fn iter<F>(&self, F) where F: Fn(T);

It is also possible to define associated types for a trait. Consider the following example of a Container trait. Notice how the type is available for use in the method signatures:

trait Container {
    type E;
    fn empty() -> Self;
    fn insert(&mut self, Self::E);

In order for a type to implement this trait, it must not only provide implementations for every method, but it must specify the type E. Here's an implementation of Container for the standard library type Vec:

impl<T> Container for Vec<T> {
    type E = T;
    fn empty() -> Vec<T> { Vec::new() }
    fn insert(&mut self, x: T) { self.push(x); }

Generic functions may use traits as bounds on their type parameters. This will have two effects:

For example:

fn draw_twice<T: Shape>(surface: Surface, sh: T) {

Traits also define a trait object with the same name as the trait. Values of this type are created by coercing from a pointer of some specific type to a pointer of trait type. For example, &T could be coerced to &Shape if T: Shape holds (and similarly for Box<T>). This coercion can either be implicit or explicit. Here is an example of an explicit coercion:

trait Shape { }
impl Shape for i32 { }
let mycircle = 0i32;
let myshape: Box<Shape> = Box::new(mycircle) as Box<Shape>;Run

The resulting value is a box containing the value that was cast, along with information that identifies the methods of the implementation that was used. Values with a trait type can have methods called on them, for any method in the trait, and can be used to instantiate type parameters that are bounded by the trait.

Trait methods may be static, which means that they lack a self argument. This means that they can only be called with function call syntax (f(x)) and not method call syntax (obj.f()). The way to refer to the name of a static method is to qualify it with the trait name, treating the trait name like a module. For example:

trait Num {
    fn from_i32(n: i32) -> Self;
impl Num for f64 {
    fn from_i32(n: i32) -> f64 { n as f64 }
let x: f64 = Num::from_i32(42);Run

Traits may inherit from other traits. Consider the following example:

trait Shape { fn area(&self) -> f64; }
trait Circle : Shape { fn radius(&self) -> f64; }Run

The syntax Circle : Shape means that types that implement Circle must also have an implementation for Shape. Multiple supertraits are separated by +, trait Circle : Shape + PartialEq { }. In an implementation of Circle for a given type T, methods can refer to Shape methods, since the typechecker checks that any type with an implementation of Circle also has an implementation of Shape:

struct Foo;

trait Shape { fn area(&self) -> f64; }
trait Circle : Shape { fn radius(&self) -> f64; }
impl Shape for Foo {
    fn area(&self) -> f64 {
impl Circle for Foo {
    fn radius(&self) -> f64 {
        println!("calling area: {}", self.area());


let c = Foo;

In type-parameterized functions, methods of the supertrait may be called on values of subtrait-bound type parameters. Referring to the previous example of trait Circle : Shape:

fn radius_times_area<T: Circle>(c: T) -> f64 {
    // `c` is both a Circle and a Shape
    c.radius() * c.area()

Likewise, supertrait methods may also be called on trait objects.

let mycircle = Box::new(mycircle) as Box<Circle>;
let nonsense = mycircle.radius() * mycircle.area();Run

6.1.10 Implementations

An implementation is an item that implements a trait for a specific type.

Implementations are defined with the keyword impl.

struct Circle {
    radius: f64,
    center: Point,

impl Copy for Circle {}

impl Clone for Circle {
    fn clone(&self) -> Circle { *self }

impl Shape for Circle {
    fn draw(&self, s: Surface) { do_draw_circle(s, *self); }
    fn bounding_box(&self) -> BoundingBox {
        let r = self.radius;
        BoundingBox {
            x: - r,
            y: - r,
            width: 2.0 * r,
            height: 2.0 * r,

It is possible to define an implementation without referring to a trait. The methods in such an implementation can only be used as direct calls on the values of the type that the implementation targets. In such an implementation, the trait type and for after impl are omitted. Such implementations are limited to nominal types (enums, structs, trait objects), and the implementation must appear in the same crate as the self type:

struct Point {x: i32, y: i32}

impl Point {
    fn log(&self) {
        println!("Point is at ({}, {})", self.x, self.y);

let my_point = Point {x: 10, y:11};

When a trait is specified in an impl, all methods declared as part of the trait must be implemented, with matching types and type parameter counts.

An implementation can take type parameters, which can be different from the type parameters taken by the trait it implements. Implementation parameters are written after the impl keyword.

impl<T> Seq<T> for Vec<T> {
    /* ... */
impl Seq<bool> for u32 {
    /* Treat the integer as a sequence of bits */

6.1.11 External blocks

External blocks form the basis for Rust's foreign function interface. Declarations in an external block describe symbols in external, non-Rust libraries.

Functions within external blocks are declared in the same way as other Rust functions, with the exception that they may not have a body and are instead terminated by a semicolon.

Functions within external blocks may be called by Rust code, just like functions defined in Rust. The Rust compiler automatically translates between the Rust ABI and the foreign ABI.

A number of attributes control the behavior of external blocks.

By default external blocks assume that the library they are calling uses the standard C ABI on the specific platform. Other ABIs may be specified using an abi string, as shown here:

// Interface to the Windows API
extern "stdcall" { }Run

There are three ABI strings which are cross-platform, and which all compilers are guaranteed to support:

There are also some platform-specific ABI strings:

Finally, there are some rustc-specific ABI strings:

The link attribute allows the name of the library to be specified. When specified the compiler will attempt to link against the native library of the specified name.

#[link(name = "crypto")]
extern { }Run

The type of a function declared in an extern block is extern "abi" fn(A1, ..., An) -> R, where A1...An are the declared types of its arguments and R is the declared return type.

It is valid to add the link attribute on an empty extern block. You can use this to satisfy the linking requirements of extern blocks elsewhere in your code (including upstream crates) instead of adding the attribute to each extern block.

6.2 Visibility and Privacy

These two terms are often used interchangeably, and what they are attempting to convey is the answer to the question "Can this item be used at this location?"

Rust's name resolution operates on a global hierarchy of namespaces. Each level in the hierarchy can be thought of as some item. The items are one of those mentioned above, but also include external crates. Declaring or defining a new module can be thought of as inserting a new tree into the hierarchy at the location of the definition.

To control whether interfaces can be used across modules, Rust checks each use of an item to see whether it should be allowed or not. This is where privacy warnings are generated, or otherwise "you used a private item of another module and weren't allowed to."

By default, everything in Rust is private, with one exception. Enum variants in a pub enum are also public by default. When an item is declared as pub, it can be thought of as being accessible to the outside world. For example:

// Declare a private struct
struct Foo;

// Declare a public struct with a private field
pub struct Bar {
    field: i32,

// Declare a public enum with two public variants
pub enum State {

With the notion of an item being either public or private, Rust allows item accesses in two cases:

  1. If an item is public, then it can be used externally through any of its public ancestors.
  2. If an item is private, it may be accessed by the current module and its descendants.

These two cases are surprisingly powerful for creating module hierarchies exposing public APIs while hiding internal implementation details. To help explain, here's a few use cases and what they would entail:

In the second case, it mentions that a private item "can be accessed" by the current module and its descendants, but the exact meaning of accessing an item depends on what the item is. Accessing a module, for example, would mean looking inside of it (to import more items). On the other hand, accessing a function would mean that it is invoked. Additionally, path expressions and import statements are considered to access an item in the sense that the import/expression is only valid if the destination is in the current visibility scope.

Here's an example of a program which exemplifies the three cases outlined above:

// This module is private, meaning that no external crate can access this
// module. Because it is private at the root of this current crate, however, any
// module in the crate may access any publicly visible item in this module.
mod crate_helper_module {

    // This function can be used by anything in the current crate
    pub fn crate_helper() {}

    // This function *cannot* be used by anything else in the crate. It is not
    // publicly visible outside of the `crate_helper_module`, so only this
    // current module and its descendants may access it.
    fn implementation_detail() {}

// This function is "public to the root" meaning that it's available to external
// crates linking against this one.
pub fn public_api() {}

// Similarly to 'public_api', this module is public so external crates may look
// inside of it.
pub mod submodule {
    use crate_helper_module;

    pub fn my_method() {
        // Any item in the local crate may invoke the helper module's public
        // interface through a combination of the two rules above.

    // This function is hidden to any module which is not a descendant of
    // `submodule`
    fn my_implementation() {}

    mod test {

        fn test_my_implementation() {
            // Because this module is a descendant of `submodule`, it's allowed
            // to access private items inside of `submodule` without a privacy
            // violation.

For a Rust program to pass the privacy checking pass, all paths must be valid accesses given the two rules above. This includes all use statements, expressions, types, etc.

6.2.1 Re-exporting and Visibility

Rust allows publicly re-exporting items through a pub use directive. Because this is a public directive, this allows the item to be used in the current module through the rules above. It essentially allows public access into the re-exported item. For example, this program is valid:

pub use self::implementation::api;

mod implementation {
    pub mod api {
        pub fn f() {}

This means that any external crate referencing implementation::api::f would receive a privacy violation, while the path api::f would be allowed.

When re-exporting a private item, it can be thought of as allowing the "privacy chain" being short-circuited through the reexport instead of passing through the namespace hierarchy as it normally would.

6.3 Attributes

Any item declaration may have an attribute applied to it. Attributes in Rust are modeled on Attributes in ECMA-335, with the syntax coming from ECMA-334 (C#). An attribute is a general, free-form metadatum that is interpreted according to name, convention, and language and compiler version. Attributes may appear as any of:

Attributes with a bang ("!") after the hash ("#") apply to the item that the attribute is declared within. Attributes that do not have a bang after the hash apply to the item that follows the attribute.

An example of attributes:

// General metadata applied to the enclosing module or crate.
#![crate_type = "lib"]

// A function marked as a unit test
fn test_foo() {
    /* ... */

// A conditionally-compiled module
mod bar {
    /* ... */

// A lint attribute used to suppress a warning/error
type int8_t = i8;Run

Note: At some point in the future, the compiler will distinguish between language-reserved and user-available attributes. Until then, there is effectively no difference between an attribute handled by a loadable syntax extension and the compiler.

6.3.1 Crate-only attributes

6.3.2 Module-only attributes

6.3.3 Function-only attributes

6.3.4 Static-only attributes

6.3.5 FFI attributes

On an extern block, the following attributes are interpreted:

On declarations inside an extern block, the following attributes are interpreted:

On enums:

On structs:

See the macros section of the book for more information on macro scope.

6.3.7 Miscellaneous attributes

6.3.8 Conditional compilation

Sometimes one wants to have different compiler outputs from the same code, depending on build target, such as targeted operating system, or to enable release builds.

There are two kinds of configuration options, one that is either defined or not (#[cfg(foo)]), and the other that contains a string that can be checked against (#[cfg(bar = "baz")]). Currently, only compiler-defined configuration options can have the latter form.

// The function is only included in the build when compiling for OSX
#[cfg(target_os = "macos")]
fn macos_only() {
  // ...

// This function is only included when either foo or bar is defined
#[cfg(any(foo, bar))]
fn needs_foo_or_bar() {
  // ...

// This function is only included when compiling for a unixish OS with a 32-bit
// architecture
#[cfg(all(unix, target_pointer_width = "32"))]
fn on_32bit_unix() {
  // ...

// This function is only included when foo is not defined
fn needs_not_foo() {
  // ...

This illustrates some conditional compilation can be achieved using the #[cfg(...)] attribute. any, all and not can be used to assemble arbitrarily complex configurations through nesting.

The following configurations must be defined by the implementation:

You can also set another attribute based on a cfg variable with cfg_attr:

#[cfg_attr(a, b)]Run

Will be the same as #[b] if a is set by cfg, and nothing otherwise.

6.3.9 Lint check attributes

A lint check names a potentially undesirable coding pattern, such as unreachable code or omitted documentation, for the static entity to which the attribute applies.

For any lint check C:

The lint checks supported by the compiler can be found via rustc -W help, along with their default settings. Compiler plugins can provide additional lint checks.

pub mod m1 {
    // Missing documentation is ignored here
    pub fn undocumented_one() -> i32 { 1 }

    // Missing documentation signals a warning here
    pub fn undocumented_too() -> i32 { 2 }

    // Missing documentation signals an error here
    pub fn undocumented_end() -> i32 { 3 }

This example shows how one can use allow and warn to toggle a particular check on and off:

pub mod m2{
    pub mod nested {
        // Missing documentation is ignored here
        pub fn undocumented_one() -> i32 { 1 }

        // Missing documentation signals a warning here,
        // despite the allow above.
        pub fn undocumented_two() -> i32 { 2 }

    // Missing documentation signals a warning here
    pub fn undocumented_too() -> i32 { 3 }

This example shows how one can use forbid to disallow uses of allow for that lint check:

pub mod m3 {
    // Attempting to toggle warning signals an error here
    /// Returns 2.
    pub fn undocumented_too() -> i32 { 2 }

6.3.10 Language items

Some primitive Rust operations are defined in Rust code, rather than being implemented directly in C or assembly language. The definitions of these operations have to be easy for the compiler to find. The lang attribute makes it possible to declare these operations. For example, the str module in the Rust standard library defines the string equality function:

#[lang = "str_eq"]
pub fn eq_slice(a: &str, b: &str) -> bool {
    // details elided

The name str_eq has a special meaning to the Rust compiler, and the presence of this definition means that it will use this definition when generating calls to the string equality function.

The set of language items is currently considered unstable. A complete list of the built-in language items will be added in the future.

6.3.11 Inline attributes

The inline attribute suggests that the compiler should place a copy of the function or static in the caller, rather than generating code to call the function or access the static where it is defined.

The compiler automatically inlines functions based on internal heuristics. Incorrectly inlining functions can actually make the program slower, so it should be used with care.

#[inline] and #[inline(always)] always cause the function to be serialized into the crate metadata to allow cross-crate inlining.

There are three different types of inline attributes:

6.3.12 derive

The derive attribute allows certain traits to be automatically implemented for data structures. For example, the following will create an impl for the PartialEq and Clone traits for Foo, the type parameter T will be given the PartialEq or Clone constraints for the appropriate impl:

#[derive(PartialEq, Clone)]
struct Foo<T> {
    a: i32,
    b: T,

The generated impl for PartialEq is equivalent to

impl<T: PartialEq> PartialEq for Foo<T> {
    fn eq(&self, other: &Foo<T>) -> bool {
        self.a == other.a && self.b == other.b

    fn ne(&self, other: &Foo<T>) -> bool {
        self.a != other.a || self.b != other.b

6.3.13 Compiler Features

Certain aspects of Rust may be implemented in the compiler, but they're not necessarily ready for every-day use. These features are often of "prototype quality" or "almost production ready", but may not be stable enough to be considered a full-fledged language feature.

For this reason, Rust recognizes a special crate-level attribute of the form:

#![feature(feature1, feature2, feature3)]Run

This directive informs the compiler that the feature list: feature1, feature2, and feature3 should all be enabled. This is only recognized at a crate-level, not at a module-level. Without this directive, all features are considered off, and using the features will result in a compiler error.

The currently implemented features of the reference compiler are:

If a feature is promoted to a language feature, then all existing programs will start to receive compilation warnings about #![feature] directives which enabled the new feature (because the directive is no longer necessary). However, if a feature is decided to be removed from the language, errors will be issued (if there isn't a parser error first). The directive in this case is no longer necessary, and it's likely that existing code will break if the feature isn't removed.

If an unknown feature is found in a directive, it results in a compiler error. An unknown feature is one which has never been recognized by the compiler.

7 Statements and expressions

Rust is primarily an expression language. This means that most forms of value-producing or effect-causing evaluation are directed by the uniform syntax category of expressions. Each kind of expression can typically nest within each other kind of expression, and rules for evaluation of expressions involve specifying both the value produced by the expression and the order in which its sub-expressions are themselves evaluated.

In contrast, statements in Rust serve mostly to contain and explicitly sequence expression evaluation.

7.1 Statements

A statement is a component of a block, which is in turn a component of an outer expression or function.

Rust has two kinds of statement: declaration statements and expression statements.

7.1.1 Declaration statements

A declaration statement is one that introduces one or more names into the enclosing statement block. The declared names may denote new variables or new items. Item declarations

An item declaration statement has a syntactic form identical to an item declaration within a module. Declaring an item — a function, enumeration, struct, type, static, trait, implementation or module — locally within a statement block is simply a way of restricting its scope to a narrow region containing all of its uses; it is otherwise identical in meaning to declaring the item outside the statement block.

Note: there is no implicit capture of the function's dynamic environment when declaring a function-local item. let statements

A let statement introduces a new set of variables, given by a pattern. The pattern may be followed by a type annotation, and/or an initializer expression. When no type annotation is given, the compiler will infer the type, or signal an error if insufficient type information is available for definite inference. Any variables introduced by a variable declaration are visible from the point of declaration until the end of the enclosing block scope.

7.1.2 Expression statements

An expression statement is one that evaluates an expression and ignores its result. The type of an expression statement e; is always (), regardless of the type of e. As a rule, an expression statement's purpose is to trigger the effects of evaluating its expression.

7.2 Expressions

An expression may have two roles: it always produces a value, and it may have effects (otherwise known as "side effects"). An expression evaluates to a value, and has effects during evaluation. Many expressions contain sub-expressions (operands). The meaning of each kind of expression dictates several things:

In this way, the structure of expressions dictates the structure of execution. Blocks are just another kind of expression, so blocks, statements, expressions, and blocks again can recursively nest inside each other to an arbitrary depth. Lvalues, rvalues and temporaries

Expressions are divided into two main categories: lvalues and rvalues. Likewise within each expression, sub-expressions may occur in lvalue context or rvalue context. The evaluation of an expression depends both on its own category and the context it occurs within.

An lvalue is an expression that represents a memory location. These expressions are paths (which refer to local variables, function and method arguments, or static variables), dereferences (*expr), indexing expressions (expr[expr]), and field references (expr.f). All other expressions are rvalues.

The left operand of an assignment or compound-assignment expression is an lvalue context, as is the single operand of a unary borrow. The discriminant or subject of a match expression may be an lvalue context, if ref bindings are made, but is otherwise an rvalue context. All other expression contexts are rvalue contexts.

When an lvalue is evaluated in an lvalue context, it denotes a memory location; when evaluated in an rvalue context, it denotes the value held in that memory location. Temporary lifetimes

When an rvalue is used in an lvalue context, a temporary un-named lvalue is created and used instead. The lifetime of temporary values is typically the innermost enclosing statement; the tail expression of a block is considered part of the statement that encloses the block.

When a temporary rvalue is being created that is assigned into a let declaration, however, the temporary is created with the lifetime of the enclosing block instead, as using the enclosing statement (the let declaration) would be a guaranteed error (since a pointer to the temporary would be stored into a variable, but the temporary would be freed before the variable could be used). The compiler uses simple syntactic rules to decide which values are being assigned into a let binding, and therefore deserve a longer temporary lifetime.

Here are some examples: Moved and copied types

When a local variable is used as an rvalue, the variable will be copied if its type implements Copy. All others are moved.

7.2.1 Literal expressions

A literal expression consists of one of the literal forms described earlier. It directly describes a number, character, string, boolean value, or the unit value.

();        // unit type
"hello";   // string type
'5';       // character type
5;         // integer type

7.2.2 Path expressions

A path used as an expression context denotes either a local variable or an item. Path expressions are lvalues.

7.2.3 Tuple expressions

Tuples are written by enclosing zero or more comma-separated expressions in parentheses. They are used to create tuple-typed values.

(0.0, 4.5);
("a", 4usize, true);

You can disambiguate a single-element tuple from a value in parentheses with a comma:

(0,); // single-element tuple
(0); // zero in parenthesesRun

7.2.4 Struct expressions

There are several forms of struct expressions. A struct expression consists of the path of a struct item, followed by a brace-enclosed list of zero or more comma-separated name-value pairs, providing the field values of a new instance of the struct. A field name can be any identifier, and is separated from its value expression by a colon. The location denoted by a struct field is mutable if and only if the enclosing struct is mutable.

A tuple struct expression consists of the path of a struct item, followed by a parenthesized list of one or more comma-separated expressions (in other words, the path of a struct item followed by a tuple expression). The struct item must be a tuple struct item.

A unit-like struct expression consists only of the path of a struct item.

The following are examples of struct expressions:

Point {x: 10.0, y: 20.0};
NothingInMe {};
TuplePoint(10.0, 20.0);
let u = game::User {name: "Joe", age: 35, score: 100_000};

A struct expression forms a new value of the named struct type. Note that for a given unit-like struct type, this will always be the same value.

A struct expression can terminate with the syntax .. followed by an expression to denote a functional update. The expression following .. (the base) must have the same struct type as the new struct type being formed. The entire expression denotes the result of constructing a new struct (with the same type as the base expression) with the given values for the fields that were explicitly specified and the values in the base expression for all other fields.

let base = Point3d {x: 1, y: 2, z: 3};
Point3d {y: 0, z: 10, .. base};Run

7.2.5 Block expressions

A block expression is similar to a module in terms of the declarations that are possible. Each block conceptually introduces a new namespace scope. Use items can bring new names into scopes and declared items are in scope for only the block itself.

A block will execute each statement sequentially, and then execute the expression (if given). If the block ends in a statement, its value is ():

let x: () = { println!("Hello."); };Run

If it ends in an expression, its value and type are that of the expression:

let x: i32 = { println!("Hello."); 5 };

assert_eq!(5, x);Run

7.2.6 Method-call expressions

A method call consists of an expression followed by a single dot, an identifier, and a parenthesized expression-list. Method calls are resolved to methods on specific traits, either statically dispatching to a method if the exact self-type of the left-hand-side is known, or dynamically dispatching if the left-hand-side expression is an indirect trait object.

7.2.7 Field expressions

A field expression consists of an expression followed by a single dot and an identifier, when not immediately followed by a parenthesized expression-list (the latter is a method call expression). A field expression denotes a field of a struct.

(Struct {a: 10, b: 20}).a;Run

A field access is an lvalue referring to the value of that field. When the type providing the field inherits mutability, it can be assigned to.

Also, if the type of the expression to the left of the dot is a pointer, it is automatically dereferenced as many times as necessary to make the field access possible. In cases of ambiguity, we prefer fewer autoderefs to more.

7.2.8 Array expressions

An array expression is written by enclosing zero or more comma-separated expressions of uniform type in square brackets.

In the [expr ';' expr] form, the expression after the ';' must be a constant expression that can be evaluated at compile time, such as a literal or a static item.

[1, 2, 3, 4];
["a", "b", "c", "d"];
[0; 128];              // array with 128 zeros
[0u8, 0u8, 0u8, 0u8];Run

7.2.9 Index expressions

Array-typed expressions can be indexed by writing a square-bracket-enclosed expression (the index) after them. When the array is mutable, the resulting lvalue can be assigned to.

Indices are zero-based, and may be of any integral type. Vector access is bounds-checked at compile-time for constant arrays being accessed with a constant index value. Otherwise a check will be performed at run-time that will put the thread in a panicked state if it fails.

([1, 2, 3, 4])[0];

let x = (["a", "b"])[10]; // compiler error: const index-expr is out of bounds

let n = 10;
let y = (["a", "b"])[n]; // panics

let arr = ["a", "b"];
arr[10]; // panics

Also, if the type of the expression to the left of the brackets is a pointer, it is automatically dereferenced as many times as necessary to make the indexing possible. In cases of ambiguity, we prefer fewer autoderefs to more.

7.2.10 Range expressions

The .. operator will construct an object of one of the std::ops::Range variants.

1..2;   // std::ops::Range
3..;    // std::ops::RangeFrom
..4;    // std::ops::RangeTo
..;     // std::ops::RangeFullRun

The following expressions are equivalent.

let x = std::ops::Range {start: 0, end: 10};
let y = 0..10;

assert_eq!(x, y);Run

Similarly, the ... operator will construct an object of one of the std::ops::RangeInclusive variants.

1...2;   // std::ops::RangeInclusive
...4;    // std::ops::RangeToInclusiveRun

The following expressions are equivalent.

let x = std::ops::RangeInclusive::NonEmpty {start: 0, end: 10};
let y = 0...10;

assert_eq!(x, y);Run

7.2.11 Unary operator expressions

Rust defines the following unary operators. With the exception of ?, they are all written as prefix operators, before the expression they apply to.

7.2.12 Binary operator expressions

Binary operators expressions are given in terms of operator precedence. Arithmetic operators

Binary arithmetic expressions are syntactic sugar for calls to built-in traits, defined in the std::ops module of the std library. This means that arithmetic operators can be overridden for user-defined types. The default meaning of the operators on standard types is given here. Bitwise operators

Like the arithmetic operators, bitwise operators are syntactic sugar for calls to methods of built-in traits. This means that bitwise operators can be overridden for user-defined types. The default meaning of the operators on standard types is given here. Bitwise &, | and ^ applied to boolean arguments are equivalent to logical &&, || and != evaluated in non-lazy fashion. Lazy boolean operators

The operators || and && may be applied to operands of boolean type. The || operator denotes logical 'or', and the && operator denotes logical 'and'. They differ from | and & in that the right-hand operand is only evaluated when the left-hand operand does not already determine the result of the expression. That is, || only evaluates its right-hand operand when the left-hand operand evaluates to false, and && only when it evaluates to true. Comparison operators

Comparison operators are, like the arithmetic operators, and bitwise operators, syntactic sugar for calls to built-in traits. This means that comparison operators can be overridden for user-defined types. The default meaning of the operators on standard types is given here. Type cast expressions

A type cast expression is denoted with the binary operator as.

Executing an as expression casts the value on the left-hand side to the type on the right-hand side.

An example of an as expression:

fn average(values: &[f64]) -> f64 {
    let sum: f64 = sum(values);
    let size: f64 = len(values) as f64;
    sum / size

Some of the conversions which can be done through the as operator can also be done implicitly at various points in the program, such as argument passing and assignment to a let binding with an explicit type. Implicit conversions are limited to "harmless" conversions that do not lose information and which have minimal or no risk of surprising side-effects on the dynamic execution semantics. Assignment expressions

An assignment expression consists of an lvalue expression followed by an equals sign (=) and an rvalue expression.

Evaluating an assignment expression either copies or moves its right-hand operand to its left-hand operand.

x = y;Run Compound assignment expressions

The +, -, *, /, %, &, |, ^, <<, and >> operators may be composed with the = operator. The expression lval OP= val is equivalent to lval = lval OP val. For example, x = x + 1 may be written as x += 1.

Any such expression always has the unit type. Operator precedence

The precedence of Rust binary operators is ordered as follows, going from strong to weak:

as :
* / %
+ -
<< >>
== != < > <= >=
.. ...

Operators at the same precedence level are evaluated left-to-right. Unary operators have the same precedence level and are stronger than any of the binary operators.

7.2.13 Grouped expressions

An expression enclosed in parentheses evaluates to the result of the enclosed expression. Parentheses can be used to explicitly specify evaluation order within an expression.

An example of a parenthesized expression:

let x: i32 = (2 + 3) * 4;Run

7.2.14 Call expressions

A call expression invokes a function, providing zero or more input variables and an optional location to move the function's output into. If the function eventually returns, then the expression completes.

Some examples of call expressions:

let x: i32 = add(1i32, 2i32);
let pi: Result<f32, _> = "3.14".parse();Run

7.2.15 Lambda expressions

A lambda expression (sometimes called an "anonymous function expression") defines a function and denotes it as a value, in a single expression. A lambda expression is a pipe-symbol-delimited (|) list of identifiers followed by an expression.

A lambda expression denotes a function that maps a list of parameters (ident_list) onto the expression that follows the ident_list. The identifiers in the ident_list are the parameters to the function. These parameters' types need not be specified, as the compiler infers them from context.

Lambda expressions are most useful when passing functions as arguments to other functions, as an abbreviation for defining and capturing a separate function.

Significantly, lambda expressions capture their environment, which regular function definitions do not. The exact type of capture depends on the function type inferred for the lambda expression. In the simplest and least-expensive form (analogous to a || { } expression), the lambda expression captures its environment by reference, effectively borrowing pointers to all outer variables mentioned inside the function. Alternately, the compiler may infer that a lambda expression should copy or move values (depending on their type) from the environment into the lambda expression's captured environment. A lambda can be forced to capture its environment by moving values by prefixing it with the move keyword.

In this example, we define a function ten_times that takes a higher-order function argument, and we then call it with a lambda expression as an argument, followed by a lambda expression that moves values from its environment.

fn ten_times<F>(f: F) where F: Fn(i32) {
    for index in 0..10 {

ten_times(|j| println!("hello, {}", j));

let word = "konnichiwa".to_owned();
ten_times(move |j| println!("{}, {}", word, j));Run

7.2.16 Infinite loops

A loop expression denotes an infinite loop.

A loop expression may optionally have a label. The label is written as a lifetime preceding the loop expression, as in 'foo: loop{ }. If a label is present, then labeled break and continue expressions nested within this loop may exit out of this loop or return control to its head. See break expressions and continue expressions.

7.2.17 break expressions

A break expression has an optional label. If the label is absent, then executing a break expression immediately terminates the innermost loop enclosing it. It is only permitted in the body of a loop. If the label is present, then break 'foo terminates the loop with label 'foo, which need not be the innermost label enclosing the break expression, but must enclose it.

7.2.18 continue expressions

A continue expression has an optional label. If the label is absent, then executing a continue expression immediately terminates the current iteration of the innermost loop enclosing it, returning control to the loop head. In the case of a while loop, the head is the conditional expression controlling the loop. In the case of a for loop, the head is the call-expression controlling the loop. If the label is present, then continue 'foo returns control to the head of the loop with label 'foo, which need not be the innermost label enclosing the continue expression, but must enclose it.

A continue expression is only permitted in the body of a loop.

7.2.19 while loops

A while loop begins by evaluating the boolean loop conditional expression. If the loop conditional expression evaluates to true, the loop body block executes and control returns to the loop conditional expression. If the loop conditional expression evaluates to false, the while expression completes.

An example:

let mut i = 0;

while i < 10 {
    i = i + 1;

Like loop expressions, while loops can be controlled with break or continue, and may optionally have a label. See infinite loops, break expressions, and continue expressions for more information.

7.2.20 for expressions

A for expression is a syntactic construct for looping over elements provided by an implementation of std::iter::IntoIterator.

An example of a for loop over the contents of an array:

let v: &[Foo] = &[a, b, c];

for e in v {

An example of a for loop over a series of integers:

for i in 0..256 {

Like loop expressions, for loops can be controlled with break or continue, and may optionally have a label. See infinite loops, break expressions, and continue expressions for more information.

7.2.21 if expressions

An if expression is a conditional branch in program control. The form of an if expression is a condition expression, followed by a consequent block, any number of else if conditions and blocks, and an optional trailing else block. The condition expressions must have type bool. If a condition expression evaluates to true, the consequent block is executed and any subsequent else if or else block is skipped. If a condition expression evaluates to false, the consequent block is skipped and any subsequent else if condition is evaluated. If all if and else if conditions evaluate to false then any else block is executed.

7.2.22 match expressions

A match expression branches on a pattern. The exact form of matching that occurs depends on the pattern. Patterns consist of some combination of literals, destructured arrays or enum constructors, structs and tuples, variable binding specifications, wildcards (..), and placeholders (_). A match expression has a head expression, which is the value to compare to the patterns. The type of the patterns must equal the type of the head expression.

In a pattern whose head expression has an enum type, a placeholder (_) stands for a single data field, whereas a wildcard .. stands for all the fields of a particular variant.

A match behaves differently depending on whether or not the head expression is an lvalue or an rvalue. If the head expression is an rvalue, it is first evaluated into a temporary location, and the resulting value is sequentially compared to the patterns in the arms until a match is found. The first arm with a matching pattern is chosen as the branch target of the match, any variables bound by the pattern are assigned to local variables in the arm's block, and control enters the block.

When the head expression is an lvalue, the match does not allocate a temporary location (however, a by-value binding may copy or move from the lvalue). When possible, it is preferable to match on lvalues, as the lifetime of these matches inherits the lifetime of the lvalue, rather than being restricted to the inside of the match.

An example of a match expression:

let x = 1;

match x {
    1 => println!("one"),
    2 => println!("two"),
    3 => println!("three"),
    4 => println!("four"),
    5 => println!("five"),
    _ => println!("something else"),

Patterns that bind variables default to binding to a copy or move of the matched value (depending on the matched value's type). This can be changed to bind to a reference by using the ref keyword, or to a mutable reference using ref mut.

Subpatterns can also be bound to variables by the use of the syntax variable @ subpattern. For example:

let x = 1;

match x {
    e @ 1 ... 5 => println!("got a range element {}", e),
    _ => println!("anything"),

Patterns can also dereference pointers by using the &, &mut and box symbols, as appropriate. For example, these two matches on x: &i32 are equivalent:

let y = match *x { 0 => "zero", _ => "some" };
let z = match x { &0 => "zero", _ => "some" };

assert_eq!(y, z);Run

Multiple match patterns may be joined with the | operator. A range of values may be specified with .... For example:

let message = match x {
    0 | 1  => "not many",
    2 ... 9 => "a few",
    _      => "lots"

Range patterns only work on scalar types (like integers and characters; not like arrays and structs, which have sub-components). A range pattern may not be a sub-range of another range pattern inside the same match.

Finally, match patterns can accept pattern guards to further refine the criteria for matching a case. Pattern guards appear after the pattern and consist of a bool-typed expression following the if keyword. A pattern guard may refer to the variables bound within the pattern they follow.

let message = match maybe_digit {
    Some(x) if x < 10 => process_digit(x),
    Some(x) => process_other(x),
    None => panic!(),

7.2.23 if let expressions

An if let expression is semantically identical to an if expression but in place of a condition expression it expects a let statement with a refutable pattern. If the value of the expression on the right hand side of the let statement matches the pattern, the corresponding block will execute, otherwise flow proceeds to the first else block that follows.

let dish = ("Ham", "Eggs");

// this body will be skipped because the pattern is refuted
if let ("Bacon", b) = dish {
    println!("Bacon is served with {}", b);

// this body will execute
if let ("Ham", b) = dish {
    println!("Ham is served with {}", b);

7.2.24 while let loops

A while let loop is semantically identical to a while loop but in place of a condition expression it expects let statement with a refutable pattern. If the value of the expression on the right hand side of the let statement matches the pattern, the loop body block executes and control returns to the pattern matching statement. Otherwise, the while expression completes.

7.2.25 return expressions

Return expressions are denoted with the keyword return. Evaluating a return expression moves its argument into the designated output location for the current function call, destroys the current function activation frame, and transfers control to the caller frame.

An example of a return expression:

fn max(a: i32, b: i32) -> i32 {
    if a > b {
        return a;
    return b;

8 Type system

8.1 Types

Every variable, item and value in a Rust program has a type. The type of a value defines the interpretation of the memory holding it.

Built-in types and type-constructors are tightly integrated into the language, in nontrivial ways that are not possible to emulate in user-defined types. User-defined types have limited capabilities.

8.1.1 Primitive types

The primitive types are the following: Machine types

The machine types are the following: Machine-dependent integer types

The usize type is an unsigned integer type with the same number of bits as the platform's pointer type. It can represent every memory address in the process.

The isize type is a signed integer type with the same number of bits as the platform's pointer type. The theoretical upper bound on object and array size is the maximum isize value. This ensures that isize can be used to calculate differences between pointers into an object or array and can address every byte within an object along with one byte past the end.

8.1.2 Textual types

The types char and str hold textual data.

A value of type char is a Unicode scalar value (i.e. a code point that is not a surrogate), represented as a 32-bit unsigned word in the 0x0000 to 0xD7FF or 0xE000 to 0x10FFFF range. A [char] array is effectively an UCS-4 / UTF-32 string.

A value of type str is a Unicode string, represented as an array of 8-bit unsigned bytes holding a sequence of UTF-8 code points. Since str is of unknown size, it is not a first-class type, but can only be instantiated through a pointer type, such as &str.

8.1.3 Tuple types

A tuple type is a heterogeneous product of other types, called the elements of the tuple. It has no nominal name and is instead structurally typed.

Tuple types and values are denoted by listing the types or values of their elements, respectively, in a parenthesized, comma-separated list.

Because tuple elements don't have a name, they can only be accessed by pattern-matching or by using N directly as a field to access the Nth element.

An example of a tuple type and its use:

type Pair<'a> = (i32, &'a str);
let p: Pair<'static> = (10, "ten");
let (a, b) = p;

assert_eq!(a, 10);
assert_eq!(b, "ten");
assert_eq!(p.0, 10);
assert_eq!(p.1, "ten");Run

For historical reasons and convenience, the tuple type with no elements (()) is often called ‘unit’ or ‘the unit type’.

8.1.4 Array, and Slice types

Rust has two different types for a list of items:

An array has a fixed size, and can be allocated on either the stack or the heap.

A slice is a 'view' into an array. It doesn't own the data it points to, it borrows it.


// A stack-allocated array
let array: [i32; 3] = [1, 2, 3];

// A heap-allocated array
let vector: Vec<i32> = vec![1, 2, 3];

// A slice into an array
let slice: &[i32] = &vector[..];Run

As you can see, the vec! macro allows you to create a Vec<T> easily. The vec! macro is also part of the standard library, rather than the language.

All in-bounds elements of arrays and slices are always initialized, and access to an array or slice is always bounds-checked.

8.1.5 Struct types

A struct type is a heterogeneous product of other types, called the fields of the type.5

New instances of a struct can be constructed with a struct expression.

The memory layout of a struct is undefined by default to allow for compiler optimizations like field reordering, but it can be fixed with the #[repr(...)] attribute. In either case, fields may be given in any order in a corresponding struct expression; the resulting struct value will always have the same memory layout.

The fields of a struct may be qualified by visibility modifiers, to allow access to data in a struct outside a module.

A tuple struct type is just like a struct type, except that the fields are anonymous.

A unit-like struct type is like a struct type, except that it has no fields. The one value constructed by the associated struct expression is the only value that inhabits such a type.

8.1.6 Enumerated types

An enumerated type is a nominal, heterogeneous disjoint union type, denoted by the name of an enum item. 6

An enum item declares both the type and a number of variant constructors, each of which is independently named and takes an optional tuple of arguments.

New instances of an enum can be constructed by calling one of the variant constructors, in a call expression.

Any enum value consumes as much memory as the largest variant constructor for its corresponding enum type.

Enum types cannot be denoted structurally as types, but must be denoted by named reference to an enum item.

8.1.7 Recursive types

Nominal types — enumerations and structs — may be recursive. That is, each enum constructor or struct field may refer, directly or indirectly, to the enclosing enum or struct type itself. Such recursion has restrictions:

An example of a recursive type and its use:

enum List<T> {
    Cons(T, Box<List<T>>)

let a: List<i32> = List::Cons(7, Box::new(List::Cons(13, Box::new(List::Nil))));Run

8.1.8 Pointer types

All pointers in Rust are explicit first-class values. They can be copied, stored into data structs, and returned from functions. There are two varieties of pointer in Rust:

The standard library contains additional 'smart pointer' types beyond references and raw pointers.

8.1.9 Function types

The function type constructor fn forms new function types. A function type consists of a possibly-empty set of function-type modifiers (such as unsafe or extern), a sequence of input types and an output type.

An example of a fn type:

fn add(x: i32, y: i32) -> i32 {
    x + y

let mut x = add(5,7);

type Binop = fn(i32, i32) -> i32;
let bo: Binop = add;
x = bo(5,7);Run Function types for specific items

Internal to the compiler, there are also function types that are specific to a particular function item. In the following snippet, for example, the internal types of the functions foo and bar are different, despite the fact that they have the same signature:

fn foo() { }
fn bar() { }Run

The types of foo and bar can both be implicitly coerced to the fn pointer type fn(). There is currently no syntax for unique fn types, though the compiler will emit a type like fn() {foo} in error messages to indicate "the unique fn type for the function foo".

8.1.10 Closure types

A lambda expression produces a closure value with a unique, anonymous type that cannot be written out.

Depending on the requirements of the closure, its type implements one or more of the closure traits:

8.1.11 Trait objects

In Rust, a type like &SomeTrait or Box<SomeTrait> is called a trait object. Each instance of a trait object includes:

The purpose of trait objects is to permit "late binding" of methods. Calling a method on a trait object results in virtual dispatch at runtime: that is, a function pointer is loaded from the trait object vtable and invoked indirectly. The actual implementation for each vtable entry can vary on an object-by-object basis.

Note that for a trait object to be instantiated, the trait must be object-safe. Object safety rules are defined in RFC 255.

Given a pointer-typed expression E of type &T or Box<T>, where T implements trait R, casting E to the corresponding pointer type &R or Box<R> results in a value of the trait object R. This result is represented as a pair of pointers: the vtable pointer for the T implementation of R, and the pointer value of E.

An example of a trait object:

trait Printable {
    fn stringify(&self) -> String;

impl Printable for i32 {
    fn stringify(&self) -> String { self.to_string() }

fn print(a: Box<Printable>) {
    println!("{}", a.stringify());

fn main() {
    print(Box::new(10) as Box<Printable>);

In this example, the trait Printable occurs as a trait object in both the type signature of print, and the cast expression in main.

8.1.12 Type parameters

Within the body of an item that has type parameter declarations, the names of its type parameters are types:

fn to_vec<A: Clone>(xs: &[A]) -> Vec<A> {
    if xs.is_empty() {
        return vec![];
    let first: A = xs[0].clone();
    let mut rest: Vec<A> = to_vec(&xs[1..]);
    rest.insert(0, first);

Here, first has type A, referring to to_vec's A type parameter; and rest has type Vec<A>, a vector with element type A.

8.1.13 Self types

The special type Self has a meaning within traits and impls. In a trait definition, it refers to an implicit type parameter representing the "implementing" type. In an impl, it is an alias for the implementing type. For example, in:

pub trait From<T> {
    fn from(T) -> Self;

impl From<i32> for String {
    fn from(x: i32) -> Self {

The notation Self in the impl refers to the implementing type: String. In another example:

trait Printable {
    fn make_string(&self) -> String;

impl Printable for String {
    fn make_string(&self) -> String {

The notation &self is a shorthand for self: &Self. In this case, in the impl, Self refers to the value of type String that is the receiver for a call to the method make_string.

8.2 Subtyping

Subtyping is implicit and can occur at any stage in type checking or inference. Subtyping in Rust is very restricted and occurs only due to variance with respect to lifetimes and between types with higher ranked lifetimes. If we were to erase lifetimes from types, then the only subtyping would be due to type equality.

Consider the following example: string literals always have 'static lifetime. Nevertheless, we can assign s to t:

fn bar<'a>() {
    let s: &'static str = "hi";
    let t: &'a str = s;

Since 'static "lives longer" than 'a, &'static str is a subtype of &'a str.

8.3 Type coercions

Coercions are defined in RFC 401. A coercion is implicit and has no syntax.

8.3.1 Coercion sites

A coercion can only occur at certain coercion sites in a program; these are typically places where the desired type is explicit or can be derived by propagation from explicit types (without type inference). Possible coercion sites are:

If the expression in one of these coercion sites is a coercion-propagating expression, then the relevant sub-expressions in that expression are also coercion sites. Propagation recurses from these new coercion sites. Propagating expressions and their relevant sub-expressions are:

8.3.2 Coercion types

Coercion is allowed between the following types:

9 Special traits

Several traits define special evaluation behavior.

9.1 The Copy trait

The Copy trait changes the semantics of a type implementing it. Values whose type implements Copy are copied rather than moved upon assignment.

9.2 The Sized trait

The Sized trait indicates that the size of this type is known at compile-time.

9.3 The Drop trait

The Drop trait provides a destructor, to be run whenever a value of this type is to be destroyed.

9.4 The Deref trait

The Deref<Target = U> trait allows a type to implicitly implement all the methods of the type U. When attempting to resolve a method call, the compiler will search the top-level type for the implementation of the called method. If no such method is found, .deref() is called and the compiler continues to search for the method implementation in the returned type U.

9.5 The Send trait

The Send trait indicates that a value of this type is safe to send from one thread to another.

9.6 The Sync trait

The Sync trait indicates that a value of this type is safe to share between multiple threads.

10 Memory model

A Rust program's memory consists of a static set of items and a heap. Immutable portions of the heap may be safely shared between threads, mutable portions may not be safely shared, but several mechanisms for effectively-safe sharing of mutable values, built on unsafe code but enforcing a safe locking discipline, exist in the standard library.

Allocations in the stack consist of variables, and allocations in the heap consist of boxes.

10.0.1 Memory allocation and lifetime

The items of a program are those functions, modules and types that have their value calculated at compile-time and stored uniquely in the memory image of the rust process. Items are neither dynamically allocated nor freed.

The heap is a general term that describes boxes. The lifetime of an allocation in the heap depends on the lifetime of the box values pointing to it. Since box values may themselves be passed in and out of frames, or stored in the heap, heap allocations may outlive the frame they are allocated within. An allocation in the heap is guaranteed to reside at a single location in the heap for the whole lifetime of the allocation - it will never be relocated as a result of moving a box value.

10.0.2 Memory ownership

When a stack frame is exited, its local allocations are all released, and its references to boxes are dropped.

10.0.3 Variables

A variable is a component of a stack frame, either a named function parameter, an anonymous temporary, or a named local variable.

A local variable (or stack-local allocation) holds a value directly, allocated within the stack's memory. The value is a part of the stack frame.

Local variables are immutable unless declared otherwise like: let mut x = ....

Function parameters are immutable unless declared with mut. The mut keyword applies only to the following parameter (so |mut x, y| and fn f(mut x: Box<i32>, y: Box<i32>) declare one mutable variable x and one immutable variable y).

Methods that take either self or Box<Self> can optionally place them in a mutable variable by prefixing them with mut (similar to regular arguments):

trait Changer: Sized {
    fn change(mut self) {}
    fn modify(mut self: Box<Self>) {}

Local variables are not initialized when allocated; the entire frame worth of local variables are allocated at once, on frame-entry, in an uninitialized state. Subsequent statements within a function may or may not initialize the local variables. Local variables can be used only after they have been initialized; this is enforced by the compiler.

11 Linkage

The Rust compiler supports various methods to link crates together both statically and dynamically. This section will explore the various methods to link Rust crates together, and more information about native libraries can be found in the FFI section of the book.

In one session of compilation, the compiler can generate multiple artifacts through the usage of either command line flags or the crate_type attribute. If one or more command line flags are specified, all crate_type attributes will be ignored in favor of only building the artifacts specified by command line.

Note that these outputs are stackable in the sense that if multiple are specified, then the compiler will produce each form of output at once without having to recompile. However, this only applies for outputs specified by the same method. If only crate_type attributes are specified, then they will all be built, but if one or more --crate-type command line flags are specified, then only those outputs will be built.

With all these different kinds of outputs, if crate A depends on crate B, then the compiler could find B in various different forms throughout the system. The only forms looked for by the compiler, however, are the rlib format and the dynamic library format. With these two options for a dependent library, the compiler must at some point make a choice between these two formats. With this in mind, the compiler follows these rules when determining what format of dependencies will be used:

  1. If a static library is being produced, all upstream dependencies are required to be available in rlib formats. This requirement stems from the reason that a dynamic library cannot be converted into a static format.

    Note that it is impossible to link in native dynamic dependencies to a static library, and in this case warnings will be printed about all unlinked native dynamic dependencies.

  2. If an rlib file is being produced, then there are no restrictions on what format the upstream dependencies are available in. It is simply required that all upstream dependencies be available for reading metadata from.

    The reason for this is that rlib files do not contain any of their upstream dependencies. It wouldn't be very efficient for all rlib files to contain a copy of libstd.rlib!

  3. If an executable is being produced and the -C prefer-dynamic flag is not specified, then dependencies are first attempted to be found in the rlib format. If some dependencies are not available in an rlib format, then dynamic linking is attempted (see below).

  4. If a dynamic library or an executable that is being dynamically linked is being produced, then the compiler will attempt to reconcile the available dependencies in either the rlib or dylib format to create a final product.

    A major goal of the compiler is to ensure that a library never appears more than once in any artifact. For example, if dynamic libraries B and C were each statically linked to library A, then a crate could not link to B and C together because there would be two copies of A. The compiler allows mixing the rlib and dylib formats, but this restriction must be satisfied.

    The compiler currently implements no method of hinting what format a library should be linked with. When dynamically linking, the compiler will attempt to maximize dynamic dependencies while still allowing some dependencies to be linked in via an rlib.

    For most situations, having all libraries available as a dylib is recommended if dynamically linking. For other situations, the compiler will emit a warning if it is unable to determine which formats to link each library with.

In general, --crate-type=bin or --crate-type=lib should be sufficient for all compilation needs, and the other options are just available if more fine-grained control is desired over the output format of a Rust crate.

12 Unsafety

Unsafe operations are those that potentially violate the memory-safety guarantees of Rust's static semantics.

The following language level features cannot be used in the safe subset of Rust:

12.1 Unsafe functions

Unsafe functions are functions that are not safe in all contexts and/or for all possible inputs. Such a function must be prefixed with the keyword unsafe and can only be called from an unsafe block or another unsafe function.

12.2 Unsafe blocks

A block of code can be prefixed with the unsafe keyword, to permit calling unsafe functions or dereferencing raw pointers within a safe function.

When a programmer has sufficient conviction that a sequence of potentially unsafe operations is actually safe, they can encapsulate that sequence (taken as a whole) within an unsafe block. The compiler will consider uses of such code safe, in the surrounding context.

Unsafe blocks are used to wrap foreign libraries, make direct use of hardware or implement features not directly present in the language. For example, Rust provides the language features necessary to implement memory-safe concurrency in the language but the implementation of threads and message passing is in the standard library.

Rust's type system is a conservative approximation of the dynamic safety requirements, so in some cases there is a performance cost to using safe code. For example, a doubly-linked list is not a tree structure and can only be represented with reference-counted pointers in safe code. By using unsafe blocks to represent the reverse links as raw pointers, it can be implemented with only boxes.

12.3 Behavior considered undefined

The following is a list of behavior which is forbidden in all Rust code, including within unsafe blocks and unsafe functions. Type checking provides the guarantee that these issues are never caused by safe code.

12.4 Behavior not considered unsafe

This is a list of behavior not considered unsafe in Rust terms, but that may be undesired.

13 Appendix: Influences

Rust is not a particularly original language, with design elements coming from a wide range of sources. Some of these are listed below (including elements that have since been removed):

  1. Substitute definitions for the special Unicode productions are provided to the grammar verifier, restricted to ASCII range, when verifying the grammar in this document. 

  2. Non-ASCII characters in identifiers are currently feature gated. This is expected to improve soon. 

  3. This distinction would also exist in an interpreter. Static checks like syntactic analysis, type checking, and lints should happen before the program is executed regardless of when it is executed. 

  4. A crate is somewhat analogous to an assembly in the ECMA-335 CLI model, a library in the SML/NJ Compilation Manager, a unit in the Owens and Flatt module system, or a configuration in Mesa. 

  5. struct types are analogous to struct types in C, the record types of the ML family, or the struct types of the Lisp family. 

  6. The enum type is analogous to a data constructor declaration in ML, or a pick ADT in Limbo.