5 This document is the primary reference for the Rust programming language. It
6 provides three kinds of material:
8 - Chapters that informally describe each language construct and their use.
9 - Chapters that informally describe the memory model, concurrency model,
10 runtime services, linkage model and debugging facilities.
11 - Appendix chapters providing rationale and references to languages that
12 influenced the design.
14 This document does not serve as an introduction to the language. Background
15 familiarity with the language is assumed. A separate [book] is available to
16 help acquire such background familiarity.
18 This document also does not serve as a reference to the [standard] library
19 included in the language distribution. Those libraries are documented
20 separately by extracting documentation attributes from their source code. Many
21 of the features that one might expect to be language features are library
22 features in Rust, so what you're looking for may be there, not here.
24 You may also be interested in the [grammar].
26 [book]: book/index.html
27 [standard]: std/index.html
28 [grammar]: grammar.html
32 Rust's grammar is defined over Unicode code points, each conventionally denoted
33 `U+XXXX`, for 4 or more hexadecimal digits `X`. _Most_ of Rust's grammar is
34 confined to the ASCII range of Unicode, and is described in this document by a
35 dialect of Extended Backus-Naur Form (EBNF), specifically a dialect of EBNF
36 supported by common automated LL(k) parsing tools such as `llgen`, rather than
37 the dialect given in ISO 14977. The dialect can be defined self-referentially
42 rule : nonterminal ':' productionrule ';' ;
43 productionrule : production [ '|' production ] * ;
45 term : element repeats ;
46 element : LITERAL | IDENTIFIER | '[' productionrule ']' ;
47 repeats : [ '*' | '+' ] NUMBER ? | NUMBER ? | '?' ;
52 - Whitespace in the grammar is ignored.
53 - Square brackets are used to group rules.
54 - `LITERAL` is a single printable ASCII character, or an escaped hexadecimal
55 ASCII code of the form `\xQQ`, in single quotes, denoting the corresponding
56 Unicode code point `U+00QQ`.
57 - `IDENTIFIER` is a nonempty string of ASCII letters and underscores.
58 - The `repeat` forms apply to the adjacent `element`, and are as follows:
59 - `?` means zero or one repetition
60 - `*` means zero or more repetitions
61 - `+` means one or more repetitions
62 - NUMBER trailing a repeat symbol gives a maximum repetition count
63 - NUMBER on its own gives an exact repetition count
65 This EBNF dialect should hopefully be familiar to many readers.
67 ## Unicode productions
69 A few productions in Rust's grammar permit Unicode code points outside the ASCII
70 range. We define these productions in terms of character properties specified
71 in the Unicode standard, rather than in terms of ASCII-range code points. The
72 section [Special Unicode Productions](#special-unicode-productions) lists these
75 ## String table productions
77 Some rules in the grammar — notably [unary
78 operators](#unary-operator-expressions), [binary
79 operators](#binary-operator-expressions), and [keywords](#keywords) — are
80 given in a simplified form: as a listing of a table of unquoted, printable
81 whitespace-separated strings. These cases form a subset of the rules regarding
82 the [token](#tokens) rule, and are assumed to be the result of a
83 lexical-analysis phase feeding the parser, driven by a DFA, operating over the
84 disjunction of all such string table entries.
86 When such a string enclosed in double-quotes (`"`) occurs inside the grammar,
87 it is an implicit reference to a single member of such a string table
88 production. See [tokens](#tokens) for more information.
94 Rust input is interpreted as a sequence of Unicode code points encoded in UTF-8.
95 Most Rust grammar rules are defined in terms of printable ASCII-range
96 code points, but a small number are defined in terms of Unicode properties or
97 explicit code point lists. [^inputformat]
99 [^inputformat]: Substitute definitions for the special Unicode productions are
100 provided to the grammar verifier, restricted to ASCII range, when verifying the
101 grammar in this document.
103 ## Special Unicode Productions
105 The following productions in the Rust grammar are defined in terms of Unicode
106 properties: `ident`, `non_null`, `non_star`, `non_eol`, `non_slash_or_star`,
107 `non_single_quote` and `non_double_quote`.
111 The `ident` production is any nonempty Unicode string of the following form:
113 - The first character has property `XID_start`
114 - The remaining characters have property `XID_continue`
116 that does _not_ occur in the set of [keywords](#keywords).
118 > **Note**: `XID_start` and `XID_continue` as character properties cover the
119 > character ranges used to form the more familiar C and Java language-family
122 ### Delimiter-restricted productions
124 Some productions are defined by exclusion of particular Unicode characters:
126 - `non_null` is any single Unicode character aside from `U+0000` (null)
127 - `non_eol` is `non_null` restricted to exclude `U+000A` (`'\n'`)
128 - `non_star` is `non_null` restricted to exclude `U+002A` (`*`)
129 - `non_slash_or_star` is `non_null` restricted to exclude `U+002F` (`/`) and `U+002A` (`*`)
130 - `non_single_quote` is `non_null` restricted to exclude `U+0027` (`'`)
131 - `non_double_quote` is `non_null` restricted to exclude `U+0022` (`"`)
136 comment : block_comment | line_comment ;
137 block_comment : "/*" block_comment_body * "*/" ;
138 block_comment_body : [block_comment | character] * ;
139 line_comment : "//" non_eol * ;
142 Comments in Rust code follow the general C++ style of line and block-comment
143 forms. Nested block comments are supported.
145 Line comments beginning with exactly _three_ slashes (`///`), and block
146 comments beginning with exactly one repeated asterisk in the block-open
147 sequence (`/**`), are interpreted as a special syntax for `doc`
148 [attributes](#attributes). That is, they are equivalent to writing
149 `#[doc="..."]` around the body of the comment (this includes the comment
150 characters themselves, i.e. `/// Foo` turns into `#[doc="/// Foo"]`).
152 Line comments beginning with `//!` and block comments beginning with `/*!` are
153 doc comments that apply to the parent of the comment, rather than the item
154 that follows. That is, they are equivalent to writing `#![doc="..."]` around
155 the body of the comment. `//!` comments are usually used to display
156 information on the crate index page.
158 Non-doc comments are interpreted as a form of whitespace.
163 whitespace_char : '\x20' | '\x09' | '\x0a' | '\x0d' ;
164 whitespace : [ whitespace_char | comment ] + ;
167 The `whitespace_char` production is any nonempty Unicode string consisting of
168 any of the following Unicode characters: `U+0020` (space, `' '`), `U+0009`
169 (tab, `'\t'`), `U+000A` (LF, `'\n'`), `U+000D` (CR, `'\r'`).
171 Rust is a "free-form" language, meaning that all forms of whitespace serve only
172 to separate _tokens_ in the grammar, and have no semantic significance.
174 A Rust program has identical meaning if each whitespace element is replaced
175 with any other legal whitespace element, such as a single space character.
180 simple_token : keyword | unop | binop ;
181 token : simple_token | ident | literal | symbol | whitespace token ;
184 Tokens are primitive productions in the grammar defined by regular
185 (non-recursive) languages. "Simple" tokens are given in [string table
186 production](#string-table-productions) form, and occur in the rest of the
187 grammar as double-quoted strings. Other tokens have exact rules given.
191 <p id="keyword-table-marker"></p>
194 |----------|----------|----------|----------|---------|
195 | abstract | alignof | as | become | box |
196 | break | const | continue | crate | do |
197 | else | enum | extern | false | final |
198 | fn | for | if | impl | in |
199 | let | loop | macro | match | mod |
200 | move | mut | offsetof | override | priv |
201 | proc | pub | pure | ref | return |
202 | Self | self | sizeof | static | struct |
203 | super | trait | true | type | typeof |
204 | unsafe | unsized | use | virtual | where |
205 | while | yield | | | |
208 Each of these keywords has special meaning in its grammar, and all of them are
209 excluded from the `ident` rule.
211 Note that some of these keywords are reserved, and do not currently do
216 A literal is an expression consisting of a single token, rather than a sequence
217 of tokens, that immediately and directly denotes the value it evaluates to,
218 rather than referring to it by name or some other evaluation rule. A literal is
219 a form of constant expression, so is evaluated (primarily) at compile time.
223 literal : [ string_lit | char_lit | byte_string_lit | byte_lit | num_lit ] lit_suffix ?;
226 The optional suffix is only used for certain numeric literals, but is
227 reserved for future extension, that is, the above gives the lexical
228 grammar, but a Rust parser will reject everything but the 12 special
229 cases mentioned in [Number literals](#number-literals) below.
233 ##### Characters and strings
235 | | Example | `#` sets | Characters | Escapes |
236 |----------------------------------------------|-----------------|------------|-------------|---------------------|
237 | [Character](#character-literals) | `'H'` | `N/A` | All Unicode | `\'` & [Byte](#byte-escapes) & [Unicode](#unicode-escapes) |
238 | [String](#string-literals) | `"hello"` | `N/A` | All Unicode | `\"` & [Byte](#byte-escapes) & [Unicode](#unicode-escapes) |
239 | [Raw](#raw-string-literals) | `r#"hello"#` | `0...` | All Unicode | `N/A` |
240 | [Byte](#byte-literals) | `b'H'` | `N/A` | All ASCII | `\'` & [Byte](#byte-escapes) |
241 | [Byte string](#byte-string-literals) | `b"hello"` | `N/A` | All ASCII | `\"` & [Byte](#byte-escapes) |
242 | [Raw byte string](#raw-byte-string-literals) | `br#"hello"#` | `0...` | All ASCII | `N/A` |
248 | `\x7F` | 8-bit character code (exactly 2 digits) |
250 | `\r` | Carriage return |
254 ##### Unicode escapes
257 | `\u{7FFF}` | 24-bit Unicode character code (up to 6 digits) |
261 | [Number literals](#number-literals)`*` | Example | Exponentiation | Suffixes |
262 |----------------------------------------|---------|----------------|----------|
263 | Decimal integer | `98_222` | `N/A` | Integer suffixes |
264 | Hex integer | `0xff` | `N/A` | Integer suffixes |
265 | Octal integer | `0o77` | `N/A` | Integer suffixes |
266 | Binary integer | `0b1111_0000` | `N/A` | Integer suffixes |
267 | Floating-point | `123.0E+77` | `Optional` | Floating-point suffixes |
269 `*` All number literals allow `_` as a visual separator: `1_234.0E+18f64`
272 | Integer | Floating-point |
273 |---------|----------------|
274 | `u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`, `i64`, `isize`, `usize` | `f32`, `f64` |
276 #### Character and string literals
279 char_lit : '\x27' char_body '\x27' ;
280 string_lit : '"' string_body * '"' | 'r' raw_string ;
282 char_body : non_single_quote
283 | '\x5c' [ '\x27' | common_escape | unicode_escape ] ;
285 string_body : non_double_quote
286 | '\x5c' [ '\x22' | common_escape | unicode_escape ] ;
287 raw_string : '"' raw_string_body '"' | '#' raw_string '#' ;
289 common_escape : '\x5c'
290 | 'n' | 'r' | 't' | '0'
293 unicode_escape : 'u' '{' hex_digit+ 6 '}';
295 hex_digit : 'a' | 'b' | 'c' | 'd' | 'e' | 'f'
296 | 'A' | 'B' | 'C' | 'D' | 'E' | 'F'
298 oct_digit : '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' ;
299 dec_digit : '0' | nonzero_dec ;
300 nonzero_dec: '1' | '2' | '3' | '4'
301 | '5' | '6' | '7' | '8' | '9' ;
304 ##### Character literals
306 A _character literal_ is a single Unicode character enclosed within two
307 `U+0027` (single-quote) characters, with the exception of `U+0027` itself,
308 which must be _escaped_ by a preceding `U+005C` character (`\`).
310 ##### String literals
312 A _string literal_ is a sequence of any Unicode characters enclosed within two
313 `U+0022` (double-quote) characters, with the exception of `U+0022` itself,
314 which must be _escaped_ by a preceding `U+005C` character (`\`), or a _raw
317 A multi-line string literal may be defined by terminating each line with a
318 `U+005C` character (`\`) immediately before the newline. This causes the
319 `U+005C` character, the newline, and all whitespace at the beginning of the
320 next line to be ignored.
330 ##### Character escapes
332 Some additional _escapes_ are available in either character or non-raw string
333 literals. An escape starts with a `U+005C` (`\`) and continues with one of the
336 * An _8-bit code point escape_ starts with `U+0078` (`x`) and is
337 followed by exactly two _hex digits_. It denotes the Unicode code point
338 equal to the provided hex value.
339 * A _24-bit code point escape_ starts with `U+0075` (`u`) and is followed
340 by up to six _hex digits_ surrounded by braces `U+007B` (`{`) and `U+007D`
341 (`}`). It denotes the Unicode code point equal to the provided hex value.
342 * A _whitespace escape_ is one of the characters `U+006E` (`n`), `U+0072`
343 (`r`), or `U+0074` (`t`), denoting the Unicode values `U+000A` (LF),
344 `U+000D` (CR) or `U+0009` (HT) respectively.
345 * The _backslash escape_ is the character `U+005C` (`\`) which must be
346 escaped in order to denote *itself*.
348 ##### Raw string literals
350 Raw string literals do not process any escapes. They start with the character
351 `U+0072` (`r`), followed by zero or more of the character `U+0023` (`#`) and a
352 `U+0022` (double-quote) character. The _raw string body_ is not defined in the
353 EBNF grammar above: it can contain any sequence of Unicode characters and is
354 terminated only by another `U+0022` (double-quote) character, followed by the
355 same number of `U+0023` (`#`) characters that preceded the opening `U+0022`
356 (double-quote) character.
358 All Unicode characters contained in the raw string body represent themselves,
359 the characters `U+0022` (double-quote) (except when followed by at least as
360 many `U+0023` (`#`) characters as were used to start the raw string literal) or
361 `U+005C` (`\`) do not have any special meaning.
363 Examples for string literals:
366 "foo"; r"foo"; // foo
367 "\"foo\""; r#""foo""#; // "foo"
370 r##"foo #"# bar"##; // foo #"# bar
372 "\x52"; "R"; r"R"; // R
373 "\\x52"; r"\x52"; // \x52
376 #### Byte and byte string literals
379 byte_lit : "b\x27" byte_body '\x27' ;
380 byte_string_lit : "b\x22" string_body * '\x22' | "br" raw_byte_string ;
382 byte_body : ascii_non_single_quote
383 | '\x5c' [ '\x27' | common_escape ] ;
385 byte_string_body : ascii_non_double_quote
386 | '\x5c' [ '\x22' | common_escape ] ;
387 raw_byte_string : '"' raw_byte_string_body '"' | '#' raw_byte_string '#' ;
393 A _byte literal_ is a single ASCII character (in the `U+0000` to `U+007F`
394 range) enclosed within two `U+0027` (single-quote) characters, with the
395 exception of `U+0027` itself, which must be _escaped_ by a preceding U+005C
396 character (`\`), or a single _escape_. It is equivalent to a `u8` unsigned
397 8-bit integer _number literal_.
399 ##### Byte string literals
401 A non-raw _byte string literal_ is a sequence of ASCII characters and _escapes_,
402 preceded by the characters `U+0062` (`b`) and `U+0022` (double-quote), and
403 followed by the character `U+0022`. If the character `U+0022` is present within
404 the literal, it must be _escaped_ by a preceding `U+005C` (`\`) character.
405 Alternatively, a byte string literal can be a _raw byte string literal_, defined
406 below. A byte string literal is equivalent to a `&'static [u8]` borrowed array
407 of unsigned 8-bit integers.
409 Some additional _escapes_ are available in either byte or non-raw byte string
410 literals. An escape starts with a `U+005C` (`\`) and continues with one of the
413 * A _byte escape_ escape starts with `U+0078` (`x`) and is
414 followed by exactly two _hex digits_. It denotes the byte
415 equal to the provided hex value.
416 * A _whitespace escape_ is one of the characters `U+006E` (`n`), `U+0072`
417 (`r`), or `U+0074` (`t`), denoting the bytes values `0x0A` (ASCII LF),
418 `0x0D` (ASCII CR) or `0x09` (ASCII HT) respectively.
419 * The _backslash escape_ is the character `U+005C` (`\`) which must be
420 escaped in order to denote its ASCII encoding `0x5C`.
422 ##### Raw byte string literals
424 Raw byte string literals do not process any escapes. They start with the
425 character `U+0062` (`b`), followed by `U+0072` (`r`), followed by zero or more
426 of the character `U+0023` (`#`), and a `U+0022` (double-quote) character. The
427 _raw string body_ is not defined in the EBNF grammar above: it can contain any
428 sequence of ASCII characters and is terminated only by another `U+0022`
429 (double-quote) character, followed by the same number of `U+0023` (`#`)
430 characters that preceded the opening `U+0022` (double-quote) character. A raw
431 byte string literal can not contain any non-ASCII byte.
433 All characters contained in the raw string body represent their ASCII encoding,
434 the characters `U+0022` (double-quote) (except when followed by at least as
435 many `U+0023` (`#`) characters as were used to start the raw string literal) or
436 `U+005C` (`\`) do not have any special meaning.
438 Examples for byte string literals:
441 b"foo"; br"foo"; // foo
442 b"\"foo\""; br#""foo""#; // "foo"
445 br##"foo #"# bar"##; // foo #"# bar
447 b"\x52"; b"R"; br"R"; // R
448 b"\\x52"; br"\x52"; // \x52
454 num_lit : nonzero_dec [ dec_digit | '_' ] * float_suffix ?
455 | '0' [ [ dec_digit | '_' ] * float_suffix ?
456 | 'b' [ '1' | '0' | '_' ] +
457 | 'o' [ oct_digit | '_' ] +
458 | 'x' [ hex_digit | '_' ] + ] ;
460 float_suffix : [ exponent | '.' dec_lit exponent ? ] ? ;
462 exponent : ['E' | 'e'] ['-' | '+' ] ? dec_lit ;
463 dec_lit : [ dec_digit | '_' ] + ;
466 A _number literal_ is either an _integer literal_ or a _floating-point
467 literal_. The grammar for recognizing the two kinds of literals is mixed.
469 ##### Integer literals
471 An _integer literal_ has one of four forms:
473 * A _decimal literal_ starts with a *decimal digit* and continues with any
474 mixture of *decimal digits* and _underscores_.
475 * A _hex literal_ starts with the character sequence `U+0030` `U+0078`
476 (`0x`) and continues as any mixture of hex digits and underscores.
477 * An _octal literal_ starts with the character sequence `U+0030` `U+006F`
478 (`0o`) and continues as any mixture of octal digits and underscores.
479 * A _binary literal_ starts with the character sequence `U+0030` `U+0062`
480 (`0b`) and continues as any mixture of binary digits and underscores.
482 Like any literal, an integer literal may be followed (immediately,
483 without any spaces) by an _integer suffix_, which forcibly sets the
484 type of the literal. The integer suffix must be the name of one of the
485 integral types: `u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`, `i64`,
488 The type of an _unsuffixed_ integer literal is determined by type inference.
489 If an integer type can be _uniquely_ determined from the surrounding program
490 context, the unsuffixed integer literal has that type. If the program context
491 underconstrains the type, it defaults to the signed 32-bit integer `i32`; if
492 the program context overconstrains the type, it is considered a static type
495 Examples of integer literals of various forms:
502 0o70_i16; // type i16
503 0b1111_1111_1001_0000_i32; // type i32
504 0usize; // type usize
507 ##### Floating-point literals
509 A _floating-point literal_ has one of two forms:
511 * A _decimal literal_ followed by a period character `U+002E` (`.`). This is
512 optionally followed by another decimal literal, with an optional _exponent_.
513 * A single _decimal literal_ followed by an _exponent_.
515 By default, a floating-point literal has a generic type, and, like integer
516 literals, the type must be uniquely determined from the context. There are two valid
517 _floating-point suffixes_, `f32` and `f64` (the 32-bit and 64-bit floating point
518 types), which explicitly determine the type of the literal.
520 Examples of floating-point literals of various forms:
523 123.0f64; // type f64
526 12E+99_f64; // type f64
527 let x: f64 = 2.; // type f64
530 This last example is different because it is not possible to use the suffix
531 syntax with a floating point literal ending in a period. `2.f64` would attempt
532 to call a method named `f64` on `2`.
534 The representation semantics of floating-point numbers are described in
535 ["Machine Types"](#machine-types).
537 #### Boolean literals
539 The two values of the boolean type are written `true` and `false`.
545 | '#' | '[' | ']' | '(' | ')' | '{' | '}'
549 Symbols are a general class of printable [token](#tokens) that play structural
550 roles in a variety of grammar productions. They are catalogued here for
551 completeness as the set of remaining miscellaneous printable tokens that do not
552 otherwise appear as [unary operators](#unary-operator-expressions), [binary
553 operators](#binary-operator-expressions), or [keywords](#keywords).
559 expr_path : [ "::" ] ident [ "::" expr_path_tail ] + ;
560 expr_path_tail : '<' type_expr [ ',' type_expr ] + '>'
563 type_path : ident [ type_path_tail ] + ;
564 type_path_tail : '<' type_expr [ ',' type_expr ] + '>'
568 A _path_ is a sequence of one or more path components _logically_ separated by
569 a namespace qualifier (`::`). If a path consists of only one component, it may
570 refer to either an [item](#items) or a [variable](#variables) in a local control
571 scope. If a path has multiple components, it refers to an item.
573 Every item has a _canonical path_ within its crate, but the path naming an item
574 is only meaningful within a given crate. There is no global namespace across
575 crates; an item's canonical path merely identifies it within the crate.
577 Two examples of simple paths consisting of only identifier components:
584 Path components are usually [identifiers](#identifiers), but the trailing
585 component of a path may be an angle-bracket-enclosed list of type arguments. In
586 [expression](#expressions) context, the type argument list is given after a
587 final (`::`) namespace qualifier in order to disambiguate it from a relational
588 expression involving the less-than symbol (`<`). In type expression context,
589 the final namespace qualifier is omitted.
591 Two examples of paths with type arguments:
594 # struct HashMap<K, V>(K,V);
596 # fn id<T>(t: T) -> T { t }
597 type T = HashMap<i32,String>; // Type arguments used in a type expression
598 let x = id::<i32>(10); // Type arguments used in a call expression
602 Paths can be denoted with various leading qualifiers to change the meaning of
605 * Paths starting with `::` are considered to be global paths where the
606 components of the path start being resolved from the crate root. Each
607 identifier in the path must resolve to an item.
615 ::a::foo(); // call a's foo function
621 * Paths starting with the keyword `super` begin resolution relative to the
622 parent module. Each further identifier must resolve to an item.
630 super::a::foo(); // call a's foo function
636 * Paths starting with the keyword `self` begin resolution relative to the
637 current module. Each further identifier must resolve to an item.
649 A number of minor features of Rust are not central enough to have their own
650 syntax, and yet are not implementable as functions. Instead, they are given
651 names, and invoked through a consistent syntax: `some_extension!(...)`.
653 Users of `rustc` can define new syntax extensions in two ways:
655 * [Compiler plugins][plugin] can include arbitrary
656 Rust code that manipulates syntax trees at compile time.
658 * [Macros](book/macros.html) define new syntax in a higher-level,
664 expr_macro_rules : "macro_rules" '!' ident '(' macro_rule * ')' ;
665 macro_rule : '(' matcher * ')' "=>" '(' transcriber * ')' ';' ;
666 matcher : '(' matcher * ')' | '[' matcher * ']'
667 | '{' matcher * '}' | '$' ident ':' ident
668 | '$' '(' matcher * ')' sep_token? [ '*' | '+' ]
669 | non_special_token ;
670 transcriber : '(' transcriber * ')' | '[' transcriber * ']'
671 | '{' transcriber * '}' | '$' ident
672 | '$' '(' transcriber * ')' sep_token? [ '*' | '+' ]
673 | non_special_token ;
676 `macro_rules` allows users to define syntax extension in a declarative way. We
677 call such extensions "macros by example" or simply "macros" — to be distinguished
678 from the "procedural macros" defined in [compiler plugins][plugin].
680 Currently, macros can expand to expressions, statements, items, or patterns.
682 (A `sep_token` is any token other than `*` and `+`. A `non_special_token` is
683 any token other than a delimiter or `$`.)
685 The macro expander looks up macro invocations by name, and tries each macro
686 rule in turn. It transcribes the first successful match. Matching and
687 transcription are closely related to each other, and we will describe them
692 The macro expander matches and transcribes every token that does not begin with
693 a `$` literally, including delimiters. For parsing reasons, delimiters must be
694 balanced, but they are otherwise not special.
696 In the matcher, `$` _name_ `:` _designator_ matches the nonterminal in the Rust
697 syntax named by _designator_. Valid designators are `item`, `block`, `stmt`,
698 `pat`, `expr`, `ty` (type), `ident`, `path`, `tt` (either side of the `=>`
699 in macro rules). In the transcriber, the designator is already known, and so
700 only the name of a matched nonterminal comes after the dollar sign.
702 In both the matcher and transcriber, the Kleene star-like operator indicates
703 repetition. The Kleene star operator consists of `$` and parentheses, optionally
704 followed by a separator token, followed by `*` or `+`. `*` means zero or more
705 repetitions, `+` means at least one repetition. The parentheses are not matched or
706 transcribed. On the matcher side, a name is bound to _all_ of the names it
707 matches, in a structure that mimics the structure of the repetition encountered
708 on a successful match. The job of the transcriber is to sort that structure
711 The rules for transcription of these repetitions are called "Macro By Example".
712 Essentially, one "layer" of repetition is discharged at a time, and all of them
713 must be discharged by the time a name is transcribed. Therefore, `( $( $i:ident
714 ),* ) => ( $i )` is an invalid macro, but `( $( $i:ident ),* ) => ( $( $i:ident
715 ),* )` is acceptable (if trivial).
717 When Macro By Example encounters a repetition, it examines all of the `$`
718 _name_ s that occur in its body. At the "current layer", they all must repeat
719 the same number of times, so ` ( $( $i:ident ),* ; $( $j:ident ),* ) => ( $(
720 ($i,$j) ),* )` is valid if given the argument `(a,b,c ; d,e,f)`, but not
721 `(a,b,c ; d,e)`. The repetition walks through the choices at that layer in
722 lockstep, so the former input transcribes to `( (a,d), (b,e), (c,f) )`.
724 Nested repetitions are allowed.
726 ### Parsing limitations
728 The parser used by the macro system is reasonably powerful, but the parsing of
729 Rust syntax is restricted in two ways:
731 1. The parser will always parse as much as possible. If it attempts to match
732 `$i:expr [ , ]` against `8 [ , ]`, it will attempt to parse `i` as an array
733 index operation and fail. Adding a separator can solve this problem.
734 2. The parser must have eliminated all ambiguity by the time it reaches a `$`
735 _name_ `:` _designator_. This requirement most often affects name-designator
736 pairs when they occur at the beginning of, or immediately after, a `$(...)*`;
737 requiring a distinctive token in front can solve the problem.
739 # Crates and source files
741 Although Rust, like any other language, can be implemented by an interpreter as
742 well as a compiler, the only existing implementation is a compiler —
743 from now on referred to as *the* Rust compiler — and the language has
744 always been designed to be compiled. For these reasons, this section assumes a
747 Rust's semantics obey a *phase distinction* between compile-time and
748 run-time.[^phase-distinction] Those semantic rules that have a *static
749 interpretation* govern the success or failure of compilation. Those semantics
750 that have a *dynamic interpretation* govern the behavior of the program at
753 [^phase-distinction]: This distinction would also exist in an interpreter.
754 Static checks like syntactic analysis, type checking, and lints should
755 happen before the program is executed regardless of when it is executed.
757 The compilation model centers on artifacts called _crates_. Each compilation
758 processes a single crate in source form, and if successful, produces a single
759 crate in binary form: either an executable or some sort of
760 library.[^cratesourcefile]
762 [^cratesourcefile]: A crate is somewhat analogous to an *assembly* in the
763 ECMA-335 CLI model, a *library* in the SML/NJ Compilation Manager, a *unit*
764 in the Owens and Flatt module system, or a *configuration* in Mesa.
766 A _crate_ is a unit of compilation and linking, as well as versioning,
767 distribution and runtime loading. A crate contains a _tree_ of nested
768 [module](#modules) scopes. The top level of this tree is a module that is
769 anonymous (from the point of view of paths within the module) and any item
770 within a crate has a canonical [module path](#paths) denoting its location
771 within the crate's module tree.
773 The Rust compiler is always invoked with a single source file as input, and
774 always produces a single output crate. The processing of that source file may
775 result in other source files being loaded as modules. Source files have the
778 A Rust source file describes a module, the name and location of which —
779 in the module tree of the current crate — are defined from outside the
780 source file: either by an explicit `mod_item` in a referencing source file, or
781 by the name of the crate itself. Every source file is a module, but not every
782 module needs its own source file: [module definitions](#modules) can be nested
785 Each source file contains a sequence of zero or more `item` definitions, and
786 may optionally begin with any number of [attributes](#Items and attributes)
787 that apply to the containing module, most of which influence the behavior of
788 the compiler. The anonymous crate module can have additional attributes that
789 apply to the crate as a whole.
792 // Specify the crate name.
793 #![crate_name = "projx"]
795 // Specify the type of output artifact.
796 #![crate_type = "lib"]
798 // Turn on a warning.
799 // This can be done in any module, not just the anonymous crate module.
800 #![warn(non_camel_case_types)]
803 A crate that contains a `main` function can be compiled to an executable. If a
804 `main` function is present, its return type must be [`unit`](#primitive-types)
805 and it must take no arguments.
807 # Items and attributes
809 Crates contain [items](#items), each of which may have some number of
810 [attributes](#attributes) attached to it.
815 item : extern_crate_decl | use_decl | mod_item | fn_item | type_item
816 | struct_item | enum_item | static_item | trait_item | impl_item
820 An _item_ is a component of a crate. Items are organized within a crate by a
821 nested set of [modules](#modules). Every crate has a single "outermost"
822 anonymous module; all further items within the crate have [paths](#paths)
823 within the module tree of the crate.
825 Items are entirely determined at compile-time, generally remain fixed during
826 execution, and may reside in read-only memory.
828 There are several kinds of item:
830 * [`extern crate` declarations](#extern-crate-declarations)
831 * [`use` declarations](#use-declarations)
832 * [modules](#modules)
833 * [functions](#functions)
834 * [type definitions](#type-definitions)
835 * [structures](#structures)
836 * [enumerations](#enumerations)
837 * [static items](#static-items)
839 * [implementations](#implementations)
841 Some items form an implicit scope for the declaration of sub-items. In other
842 words, within a function or module, declarations of items can (in many cases)
843 be mixed with the statements, control blocks, and similar artifacts that
844 otherwise compose the item body. The meaning of these scoped items is the same
845 as if the item was declared outside the scope — it is still a static item
846 — except that the item's *path name* within the module namespace is
847 qualified by the name of the enclosing item, or is private to the enclosing
848 item (in the case of functions). The grammar specifies the exact locations in
849 which sub-item declarations may appear.
853 All items except modules may be *parameterized* by type. Type parameters are
854 given as a comma-separated list of identifiers enclosed in angle brackets
855 (`<...>`), after the name of the item and before its definition. The type
856 parameters of an item are considered "part of the name", not part of the type
857 of the item. A referencing [path](#paths) must (in principle) provide type
858 arguments as a list of comma-separated types enclosed within angle brackets, in
859 order to refer to the type-parameterized item. In practice, the type-inference
860 system can usually infer such argument types from context. There are no
861 general type-parametric types, only type-parametric items. That is, Rust has
862 no notion of type abstraction: there are no first-class "forall" types.
867 mod_item : "mod" ident ( ';' | '{' mod '}' );
871 A module is a container for zero or more [items](#items).
873 A _module item_ is a module, surrounded in braces, named, and prefixed with the
874 keyword `mod`. A module item introduces a new, named module into the tree of
875 modules making up a crate. Modules can nest arbitrarily.
877 An example of a module:
881 type Complex = (f64, f64);
882 fn sin(f: f64) -> f64 {
886 fn cos(f: f64) -> f64 {
890 fn tan(f: f64) -> f64 {
897 Modules and types share the same namespace. Declaring a named type with
898 the same name as a module in scope is forbidden: that is, a type definition,
899 trait, struct, enumeration, or type parameter can't shadow the name of a module
900 in scope, or vice versa.
902 A module without a body is loaded from an external file, by default with the
903 same name as the module, plus the `.rs` extension. When a nested submodule is
904 loaded from an external file, it is loaded from a subdirectory path that
905 mirrors the module hierarchy.
908 // Load the `vec` module from `vec.rs`
912 // Load the `local_data` module from `thread/local_data.rs`
917 The directories and files used for loading external file modules can be
918 influenced with the `path` attribute.
921 #[path = "thread_files"]
923 // Load the `local_data` module from `thread_files/tls.rs`
929 ##### Extern crate declarations
932 extern_crate_decl : "extern" "crate" crate_name
933 crate_name: ident | ( string_lit "as" ident )
936 An _`extern crate` declaration_ specifies a dependency on an external crate.
937 The external crate is then bound into the declaring scope as the `ident`
938 provided in the `extern_crate_decl`.
940 The external crate is resolved to a specific `soname` at compile time, and a
941 runtime linkage requirement to that `soname` is passed to the linker for
942 loading at runtime. The `soname` is resolved at compile time by scanning the
943 compiler's library path and matching the optional `crateid` provided as a
944 string literal against the `crateid` attributes that were declared on the
945 external crate when it was compiled. If no `crateid` is provided, a default
946 `name` attribute is assumed, equal to the `ident` given in the
949 Three examples of `extern crate` declarations:
954 extern crate std; // equivalent to: extern crate std as std;
956 extern crate std as ruststd; // linking to 'std' under another name
959 ##### Use declarations
962 use_decl : "pub" ? "use" [ path "as" ident
965 path_glob : ident [ "::" [ path_glob
967 | '{' path_item [ ',' path_item ] * '}' ;
969 path_item : ident | "self" ;
972 A _use declaration_ creates one or more local name bindings synonymous with
973 some other [path](#paths). Usually a `use` declaration is used to shorten the
974 path required to refer to a module item. These declarations may appear at the
975 top of [modules](#modules) and [blocks](#blocks).
977 > **Note**: Unlike in many languages,
978 > `use` declarations in Rust do *not* declare linkage dependency with external crates.
979 > Rather, [`extern crate` declarations](#extern-crate-declarations) declare linkage dependencies.
981 Use declarations support a number of convenient shortcuts:
983 * Rebinding the target name as a new local name, using the syntax `use p::q::r as x;`
984 * Simultaneously binding a list of paths differing only in their final element,
985 using the glob-like brace syntax `use a::b::{c,d,e,f};`
986 * Binding all paths matching a given prefix, using the asterisk wildcard syntax
988 * Simultaneously binding a list of paths differing only in their final element
989 and their immediate parent module, using the `self` keyword, such as
990 `use a::b::{self, c, d};`
992 An example of `use` declarations:
995 use std::option::Option::{Some, None};
996 use std::collections::hash_map::{self, HashMap};
999 fn bar(map1: HashMap<String, usize>, map2: hash_map::HashMap<String, usize>){}
1002 // Equivalent to 'foo(vec![std::option::Option::Some(1.0f64),
1003 // std::option::Option::None]);'
1004 foo(vec![Some(1.0f64), None]);
1006 // Both `hash_map` and `HashMap` are in scope.
1007 let map1 = HashMap::new();
1008 let map2 = hash_map::HashMap::new();
1013 Like items, `use` declarations are private to the containing module, by
1014 default. Also like items, a `use` declaration can be public, if qualified by
1015 the `pub` keyword. Such a `use` declaration serves to _re-export_ a name. A
1016 public `use` declaration can therefore _redirect_ some public name to a
1017 different target definition: even a definition with a private canonical path,
1018 inside a different module. If a sequence of such redirections form a cycle or
1019 cannot be resolved unambiguously, they represent a compile-time error.
1021 An example of re-exporting:
1026 pub use quux::foo::{bar, baz};
1035 In this example, the module `quux` re-exports two public names defined in
1038 Also note that the paths contained in `use` items are relative to the crate
1039 root. So, in the previous example, the `use` refers to `quux::foo::{bar,
1040 baz}`, and not simply to `foo::{bar, baz}`. This also means that top-level
1041 module declarations should be at the crate root if direct usage of the declared
1042 modules within `use` items is desired. It is also possible to use `self` and
1043 `super` at the beginning of a `use` item to refer to the current and direct
1044 parent modules respectively. All rules regarding accessing declared modules in
1045 `use` declarations applies to both module declarations and `extern crate`
1048 An example of what will and will not work for `use` items:
1051 # #![allow(unused_imports)]
1052 use foo::baz::foobaz; // good: foo is at the root of the crate
1060 use foo::example::iter; // good: foo is at crate root
1061 // use example::iter; // bad: core is not at the crate root
1062 use self::baz::foobaz; // good: self refers to module 'foo'
1063 use foo::bar::foobar; // good: foo is at crate root
1070 use super::bar::foobar; // good: super refers to module 'foo'
1080 A _function item_ defines a sequence of [statements](#statements) and an
1081 optional final [expression](#expressions), along with a name and a set of
1082 parameters. Functions are declared with the keyword `fn`. Functions declare a
1083 set of *input* [*variables*](#variables) as parameters, through which the caller
1084 passes arguments into the function, and the *output* [*type*](#types)
1085 of the value the function will return to its caller on completion.
1087 A function may also be copied into a first-class *value*, in which case the
1088 value has the corresponding [*function type*](#function-types), and can be used
1089 otherwise exactly as a function item (with a minor additional cost of calling
1090 the function indirectly).
1092 Every control path in a function logically ends with a `return` expression or a
1093 diverging expression. If the outermost block of a function has a
1094 value-producing expression in its final-expression position, that expression is
1095 interpreted as an implicit `return` expression applied to the final-expression.
1097 An example of a function:
1100 fn add(x: i32, y: i32) -> i32 {
1105 As with `let` bindings, function arguments are irrefutable patterns, so any
1106 pattern that is valid in a let binding is also valid as an argument.
1109 fn first((value, _): (i32, i32)) -> i32 { value }
1113 #### Generic functions
1115 A _generic function_ allows one or more _parameterized types_ to appear in its
1116 signature. Each type parameter must be explicitly declared, in an
1117 angle-bracket-enclosed, comma-separated list following the function name.
1120 fn iter<T, F>(seq: &[T], f: F) where T: Copy, F: Fn(T) {
1121 for elt in seq { f(*elt); }
1123 fn map<T, U, F>(seq: &[T], f: F) -> Vec<U> where T: Copy, U: Copy, F: Fn(T) -> U {
1124 let mut acc = vec![];
1125 for elt in seq { acc.push(f(*elt)); }
1130 Inside the function signature and body, the name of the type parameter can be
1131 used as a type name. [Trait](#traits) bounds can be specified for type parameters
1132 to allow methods with that trait to be called on values of that type. This is
1133 specified using the `where` syntax, as in the above example.
1135 When a generic function is referenced, its type is instantiated based on the
1136 context of the reference. For example, calling the `iter` function defined
1137 above on `[1, 2]` will instantiate type parameter `T` with `i32`, and require
1138 the closure parameter to have type `Fn(i32)`.
1140 The type parameters can also be explicitly supplied in a trailing
1141 [path](#paths) component after the function name. This might be necessary if
1142 there is not sufficient context to determine the type parameters. For example,
1143 `mem::size_of::<u32>() == 4`.
1147 Unsafe operations are those that potentially violate the memory-safety
1148 guarantees of Rust's static semantics.
1150 The following language level features cannot be used in the safe subset of
1153 - Dereferencing a [raw pointer](#pointer-types).
1154 - Reading or writing a [mutable static variable](#mutable-statics).
1155 - Calling an unsafe function (including an intrinsic or foreign function).
1157 ##### Unsafe functions
1159 Unsafe functions are functions that are not safe in all contexts and/or for all
1160 possible inputs. Such a function must be prefixed with the keyword `unsafe` and
1161 can only be called from an `unsafe` block or another `unsafe` function.
1165 A block of code can be prefixed with the `unsafe` keyword, to permit calling
1166 `unsafe` functions or dereferencing raw pointers within a safe function.
1168 When a programmer has sufficient conviction that a sequence of potentially
1169 unsafe operations is actually safe, they can encapsulate that sequence (taken
1170 as a whole) within an `unsafe` block. The compiler will consider uses of such
1171 code safe, in the surrounding context.
1173 Unsafe blocks are used to wrap foreign libraries, make direct use of hardware
1174 or implement features not directly present in the language. For example, Rust
1175 provides the language features necessary to implement memory-safe concurrency
1176 in the language but the implementation of threads and message passing is in the
1179 Rust's type system is a conservative approximation of the dynamic safety
1180 requirements, so in some cases there is a performance cost to using safe code.
1181 For example, a doubly-linked list is not a tree structure and can only be
1182 represented with reference-counted pointers in safe code. By using `unsafe`
1183 blocks to represent the reverse links as raw pointers, it can be implemented
1186 ##### Behavior considered undefined
1188 The following is a list of behavior which is forbidden in all Rust code,
1189 including within `unsafe` blocks and `unsafe` functions. Type checking provides
1190 the guarantee that these issues are never caused by safe code.
1193 * Dereferencing a null/dangling raw pointer
1194 * Reads of [undef](http://llvm.org/docs/LangRef.html#undefined-values)
1195 (uninitialized) memory
1196 * Breaking the [pointer aliasing
1197 rules](http://llvm.org/docs/LangRef.html#pointer-aliasing-rules)
1198 with raw pointers (a subset of the rules used by C)
1199 * `&mut` and `&` follow LLVM’s scoped [noalias] model, except if the `&T`
1200 contains an `UnsafeCell<U>`. Unsafe code must not violate these aliasing
1202 * Mutating an immutable value/reference without `UnsafeCell<U>`
1203 * Invoking undefined behavior via compiler intrinsics:
1204 * Indexing outside of the bounds of an object with `std::ptr::offset`
1205 (`offset` intrinsic), with
1206 the exception of one byte past the end which is permitted.
1207 * Using `std::ptr::copy_nonoverlapping_memory` (`memcpy32`/`memcpy64`
1208 intrinsics) on overlapping buffers
1209 * Invalid values in primitive types, even in private fields/locals:
1210 * Dangling/null references or boxes
1211 * A value other than `false` (0) or `true` (1) in a `bool`
1212 * A discriminant in an `enum` not included in the type definition
1213 * A value in a `char` which is a surrogate or above `char::MAX`
1214 * Non-UTF-8 byte sequences in a `str`
1215 * Unwinding into Rust from foreign code or unwinding from Rust into foreign
1216 code. Rust's failure system is not compatible with exception handling in
1217 other languages. Unwinding must be caught and handled at FFI boundaries.
1219 [noalias]: http://llvm.org/docs/LangRef.html#noalias
1221 ##### Behavior not considered unsafe
1223 This is a list of behavior not considered *unsafe* in Rust terms, but that may
1227 * Reading data from private fields (`std::repr`)
1228 * Leaks due to reference count cycles, even in the global heap
1229 * Exiting without calling destructors
1231 * Accessing/modifying the file system
1232 * Unsigned integer overflow (well-defined as wrapping)
1233 * Signed integer overflow (well-defined as two’s complement representation
1236 #### Diverging functions
1238 A special kind of function can be declared with a `!` character where the
1239 output type would normally be. For example:
1242 fn my_err(s: &str) -> ! {
1248 We call such functions "diverging" because they never return a value to the
1249 caller. Every control path in a diverging function must end with a `panic!()` or
1250 a call to another diverging function on every control path. The `!` annotation
1251 does *not* denote a type.
1253 It might be necessary to declare a diverging function because as mentioned
1254 previously, the typechecker checks that every control path in a function ends
1255 with a [`return`](#return-expressions) or diverging expression. So, if `my_err`
1256 were declared without the `!` annotation, the following code would not
1260 # fn my_err(s: &str) -> ! { panic!() }
1262 fn f(i: i32) -> i32 {
1267 my_err("Bad number!");
1272 This will not compile without the `!` annotation on `my_err`, since the `else`
1273 branch of the conditional in `f` does not return an `i32`, as required by the
1274 signature of `f`. Adding the `!` annotation to `my_err` informs the
1275 typechecker that, should control ever enter `my_err`, no further type judgments
1276 about `f` need to hold, since control will never resume in any context that
1277 relies on those judgments. Thus the return type on `f` only needs to reflect
1278 the `if` branch of the conditional.
1280 #### Extern functions
1282 Extern functions are part of Rust's foreign function interface, providing the
1283 opposite functionality to [external blocks](#external-blocks). Whereas
1284 external blocks allow Rust code to call foreign code, extern functions with
1285 bodies defined in Rust code _can be called by foreign code_. They are defined
1286 in the same way as any other Rust function, except that they have the `extern`
1290 // Declares an extern fn, the ABI defaults to "C"
1291 extern fn new_i32() -> i32 { 0 }
1293 // Declares an extern fn with "stdcall" ABI
1294 extern "stdcall" fn new_i32_stdcall() -> i32 { 0 }
1297 Unlike normal functions, extern fns have an `extern "ABI" fn()`. This is the
1298 same type as the functions declared in an extern block.
1301 # extern fn new_i32() -> i32 { 0 }
1302 let fptr: extern "C" fn() -> i32 = new_i32;
1305 Extern functions may be called directly from Rust code as Rust uses large,
1306 contiguous stack segments like C.
1310 A _type alias_ defines a new name for an existing [type](#types). Type
1311 aliases are declared with the keyword `type`. Every value has a single,
1312 specific type, but may implement several different traits, or be compatible with
1313 several different type constraints.
1315 For example, the following defines the type `Point` as a synonym for the type
1316 `(u8, u8)`, the type of pairs of unsigned 8 bit integers:
1319 type Point = (u8, u8);
1320 let p: Point = (41, 68);
1325 A _structure_ is a nominal [structure type](#structure-types) defined with the
1328 An example of a `struct` item and its use:
1331 struct Point {x: i32, y: i32}
1332 let p = Point {x: 10, y: 11};
1336 A _tuple structure_ is a nominal [tuple type](#tuple-types), also defined with
1337 the keyword `struct`. For example:
1340 struct Point(i32, i32);
1341 let p = Point(10, 11);
1342 let px: i32 = match p { Point(x, _) => x };
1345 A _unit-like struct_ is a structure without any fields, defined by leaving off
1346 the list of fields entirely. Such types will have a single value, just like
1347 the [unit value `()`](#unit-and-boolean-literals) of the unit type. For
1352 let c = [Cookie, Cookie, Cookie, Cookie];
1355 The precise memory layout of a structure is not specified. One can specify a
1356 particular layout using the [`repr` attribute](#ffi-attributes).
1360 An _enumeration_ is a simultaneous definition of a nominal [enumerated
1361 type](#enumerated-types) as well as a set of *constructors*, that can be used
1362 to create or pattern-match values of the corresponding enumerated type.
1364 Enumerations are declared with the keyword `enum`.
1366 An example of an `enum` item and its use:
1374 let mut a: Animal = Animal::Dog;
1378 Enumeration constructors can have either named or unnamed fields:
1383 Cat { name: String, weight: f64 }
1386 let mut a: Animal = Animal::Dog("Cocoa".to_string(), 37.2);
1387 a = Animal::Cat { name: "Spotty".to_string(), weight: 2.7 };
1390 In this example, `Cat` is a _struct-like enum variant_,
1391 whereas `Dog` is simply called an enum variant.
1393 Enums have a discriminant. You can assign them explicitly:
1401 If a discriminant isn't assigned, they start at zero, and add one for each
1404 You can cast an enum to get this value:
1407 # enum Foo { Bar = 123 }
1408 let x = Foo::Bar as u32; // x is now 123u32
1411 This only works as long as none of the variants have data attached. If
1412 it were `Bar(i32)`, this is disallowed.
1417 const_item : "const" ident ':' type '=' expr ';' ;
1420 A *constant item* is a named _constant value_ which is not associated with a
1421 specific memory location in the program. Constants are essentially inlined
1422 wherever they are used, meaning that they are copied directly into the relevant
1423 context when used. References to the same constant are not necessarily
1424 guaranteed to refer to the same memory address.
1426 Constant values must not have destructors, and otherwise permit most forms of
1427 data. Constants may refer to the address of other constants, in which case the
1428 address will have the `static` lifetime. The compiler is, however, still at
1429 liberty to translate the constant many times, so the address referred to may not
1432 Constants must be explicitly typed. The type may be `bool`, `char`, a number, or
1433 a type derived from those primitive types. The derived types are references with
1434 the `static` lifetime, fixed-size arrays, tuples, enum variants, and structs.
1437 const BIT1: u32 = 1 << 0;
1438 const BIT2: u32 = 1 << 1;
1440 const BITS: [u32; 2] = [BIT1, BIT2];
1441 const STRING: &'static str = "bitstring";
1443 struct BitsNStrings<'a> {
1448 const BITS_N_STRINGS: BitsNStrings<'static> = BitsNStrings {
1457 static_item : "static" ident ':' type '=' expr ';' ;
1460 A *static item* is similar to a *constant*, except that it represents a precise
1461 memory location in the program. A static is never "inlined" at the usage site,
1462 and all references to it refer to the same memory location. Static items have
1463 the `static` lifetime, which outlives all other lifetimes in a Rust program.
1464 Static items may be placed in read-only memory if they do not contain any
1465 interior mutability.
1467 Statics may contain interior mutability through the `UnsafeCell` language item.
1468 All access to a static is safe, but there are a number of restrictions on
1471 * Statics may not contain any destructors.
1472 * The types of static values must ascribe to `Sync` to allow threadsafe access.
1473 * Statics may not refer to other statics by value, only by reference.
1474 * Constants cannot refer to statics.
1476 Constants should in general be preferred over statics, unless large amounts of
1477 data are being stored, or single-address and mutability properties are required.
1479 #### Mutable statics
1481 If a static item is declared with the `mut` keyword, then it is allowed to
1482 be modified by the program. One of Rust's goals is to make concurrency bugs
1483 hard to run into, and this is obviously a very large source of race conditions
1484 or other bugs. For this reason, an `unsafe` block is required when either
1485 reading or writing a mutable static variable. Care should be taken to ensure
1486 that modifications to a mutable static are safe with respect to other threads
1487 running in the same process.
1489 Mutable statics are still very useful, however. They can be used with C
1490 libraries and can also be bound from C libraries (in an `extern` block).
1493 # fn atomic_add(_: &mut u32, _: u32) -> u32 { 2 }
1495 static mut LEVELS: u32 = 0;
1497 // This violates the idea of no shared state, and this doesn't internally
1498 // protect against races, so this function is `unsafe`
1499 unsafe fn bump_levels_unsafe1() -> u32 {
1505 // Assuming that we have an atomic_add function which returns the old value,
1506 // this function is "safe" but the meaning of the return value may not be what
1507 // callers expect, so it's still marked as `unsafe`
1508 unsafe fn bump_levels_unsafe2() -> u32 {
1509 return atomic_add(&mut LEVELS, 1);
1513 Mutable statics have the same restrictions as normal statics, except that the
1514 type of the value is not required to ascribe to `Sync`.
1518 A _trait_ describes a set of method types.
1520 Traits can include default implementations of methods, written in terms of some
1521 unknown [`self` type](#self-types); the `self` type may either be completely
1522 unspecified, or constrained by some other trait.
1524 Traits are implemented for specific types through separate
1525 [implementations](#implementations).
1528 # type Surface = i32;
1529 # type BoundingBox = i32;
1531 fn draw(&self, Surface);
1532 fn bounding_box(&self) -> BoundingBox;
1536 This defines a trait with two methods. All values that have
1537 [implementations](#implementations) of this trait in scope can have their
1538 `draw` and `bounding_box` methods called, using `value.bounding_box()`
1539 [syntax](#method-call-expressions).
1541 Type parameters can be specified for a trait to make it generic. These appear
1542 after the trait name, using the same syntax used in [generic
1543 functions](#generic-functions).
1547 fn len(&self) -> u32;
1548 fn elt_at(&self, n: u32) -> T;
1549 fn iter<F>(&self, F) where F: Fn(T);
1553 Generic functions may use traits as _bounds_ on their type parameters. This
1554 will have two effects: only types that have the trait may instantiate the
1555 parameter, and within the generic function, the methods of the trait can be
1556 called on values that have the parameter's type. For example:
1559 # type Surface = i32;
1560 # trait Shape { fn draw(&self, Surface); }
1561 fn draw_twice<T: Shape>(surface: Surface, sh: T) {
1567 Traits also define an [trait object](#trait-objects) with the same name as the
1568 trait. Values of this type are created by [casting](#type-cast-expressions)
1569 pointer values (pointing to a type for which an implementation of the given
1570 trait is in scope) to pointers to the trait name, used as a type.
1573 # trait Shape { fn dummy(&self) { } }
1574 # impl Shape for i32 { }
1575 # let mycircle = 0i32;
1576 let myshape: Box<Shape> = Box::new(mycircle) as Box<Shape>;
1579 The resulting value is a box containing the value that was cast, along with
1580 information that identifies the methods of the implementation that was used.
1581 Values with a trait type can have [methods called](#method-call-expressions) on
1582 them, for any method in the trait, and can be used to instantiate type
1583 parameters that are bounded by the trait.
1585 Trait methods may be static, which means that they lack a `self` argument.
1586 This means that they can only be called with function call syntax (`f(x)`) and
1587 not method call syntax (`obj.f()`). The way to refer to the name of a static
1588 method is to qualify it with the trait name, treating the trait name like a
1589 module. For example:
1593 fn from_i32(n: i32) -> Self;
1596 fn from_i32(n: i32) -> f64 { n as f64 }
1598 let x: f64 = Num::from_i32(42);
1601 Traits may inherit from other traits. For example, in
1604 trait Shape { fn area(&self) -> f64; }
1605 trait Circle : Shape { fn radius(&self) -> f64; }
1608 the syntax `Circle : Shape` means that types that implement `Circle` must also
1609 have an implementation for `Shape`. Multiple supertraits are separated by `+`,
1610 `trait Circle : Shape + PartialEq { }`. In an implementation of `Circle` for a
1611 given type `T`, methods can refer to `Shape` methods, since the typechecker
1612 checks that any type with an implementation of `Circle` also has an
1613 implementation of `Shape`.
1615 In type-parameterized functions, methods of the supertrait may be called on
1616 values of subtrait-bound type parameters. Referring to the previous example of
1617 `trait Circle : Shape`:
1620 # trait Shape { fn area(&self) -> f64; }
1621 # trait Circle : Shape { fn radius(&self) -> f64; }
1622 fn radius_times_area<T: Circle>(c: T) -> f64 {
1623 // `c` is both a Circle and a Shape
1624 c.radius() * c.area()
1628 Likewise, supertrait methods may also be called on trait objects.
1631 # trait Shape { fn area(&self) -> f64; }
1632 # trait Circle : Shape { fn radius(&self) -> f64; }
1633 # impl Shape for i32 { fn area(&self) -> f64 { 0.0 } }
1634 # impl Circle for i32 { fn radius(&self) -> f64 { 0.0 } }
1635 # let mycircle = 0i32;
1636 let mycircle = Box::new(mycircle) as Box<Circle>;
1637 let nonsense = mycircle.radius() * mycircle.area();
1642 An _implementation_ is an item that implements a [trait](#traits) for a
1645 Implementations are defined with the keyword `impl`.
1648 # #[derive(Copy, Clone)]
1649 # struct Point {x: f64, y: f64};
1650 # type Surface = i32;
1651 # struct BoundingBox {x: f64, y: f64, width: f64, height: f64};
1652 # trait Shape { fn draw(&self, Surface); fn bounding_box(&self) -> BoundingBox; }
1653 # fn do_draw_circle(s: Surface, c: Circle) { }
1659 impl Copy for Circle {}
1661 impl Clone for Circle {
1662 fn clone(&self) -> Circle { *self }
1665 impl Shape for Circle {
1666 fn draw(&self, s: Surface) { do_draw_circle(s, *self); }
1667 fn bounding_box(&self) -> BoundingBox {
1668 let r = self.radius;
1669 BoundingBox{x: self.center.x - r, y: self.center.y - r,
1670 width: 2.0 * r, height: 2.0 * r}
1675 It is possible to define an implementation without referring to a trait. The
1676 methods in such an implementation can only be used as direct calls on the
1677 values of the type that the implementation targets. In such an implementation,
1678 the trait type and `for` after `impl` are omitted. Such implementations are
1679 limited to nominal types (enums, structs), and the implementation must appear
1680 in the same module or a sub-module as the `self` type:
1683 struct Point {x: i32, y: i32}
1687 println!("Point is at ({}, {})", self.x, self.y);
1691 let my_point = Point {x: 10, y:11};
1695 When a trait _is_ specified in an `impl`, all methods declared as part of the
1696 trait must be implemented, with matching types and type parameter counts.
1698 An implementation can take type parameters, which can be different from the
1699 type parameters taken by the trait it implements. Implementation parameters
1700 are written after the `impl` keyword.
1703 # trait Seq<T> { fn dummy(&self, _: T) { } }
1704 impl<T> Seq<T> for Vec<T> {
1707 impl Seq<bool> for u32 {
1708 /* Treat the integer as a sequence of bits */
1715 extern_block_item : "extern" '{' extern_block '}' ;
1716 extern_block : [ foreign_fn ] * ;
1719 External blocks form the basis for Rust's foreign function interface.
1720 Declarations in an external block describe symbols in external, non-Rust
1723 Functions within external blocks are declared in the same way as other Rust
1724 functions, with the exception that they may not have a body and are instead
1725 terminated by a semicolon.
1727 Functions within external blocks may be called by Rust code, just like
1728 functions defined in Rust. The Rust compiler automatically translates between
1729 the Rust ABI and the foreign ABI.
1731 A number of [attributes](#attributes) control the behavior of external blocks.
1733 By default external blocks assume that the library they are calling uses the
1734 standard C "cdecl" ABI. Other ABIs may be specified using an `abi` string, as
1738 // Interface to the Windows API
1739 extern "stdcall" { }
1742 The `link` attribute allows the name of the library to be specified. When
1743 specified the compiler will attempt to link against the native library of the
1747 #[link(name = "crypto")]
1751 The type of a function declared in an extern block is `extern "abi" fn(A1, ...,
1752 An) -> R`, where `A1...An` are the declared types of its arguments and `R` is
1753 the declared return type.
1755 ## Visibility and Privacy
1757 These two terms are often used interchangeably, and what they are attempting to
1758 convey is the answer to the question "Can this item be used at this location?"
1760 Rust's name resolution operates on a global hierarchy of namespaces. Each level
1761 in the hierarchy can be thought of as some item. The items are one of those
1762 mentioned above, but also include external crates. Declaring or defining a new
1763 module can be thought of as inserting a new tree into the hierarchy at the
1764 location of the definition.
1766 To control whether interfaces can be used across modules, Rust checks each use
1767 of an item to see whether it should be allowed or not. This is where privacy
1768 warnings are generated, or otherwise "you used a private item of another module
1769 and weren't allowed to."
1771 By default, everything in Rust is *private*, with one exception. Enum variants
1772 in a `pub` enum are also public by default. You are allowed to alter this
1773 default visibility with the `priv` keyword. When an item is declared as `pub`,
1774 it can be thought of as being accessible to the outside world. For example:
1778 // Declare a private struct
1781 // Declare a public struct with a private field
1786 // Declare a public enum with two public variants
1788 PubliclyAccessibleState,
1789 PubliclyAccessibleState2,
1793 With the notion of an item being either public or private, Rust allows item
1794 accesses in two cases:
1796 1. If an item is public, then it can be used externally through any of its
1798 2. If an item is private, it may be accessed by the current module and its
1801 These two cases are surprisingly powerful for creating module hierarchies
1802 exposing public APIs while hiding internal implementation details. To help
1803 explain, here's a few use cases and what they would entail:
1805 * A library developer needs to expose functionality to crates which link
1806 against their library. As a consequence of the first case, this means that
1807 anything which is usable externally must be `pub` from the root down to the
1808 destination item. Any private item in the chain will disallow external
1811 * A crate needs a global available "helper module" to itself, but it doesn't
1812 want to expose the helper module as a public API. To accomplish this, the
1813 root of the crate's hierarchy would have a private module which then
1814 internally has a "public api". Because the entire crate is a descendant of
1815 the root, then the entire local crate can access this private module through
1818 * When writing unit tests for a module, it's often a common idiom to have an
1819 immediate child of the module to-be-tested named `mod test`. This module
1820 could access any items of the parent module through the second case, meaning
1821 that internal implementation details could also be seamlessly tested from the
1824 In the second case, it mentions that a private item "can be accessed" by the
1825 current module and its descendants, but the exact meaning of accessing an item
1826 depends on what the item is. Accessing a module, for example, would mean
1827 looking inside of it (to import more items). On the other hand, accessing a
1828 function would mean that it is invoked. Additionally, path expressions and
1829 import statements are considered to access an item in the sense that the
1830 import/expression is only valid if the destination is in the current visibility
1833 Here's an example of a program which exemplifies the three cases outlined
1837 // This module is private, meaning that no external crate can access this
1838 // module. Because it is private at the root of this current crate, however, any
1839 // module in the crate may access any publicly visible item in this module.
1840 mod crate_helper_module {
1842 // This function can be used by anything in the current crate
1843 pub fn crate_helper() {}
1845 // This function *cannot* be used by anything else in the crate. It is not
1846 // publicly visible outside of the `crate_helper_module`, so only this
1847 // current module and its descendants may access it.
1848 fn implementation_detail() {}
1851 // This function is "public to the root" meaning that it's available to external
1852 // crates linking against this one.
1853 pub fn public_api() {}
1855 // Similarly to 'public_api', this module is public so external crates may look
1858 use crate_helper_module;
1860 pub fn my_method() {
1861 // Any item in the local crate may invoke the helper module's public
1862 // interface through a combination of the two rules above.
1863 crate_helper_module::crate_helper();
1866 // This function is hidden to any module which is not a descendant of
1868 fn my_implementation() {}
1874 fn test_my_implementation() {
1875 // Because this module is a descendant of `submodule`, it's allowed
1876 // to access private items inside of `submodule` without a privacy
1878 super::my_implementation();
1886 For a rust program to pass the privacy checking pass, all paths must be valid
1887 accesses given the two rules above. This includes all use statements,
1888 expressions, types, etc.
1890 ### Re-exporting and Visibility
1892 Rust allows publicly re-exporting items through a `pub use` directive. Because
1893 this is a public directive, this allows the item to be used in the current
1894 module through the rules above. It essentially allows public access into the
1895 re-exported item. For example, this program is valid:
1898 pub use self::implementation::api;
1900 mod implementation {
1909 This means that any external crate referencing `implementation::api::f` would
1910 receive a privacy violation, while the path `api::f` would be allowed.
1912 When re-exporting a private item, it can be thought of as allowing the "privacy
1913 chain" being short-circuited through the reexport instead of passing through
1914 the namespace hierarchy as it normally would.
1919 attribute : '#' '!' ? '[' meta_item ']' ;
1920 meta_item : ident [ '=' literal
1921 | '(' meta_seq ')' ] ? ;
1922 meta_seq : meta_item [ ',' meta_seq ] ? ;
1925 Any item declaration may have an _attribute_ applied to it. Attributes in Rust
1926 are modeled on Attributes in ECMA-335, with the syntax coming from ECMA-334
1927 (C#). An attribute is a general, free-form metadatum that is interpreted
1928 according to name, convention, and language and compiler version. Attributes
1929 may appear as any of:
1931 * A single identifier, the attribute name
1932 * An identifier followed by the equals sign '=' and a literal, providing a
1934 * An identifier followed by a parenthesized list of sub-attribute arguments
1936 Attributes with a bang ("!") after the hash ("#") apply to the item that the
1937 attribute is declared within. Attributes that do not have a bang after the hash
1938 apply to the item that follows the attribute.
1940 An example of attributes:
1943 // General metadata applied to the enclosing module or crate.
1944 #![crate_type = "lib"]
1946 // A function marked as a unit test
1952 // A conditionally-compiled module
1953 #[cfg(target_os="linux")]
1958 // A lint attribute used to suppress a warning/error
1959 #[allow(non_camel_case_types)]
1963 > **Note:** At some point in the future, the compiler will distinguish between
1964 > language-reserved and user-available attributes. Until then, there is
1965 > effectively no difference between an attribute handled by a loadable syntax
1966 > extension and the compiler.
1968 ### Crate-only attributes
1970 - `crate_name` - specify the crate's crate name.
1971 - `crate_type` - see [linkage](#linkage).
1972 - `feature` - see [compiler features](#compiler-features).
1973 - `no_builtins` - disable optimizing certain code patterns to invocations of
1974 library functions that are assumed to exist
1975 - `no_main` - disable emitting the `main` symbol. Useful when some other
1976 object being linked to defines `main`.
1977 - `no_start` - disable linking to the `native` crate, which specifies the
1978 "start" language item.
1979 - `no_std` - disable linking to the `std` crate.
1980 - `plugin` — load a list of named crates as compiler plugins, e.g.
1981 `#![plugin(foo, bar)]`. Optional arguments for each plugin,
1982 i.e. `#![plugin(foo(... args ...))]`, are provided to the plugin's
1983 registrar function. The `plugin` feature gate is required to use
1986 ### Module-only attributes
1988 - `no_implicit_prelude` - disable injecting `use std::prelude::*` in this
1990 - `path` - specifies the file to load the module from. `#[path="foo.rs"] mod
1991 bar;` is equivalent to `mod bar { /* contents of foo.rs */ }`. The path is
1992 taken relative to the directory that the current module is in.
1994 ### Function-only attributes
1996 - `main` - indicates that this function should be passed to the entry point,
1997 rather than the function in the crate root named `main`.
1998 - `plugin_registrar` - mark this function as the registration point for
1999 [compiler plugins][plugin], such as loadable syntax extensions.
2000 - `start` - indicates that this function should be used as the entry point,
2001 overriding the "start" language item. See the "start" [language
2002 item](#language-items) for more details.
2003 - `test` - indicates that this function is a test function, to only be compiled
2004 in case of `--test`.
2005 - `should_panic` - indicates that this test function should panic, inverting the success condition.
2006 - `cold` - The function is unlikely to be executed, so optimize it (and calls
2009 ### Static-only attributes
2011 - `thread_local` - on a `static mut`, this signals that the value of this
2012 static may change depending on the current thread. The exact consequences of
2013 this are implementation-defined.
2017 On an `extern` block, the following attributes are interpreted:
2019 - `link_args` - specify arguments to the linker, rather than just the library
2020 name and type. This is feature gated and the exact behavior is
2021 implementation-defined (due to variety of linker invocation syntax).
2022 - `link` - indicate that a native library should be linked to for the
2023 declarations in this block to be linked correctly. `link` supports an optional `kind`
2024 key with three possible values: `dylib`, `static`, and `framework`. See [external blocks](#external-blocks) for more about external blocks. Two
2025 examples: `#[link(name = "readline")]` and
2026 `#[link(name = "CoreFoundation", kind = "framework")]`.
2028 On declarations inside an `extern` block, the following attributes are
2031 - `link_name` - the name of the symbol that this function or static should be
2033 - `linkage` - on a static, this specifies the [linkage
2034 type](http://llvm.org/docs/LangRef.html#linkage-types).
2038 - `repr` - on C-like enums, this sets the underlying type used for
2039 representation. Takes one argument, which is the primitive
2040 type this enum should be represented for, or `C`, which specifies that it
2041 should be the default `enum` size of the C ABI for that platform. Note that
2042 enum representation in C is undefined, and this may be incorrect when the C
2043 code is compiled with certain flags.
2047 - `repr` - specifies the representation to use for this struct. Takes a list
2048 of options. The currently accepted ones are `C` and `packed`, which may be
2049 combined. `C` will use a C ABI compatible struct layout, and `packed` will
2050 remove any padding between fields (note that this is very fragile and may
2051 break platforms which require aligned access).
2053 ### Macro-related attributes
2055 - `macro_use` on a `mod` — macros defined in this module will be visible in the
2056 module's parent, after this module has been included.
2058 - `macro_use` on an `extern crate` — load macros from this crate. An optional
2059 list of names `#[macro_use(foo, bar)]` restricts the import to just those
2060 macros named. The `extern crate` must appear at the crate root, not inside
2061 `mod`, which ensures proper function of the [`$crate` macro
2062 variable](book/macros.html#the-variable-$crate).
2064 - `macro_reexport` on an `extern crate` — re-export the named macros.
2066 - `macro_export` - export a macro for cross-crate usage.
2068 - `no_link` on an `extern crate` — even if we load this crate for macros, don't
2069 link it into the output.
2071 See the [macros section of the
2072 book](book/macros.html#scoping-and-macro-import/export) for more information on
2076 ### Miscellaneous attributes
2078 - `export_name` - on statics and functions, this determines the name of the
2080 - `link_section` - on statics and functions, this specifies the section of the
2081 object file that this item's contents will be placed into.
2082 - `no_mangle` - on any item, do not apply the standard name mangling. Set the
2083 symbol for this item to its identifier.
2084 - `packed` - on structs or enums, eliminate any padding that would be used to
2086 - `simd` - on certain tuple structs, derive the arithmetic operators, which
2087 lower to the target's SIMD instructions, if any; the `simd` feature gate
2088 is necessary to use this attribute.
2089 - `static_assert` - on statics whose type is `bool`, terminates compilation
2090 with an error if it is not initialized to `true`.
2091 - `unsafe_destructor` - allow implementations of the "drop" language item
2092 where the type it is implemented for does not implement the "send" language
2093 item; the `unsafe_destructor` feature gate is needed to use this attribute
2094 - `unsafe_no_drop_flag` - on structs, remove the flag that prevents
2095 destructors from being run twice. Destructors might be run multiple times on
2096 the same object with this attribute.
2097 - `doc` - Doc comments such as `/// foo` are equivalent to `#[doc = "foo"]`.
2098 - `rustc_on_unimplemented` - Write a custom note to be shown along with the error
2099 when the trait is found to be unimplemented on a type.
2100 You may use format arguments like `{T}`, `{A}` to correspond to the
2101 types at the point of use corresponding to the type parameters of the
2102 trait of the same name. `{Self}` will be replaced with the type that is supposed
2103 to implement the trait but doesn't. To use this, the `on_unimplemented` feature gate
2106 ### Conditional compilation
2108 Sometimes one wants to have different compiler outputs from the same code,
2109 depending on build target, such as targeted operating system, or to enable
2112 There are two kinds of configuration options, one that is either defined or not
2113 (`#[cfg(foo)]`), and the other that contains a string that can be checked
2114 against (`#[cfg(bar = "baz")]` (currently only compiler-defined configuration
2115 options can have the latter form).
2118 // The function is only included in the build when compiling for OSX
2119 #[cfg(target_os = "macos")]
2124 // This function is only included when either foo or bar is defined
2125 #[cfg(any(foo, bar))]
2126 fn needs_foo_or_bar() {
2130 // This function is only included when compiling for a unixish OS with a 32-bit
2132 #[cfg(all(unix, target_pointer_width = "32"))]
2133 fn on_32bit_unix() {
2137 // This function is only included when foo is not defined
2139 fn needs_not_foo() {
2144 This illustrates some conditional compilation can be achieved using the
2145 `#[cfg(...)]` attribute. `any`, `all` and `not` can be used to assemble
2146 arbitrarily complex configurations through nesting.
2148 The following configurations must be defined by the implementation:
2150 * `target_arch = "..."`. Target CPU architecture, such as `"x86"`, `"x86_64"`
2151 `"mips"`, `"powerpc"`, `"arm"`, or `"aarch64"`.
2152 * `target_endian = "..."`. Endianness of the target CPU, either `"little"` or
2154 * `target_family = "..."`. Operating system family of the target, e. g.
2155 `"unix"` or `"windows"`. The value of this configuration option is defined
2156 as a configuration itself, like `unix` or `windows`.
2157 * `target_os = "..."`. Operating system of the target, examples include
2158 `"windows"`, `"macos"`, `"ios"`, `"linux"`, `"android"`, `"freebsd"`, `"dragonfly"`,
2159 `"bitrig"` or `"openbsd"`.
2160 * `target_pointer_width = "..."`. Target pointer width in bits. This is set
2161 to `"32"` for targets with 32-bit pointers, and likewise set to `"64"` for
2163 * `unix`. See `target_family`.
2164 * `windows`. See `target_family`.
2166 You can also set another attribute based on a `cfg` variable with `cfg_attr`:
2172 Will be the same as `#[b]` if `a` is set by `cfg`, and nothing otherwise.
2174 ### Lint check attributes
2176 A lint check names a potentially undesirable coding pattern, such as
2177 unreachable code or omitted documentation, for the static entity to which the
2180 For any lint check `C`:
2182 * `allow(C)` overrides the check for `C` so that violations will go
2184 * `deny(C)` signals an error after encountering a violation of `C`,
2185 * `forbid(C)` is the same as `deny(C)`, but also forbids changing the lint
2187 * `warn(C)` warns about violations of `C` but continues compilation.
2189 The lint checks supported by the compiler can be found via `rustc -W help`,
2190 along with their default settings. [Compiler
2191 plugins](book/plugins.html#lint-plugins) can provide additional lint checks.
2195 // Missing documentation is ignored here
2196 #[allow(missing_docs)]
2197 pub fn undocumented_one() -> i32 { 1 }
2199 // Missing documentation signals a warning here
2200 #[warn(missing_docs)]
2201 pub fn undocumented_too() -> i32 { 2 }
2203 // Missing documentation signals an error here
2204 #[deny(missing_docs)]
2205 pub fn undocumented_end() -> i32 { 3 }
2209 This example shows how one can use `allow` and `warn` to toggle a particular
2213 #[warn(missing_docs)]
2215 #[allow(missing_docs)]
2217 // Missing documentation is ignored here
2218 pub fn undocumented_one() -> i32 { 1 }
2220 // Missing documentation signals a warning here,
2221 // despite the allow above.
2222 #[warn(missing_docs)]
2223 pub fn undocumented_two() -> i32 { 2 }
2226 // Missing documentation signals a warning here
2227 pub fn undocumented_too() -> i32 { 3 }
2231 This example shows how one can use `forbid` to disallow uses of `allow` for
2235 #[forbid(missing_docs)]
2237 // Attempting to toggle warning signals an error here
2238 #[allow(missing_docs)]
2240 pub fn undocumented_too() -> i32 { 2 }
2246 Some primitive Rust operations are defined in Rust code, rather than being
2247 implemented directly in C or assembly language. The definitions of these
2248 operations have to be easy for the compiler to find. The `lang` attribute
2249 makes it possible to declare these operations. For example, the `str` module
2250 in the Rust standard library defines the string equality function:
2254 pub fn eq_slice(a: &str, b: &str) -> bool {
2259 The name `str_eq` has a special meaning to the Rust compiler, and the presence
2260 of this definition means that it will use this definition when generating calls
2261 to the string equality function.
2263 A complete list of the built-in language items will be added in the future.
2265 ### Inline attributes
2267 The inline attribute is used to suggest to the compiler to perform an inline
2268 expansion and place a copy of the function or static in the caller rather than
2269 generating code to call the function or access the static where it is defined.
2271 The compiler automatically inlines functions based on internal heuristics.
2272 Incorrectly inlining functions can actually making the program slower, so it
2273 should be used with care.
2275 Immutable statics are always considered inlineable unless marked with
2276 `#[inline(never)]`. It is undefined whether two different inlineable statics
2277 have the same memory address. In other words, the compiler is free to collapse
2278 duplicate inlineable statics together.
2280 `#[inline]` and `#[inline(always)]` always causes the function to be serialized
2281 into crate metadata to allow cross-crate inlining.
2283 There are three different types of inline attributes:
2285 * `#[inline]` hints the compiler to perform an inline expansion.
2286 * `#[inline(always)]` asks the compiler to always perform an inline expansion.
2287 * `#[inline(never)]` asks the compiler to never perform an inline expansion.
2291 The `derive` attribute allows certain traits to be automatically implemented
2292 for data structures. For example, the following will create an `impl` for the
2293 `PartialEq` and `Clone` traits for `Foo`, the type parameter `T` will be given
2294 the `PartialEq` or `Clone` constraints for the appropriate `impl`:
2297 #[derive(PartialEq, Clone)]
2304 The generated `impl` for `PartialEq` is equivalent to
2307 # struct Foo<T> { a: i32, b: T }
2308 impl<T: PartialEq> PartialEq for Foo<T> {
2309 fn eq(&self, other: &Foo<T>) -> bool {
2310 self.a == other.a && self.b == other.b
2313 fn ne(&self, other: &Foo<T>) -> bool {
2314 self.a != other.a || self.b != other.b
2319 ### Compiler Features
2321 Certain aspects of Rust may be implemented in the compiler, but they're not
2322 necessarily ready for every-day use. These features are often of "prototype
2323 quality" or "almost production ready", but may not be stable enough to be
2324 considered a full-fledged language feature.
2326 For this reason, Rust recognizes a special crate-level attribute of the form:
2329 #![feature(feature1, feature2, feature3)]
2332 This directive informs the compiler that the feature list: `feature1`,
2333 `feature2`, and `feature3` should all be enabled. This is only recognized at a
2334 crate-level, not at a module-level. Without this directive, all features are
2335 considered off, and using the features will result in a compiler error.
2337 The currently implemented features of the reference compiler are:
2339 * `advanced_slice_patterns` - See the [match expressions](#match-expressions)
2340 section for discussion; the exact semantics of
2341 slice patterns are subject to change, so some types
2344 * `slice_patterns` - OK, actually, slice patterns are just scary and
2345 completely unstable.
2347 * `asm` - The `asm!` macro provides a means for inline assembly. This is often
2348 useful, but the exact syntax for this feature along with its
2349 semantics are likely to change, so this macro usage must be opted
2352 * `associated_types` - Allows type aliases in traits. Experimental.
2354 * `box_patterns` - Allows `box` patterns, the exact semantics of which
2355 is subject to change.
2357 * `box_syntax` - Allows use of `box` expressions, the exact semantics of which
2358 is subject to change.
2360 * `concat_idents` - Allows use of the `concat_idents` macro, which is in many
2361 ways insufficient for concatenating identifiers, and may be
2362 removed entirely for something more wholesome.
2364 * `custom_attribute` - Allows the usage of attributes unknown to the compiler
2365 so that new attributes can be added in a backwards compatible
2368 * `custom_derive` - Allows the use of `#[derive(Foo,Bar)]` as sugar for
2369 `#[derive_Foo] #[derive_Bar]`, which can be user-defined syntax
2372 * `intrinsics` - Allows use of the "rust-intrinsics" ABI. Compiler intrinsics
2373 are inherently unstable and no promise about them is made.
2375 * `lang_items` - Allows use of the `#[lang]` attribute. Like `intrinsics`,
2376 lang items are inherently unstable and no promise about them
2379 * `link_args` - This attribute is used to specify custom flags to the linker,
2380 but usage is strongly discouraged. The compiler's usage of the
2381 system linker is not guaranteed to continue in the future, and
2382 if the system linker is not used then specifying custom flags
2383 doesn't have much meaning.
2385 * `link_llvm_intrinsics` – Allows linking to LLVM intrinsics via
2386 `#[link_name="llvm.*"]`.
2388 * `linkage` - Allows use of the `linkage` attribute, which is not portable.
2390 * `log_syntax` - Allows use of the `log_syntax` macro attribute, which is a
2391 nasty hack that will certainly be removed.
2393 * `main` - Allows use of the `#[main]` attribute, which changes the entry point
2394 into a Rust program. This capability is subject to change.
2396 * `macro_reexport` - Allows macros to be re-exported from one crate after being imported
2397 from another. This feature was originally designed with the sole
2398 use case of the Rust standard library in mind, and is subject to
2401 * `non_ascii_idents` - The compiler supports the use of non-ascii identifiers,
2402 but the implementation is a little rough around the
2403 edges, so this can be seen as an experimental feature
2404 for now until the specification of identifiers is fully
2407 * `no_std` - Allows the `#![no_std]` crate attribute, which disables the implicit
2408 `extern crate std`. This typically requires use of the unstable APIs
2409 behind the libstd "facade", such as libcore and libcollections. It
2410 may also cause problems when using syntax extensions, including
2413 * `on_unimplemented` - Allows the `#[rustc_on_unimplemented]` attribute, which allows
2414 trait definitions to add specialized notes to error messages
2415 when an implementation was expected but not found.
2417 * `optin_builtin_traits` - Allows the definition of default and negative trait
2418 implementations. Experimental.
2420 * `plugin` - Usage of [compiler plugins][plugin] for custom lints or syntax extensions.
2421 These depend on compiler internals and are subject to change.
2423 * `plugin_registrar` - Indicates that a crate provides [compiler plugins][plugin].
2425 * `quote` - Allows use of the `quote_*!` family of macros, which are
2426 implemented very poorly and will likely change significantly
2427 with a proper implementation.
2429 * `rustc_attrs` - Gates internal `#[rustc_*]` attributes which may be
2430 for internal use only or have meaning added to them in the future.
2432 * `rustc_diagnostic_macros`- A mysterious feature, used in the implementation
2433 of rustc, not meant for mortals.
2435 * `simd` - Allows use of the `#[simd]` attribute, which is overly simple and
2436 not the SIMD interface we want to expose in the long term.
2438 * `simd_ffi` - Allows use of SIMD vectors in signatures for foreign functions.
2439 The SIMD interface is subject to change.
2441 * `staged_api` - Allows usage of stability markers and `#![staged_api]` in a
2442 crate. Stability markers are also attributes: `#[stable]`,
2443 `#[unstable]`, and `#[deprecated]` are the three levels.
2445 * `static_assert` - The `#[static_assert]` functionality is experimental and
2446 unstable. The attribute can be attached to a `static` of
2447 type `bool` and the compiler will error if the `bool` is
2448 `false` at compile time. This version of this functionality
2449 is unintuitive and suboptimal.
2451 * `start` - Allows use of the `#[start]` attribute, which changes the entry point
2452 into a Rust program. This capability, especially the signature for the
2453 annotated function, is subject to change.
2455 * `struct_inherit` - Allows using struct inheritance, which is barely
2456 implemented and will probably be removed. Don't use this.
2458 * `struct_variant` - Structural enum variants (those with named fields). It is
2459 currently unknown whether this style of enum variant is as
2460 fully supported as the tuple-forms, and it's not certain
2461 that this style of variant should remain in the language.
2462 For now this style of variant is hidden behind a feature
2465 * `thread_local` - The usage of the `#[thread_local]` attribute is experimental
2466 and should be seen as unstable. This attribute is used to
2467 declare a `static` as being unique per-thread leveraging
2468 LLVM's implementation which works in concert with the kernel
2469 loader and dynamic linker. This is not necessarily available
2470 on all platforms, and usage of it is discouraged.
2472 * `trace_macros` - Allows use of the `trace_macros` macro, which is a nasty
2473 hack that will certainly be removed.
2475 * `unboxed_closures` - Rust's new closure design, which is currently a work in
2476 progress feature with many known bugs.
2478 * `unsafe_destructor` - Allows use of the `#[unsafe_destructor]` attribute,
2479 which is considered wildly unsafe and will be
2480 obsoleted by language improvements.
2482 * `unsafe_no_drop_flag` - Allows use of the `#[unsafe_no_drop_flag]` attribute,
2483 which removes hidden flag added to a type that
2484 implements the `Drop` trait. The design for the
2485 `Drop` flag is subject to change, and this feature
2486 may be removed in the future.
2488 * `unmarked_api` - Allows use of items within a `#![staged_api]` crate
2489 which have not been marked with a stability marker.
2490 Such items should not be allowed by the compiler to exist,
2491 so if you need this there probably is a compiler bug.
2493 * `visible_private_types` - Allows public APIs to expose otherwise private
2494 types, e.g. as the return type of a public function.
2495 This capability may be removed in the future.
2497 * `allow_internal_unstable` - Allows `macro_rules!` macros to be tagged with the
2498 `#[allow_internal_unstable]` attribute, designed
2499 to allow `std` macros to call
2500 `#[unstable]`/feature-gated functionality
2501 internally without imposing on callers
2502 (i.e. making them behave like function calls in
2503 terms of encapsulation).
2505 If a feature is promoted to a language feature, then all existing programs will
2506 start to receive compilation warnings about `#![feature]` directives which enabled
2507 the new feature (because the directive is no longer necessary). However, if a
2508 feature is decided to be removed from the language, errors will be issued (if
2509 there isn't a parser error first). The directive in this case is no longer
2510 necessary, and it's likely that existing code will break if the feature isn't
2513 If an unknown feature is found in a directive, it results in a compiler error.
2514 An unknown feature is one which has never been recognized by the compiler.
2516 # Statements and expressions
2518 Rust is _primarily_ an expression language. This means that most forms of
2519 value-producing or effect-causing evaluation are directed by the uniform syntax
2520 category of _expressions_. Each kind of expression can typically _nest_ within
2521 each other kind of expression, and rules for evaluation of expressions involve
2522 specifying both the value produced by the expression and the order in which its
2523 sub-expressions are themselves evaluated.
2525 In contrast, statements in Rust serve _mostly_ to contain and explicitly
2526 sequence expression evaluation.
2530 A _statement_ is a component of a block, which is in turn a component of an
2531 outer [expression](#expressions) or [function](#functions).
2533 Rust has two kinds of statement: [declaration
2534 statements](#declaration-statements) and [expression
2535 statements](#expression-statements).
2537 ### Declaration statements
2539 A _declaration statement_ is one that introduces one or more *names* into the
2540 enclosing statement block. The declared names may denote new variables or new
2543 #### Item declarations
2545 An _item declaration statement_ has a syntactic form identical to an
2546 [item](#items) declaration within a module. Declaring an item — a
2547 function, enumeration, structure, type, static, trait, implementation or module
2548 — locally within a statement block is simply a way of restricting its
2549 scope to a narrow region containing all of its uses; it is otherwise identical
2550 in meaning to declaring the item outside the statement block.
2552 > **Note**: there is no implicit capture of the function's dynamic environment when
2553 > declaring a function-local item.
2555 #### Variable declarations
2558 let_decl : "let" pat [':' type ] ? [ init ] ? ';' ;
2559 init : [ '=' ] expr ;
2562 A _variable declaration_ introduces a new set of variable, given by a pattern. The
2563 pattern may be followed by a type annotation, and/or an initializer expression.
2564 When no type annotation is given, the compiler will infer the type, or signal
2565 an error if insufficient type information is available for definite inference.
2566 Any variables introduced by a variable declaration are visible from the point of
2567 declaration until the end of the enclosing block scope.
2569 ### Expression statements
2571 An _expression statement_ is one that evaluates an [expression](#expressions)
2572 and ignores its result. The type of an expression statement `e;` is always
2573 `()`, regardless of the type of `e`. As a rule, an expression statement's
2574 purpose is to trigger the effects of evaluating its expression.
2578 An expression may have two roles: it always produces a *value*, and it may have
2579 *effects* (otherwise known as "side effects"). An expression *evaluates to* a
2580 value, and has effects during *evaluation*. Many expressions contain
2581 sub-expressions (operands). The meaning of each kind of expression dictates
2584 * Whether or not to evaluate the sub-expressions when evaluating the expression
2585 * The order in which to evaluate the sub-expressions
2586 * How to combine the sub-expressions' values to obtain the value of the expression
2588 In this way, the structure of expressions dictates the structure of execution.
2589 Blocks are just another kind of expression, so blocks, statements, expressions,
2590 and blocks again can recursively nest inside each other to an arbitrary depth.
2592 #### Lvalues, rvalues and temporaries
2594 Expressions are divided into two main categories: _lvalues_ and _rvalues_.
2595 Likewise within each expression, sub-expressions may occur in _lvalue context_
2596 or _rvalue context_. The evaluation of an expression depends both on its own
2597 category and the context it occurs within.
2599 An lvalue is an expression that represents a memory location. These expressions
2600 are [paths](#path-expressions) (which refer to local variables, function and
2601 method arguments, or static variables), dereferences (`*expr`), [indexing
2602 expressions](#index-expressions) (`expr[expr]`), and [field
2603 references](#field-expressions) (`expr.f`). All other expressions are rvalues.
2605 The left operand of an [assignment](#assignment-expressions) or
2606 [compound-assignment](#compound-assignment-expressions) expression is an lvalue
2607 context, as is the single operand of a unary
2608 [borrow](#unary-operator-expressions). All other expression contexts are
2611 When an lvalue is evaluated in an _lvalue context_, it denotes a memory
2612 location; when evaluated in an _rvalue context_, it denotes the value held _in_
2613 that memory location.
2615 When an rvalue is used in an lvalue context, a temporary un-named lvalue is
2616 created and used instead. A temporary's lifetime equals the largest lifetime
2617 of any reference that points to it.
2619 #### Moved and copied types
2621 When a [local variable](#variables) is used as an
2622 [rvalue](#lvalues,-rvalues-and-temporaries) the variable will either be moved
2623 or copied, depending on its type. All values whose type implements `Copy` are
2624 copied, all others are moved.
2626 ### Literal expressions
2628 A _literal expression_ consists of one of the [literal](#literals) forms
2629 described earlier. It directly describes a number, character, string, boolean
2630 value, or the unit value.
2634 "hello"; // string type
2635 '5'; // character type
2639 ### Path expressions
2641 A [path](#paths) used as an expression context denotes either a local variable
2642 or an item. Path expressions are [lvalues](#lvalues,-rvalues-and-temporaries).
2644 ### Tuple expressions
2646 Tuples are written by enclosing zero or more comma-separated expressions in
2647 parentheses. They are used to create [tuple-typed](#tuple-types) values.
2655 ### Unit expressions
2657 The expression `()` denotes the _unit value_, the only value of the type with
2660 ### Structure expressions
2663 struct_expr : expr_path '{' ident ':' expr
2664 [ ',' ident ':' expr ] *
2667 [ ',' expr ] * ')' |
2671 There are several forms of structure expressions. A _structure expression_
2672 consists of the [path](#paths) of a [structure item](#structures), followed by
2673 a brace-enclosed list of one or more comma-separated name-value pairs,
2674 providing the field values of a new instance of the structure. A field name
2675 can be any identifier, and is separated from its value expression by a colon.
2676 The location denoted by a structure field is mutable if and only if the
2677 enclosing structure is mutable.
2679 A _tuple structure expression_ consists of the [path](#paths) of a [structure
2680 item](#structures), followed by a parenthesized list of one or more
2681 comma-separated expressions (in other words, the path of a structure item
2682 followed by a tuple expression). The structure item must be a tuple structure
2685 A _unit-like structure expression_ consists only of the [path](#paths) of a
2686 [structure item](#structures).
2688 The following are examples of structure expressions:
2691 # struct Point { x: f64, y: f64 }
2692 # struct TuplePoint(f64, f64);
2693 # mod game { pub struct User<'a> { pub name: &'a str, pub age: u32, pub score: usize } }
2694 # struct Cookie; fn some_fn<T>(t: T) {}
2695 Point {x: 10.0, y: 20.0};
2696 TuplePoint(10.0, 20.0);
2697 let u = game::User {name: "Joe", age: 35, score: 100_000};
2698 some_fn::<Cookie>(Cookie);
2701 A structure expression forms a new value of the named structure type. Note
2702 that for a given *unit-like* structure type, this will always be the same
2705 A structure expression can terminate with the syntax `..` followed by an
2706 expression to denote a functional update. The expression following `..` (the
2707 base) must have the same structure type as the new structure type being formed.
2708 The entire expression denotes the result of constructing a new structure (with
2709 the same type as the base expression) with the given values for the fields that
2710 were explicitly specified and the values in the base expression for all other
2714 # struct Point3d { x: i32, y: i32, z: i32 }
2715 let base = Point3d {x: 1, y: 2, z: 3};
2716 Point3d {y: 0, z: 10, .. base};
2719 ### Block expressions
2722 block_expr : '{' [ stmt ';' | item ] *
2726 A _block expression_ is similar to a module in terms of the declarations that
2727 are possible. Each block conceptually introduces a new namespace scope. Use
2728 items can bring new names into scopes and declared items are in scope for only
2731 A block will execute each statement sequentially, and then execute the
2732 expression (if given). If the block ends in a statement, its value is `()`:
2735 let x: () = { println!("Hello."); };
2738 If it ends in an expression, its value and type are that of the expression:
2741 let x: i32 = { println!("Hello."); 5 };
2746 ### Method-call expressions
2749 method_call_expr : expr '.' ident paren_expr_list ;
2752 A _method call_ consists of an expression followed by a single dot, an
2753 identifier, and a parenthesized expression-list. Method calls are resolved to
2754 methods on specific traits, either statically dispatching to a method if the
2755 exact `self`-type of the left-hand-side is known, or dynamically dispatching if
2756 the left-hand-side expression is an indirect [trait object](#trait-objects).
2758 ### Field expressions
2761 field_expr : expr '.' ident ;
2764 A _field expression_ consists of an expression followed by a single dot and an
2765 identifier, when not immediately followed by a parenthesized expression-list
2766 (the latter is a [method call expression](#method-call-expressions)). A field
2767 expression denotes a field of a [structure](#structure-types).
2772 (Struct {a: 10, b: 20}).a;
2775 A field access is an [lvalue](#lvalues,-rvalues-and-temporaries) referring to
2776 the value of that field. When the type providing the field inherits mutability,
2777 it can be [assigned](#assignment-expressions) to.
2779 Also, if the type of the expression to the left of the dot is a pointer, it is
2780 automatically dereferenced to make the field access possible.
2782 ### Array expressions
2785 array_expr : '[' "mut" ? array_elems? ']' ;
2787 array_elems : [expr [',' expr]*] | [expr ';' expr] ;
2790 An [array](#array,-and-slice-types) _expression_ is written by enclosing zero
2791 or more comma-separated expressions of uniform type in square brackets.
2793 In the `[expr ';' expr]` form, the expression after the `';'` must be a
2794 constant expression that can be evaluated at compile time, such as a
2795 [literal](#literals) or a [static item](#static-items).
2799 ["a", "b", "c", "d"];
2800 [0; 128]; // array with 128 zeros
2801 [0u8, 0u8, 0u8, 0u8];
2804 ### Index expressions
2807 idx_expr : expr '[' expr ']' ;
2810 [Array](#array,-and-slice-types)-typed expressions can be indexed by
2811 writing a square-bracket-enclosed expression (the index) after them. When the
2812 array is mutable, the resulting [lvalue](#lvalues,-rvalues-and-temporaries) can
2815 Indices are zero-based, and may be of any integral type. Vector access is
2816 bounds-checked at run-time. When the check fails, it will put the thread in a
2821 (["a", "b"])[10]; // panics
2824 ### Range expressions
2827 range_expr : expr ".." expr |
2833 The `..` operator will construct an object of one of the `std::ops::Range` variants.
2836 1..2; // std::ops::Range
2837 3..; // std::ops::RangeFrom
2838 ..4; // std::ops::RangeTo
2839 ..; // std::ops::RangeFull
2842 The following expressions are equivalent.
2845 let x = std::ops::Range {start: 0, end: 10};
2851 ### Unary operator expressions
2853 Rust defines three unary operators. They are all written as prefix operators,
2854 before the expression they apply to.
2857 : Negation. May only be applied to numeric types.
2859 : Dereference. When applied to a [pointer](#pointer-types) it denotes the
2860 pointed-to location. For pointers to mutable locations, the resulting
2861 [lvalue](#lvalues,-rvalues-and-temporaries) can be assigned to.
2862 On non-pointer types, it calls the `deref` method of the `std::ops::Deref`
2863 trait, or the `deref_mut` method of the `std::ops::DerefMut` trait (if
2864 implemented by the type and required for an outer expression that will or
2865 could mutate the dereference), and produces the result of dereferencing the
2866 `&` or `&mut` borrowed pointer returned from the overload method.
2869 : Logical negation. On the boolean type, this flips between `true` and
2870 `false`. On integer types, this inverts the individual bits in the
2871 two's complement representation of the value.
2873 ### Binary operator expressions
2876 binop_expr : expr binop expr ;
2879 Binary operators expressions are given in terms of [operator
2880 precedence](#operator-precedence).
2882 #### Arithmetic operators
2884 Binary arithmetic expressions are syntactic sugar for calls to built-in traits,
2885 defined in the `std::ops` module of the `std` library. This means that
2886 arithmetic operators can be overridden for user-defined types. The default
2887 meaning of the operators on standard types is given here.
2890 : Addition and array/string concatenation.
2891 Calls the `add` method on the `std::ops::Add` trait.
2894 Calls the `sub` method on the `std::ops::Sub` trait.
2897 Calls the `mul` method on the `std::ops::Mul` trait.
2900 Calls the `div` method on the `std::ops::Div` trait.
2903 Calls the `rem` method on the `std::ops::Rem` trait.
2905 #### Bitwise operators
2907 Like the [arithmetic operators](#arithmetic-operators), bitwise operators are
2908 syntactic sugar for calls to methods of built-in traits. This means that
2909 bitwise operators can be overridden for user-defined types. The default
2910 meaning of the operators on standard types is given here.
2914 Calls the `bitand` method of the `std::ops::BitAnd` trait.
2917 Calls the `bitor` method of the `std::ops::BitOr` trait.
2920 Calls the `bitxor` method of the `std::ops::BitXor` trait.
2923 Calls the `shl` method of the `std::ops::Shl` trait.
2926 Calls the `shr` method of the `std::ops::Shr` trait.
2928 #### Lazy boolean operators
2930 The operators `||` and `&&` may be applied to operands of boolean type. The
2931 `||` operator denotes logical 'or', and the `&&` operator denotes logical
2932 'and'. They differ from `|` and `&` in that the right-hand operand is only
2933 evaluated when the left-hand operand does not already determine the result of
2934 the expression. That is, `||` only evaluates its right-hand operand when the
2935 left-hand operand evaluates to `false`, and `&&` only when it evaluates to
2938 #### Comparison operators
2940 Comparison operators are, like the [arithmetic
2941 operators](#arithmetic-operators), and [bitwise operators](#bitwise-operators),
2942 syntactic sugar for calls to built-in traits. This means that comparison
2943 operators can be overridden for user-defined types. The default meaning of the
2944 operators on standard types is given here.
2948 Calls the `eq` method on the `std::cmp::PartialEq` trait.
2951 Calls the `ne` method on the `std::cmp::PartialEq` trait.
2954 Calls the `lt` method on the `std::cmp::PartialOrd` trait.
2957 Calls the `gt` method on the `std::cmp::PartialOrd` trait.
2959 : Less than or equal.
2960 Calls the `le` method on the `std::cmp::PartialOrd` trait.
2962 : Greater than or equal.
2963 Calls the `ge` method on the `std::cmp::PartialOrd` trait.
2965 #### Type cast expressions
2967 A type cast expression is denoted with the binary operator `as`.
2969 Executing an `as` expression casts the value on the left-hand side to the type
2970 on the right-hand side.
2972 An example of an `as` expression:
2975 # fn sum(v: &[f64]) -> f64 { 0.0 }
2976 # fn len(v: &[f64]) -> i32 { 0 }
2978 fn avg(v: &[f64]) -> f64 {
2979 let sum: f64 = sum(v);
2980 let sz: f64 = len(v) as f64;
2985 #### Assignment expressions
2987 An _assignment expression_ consists of an
2988 [lvalue](#lvalues,-rvalues-and-temporaries) expression followed by an equals
2989 sign (`=`) and an [rvalue](#lvalues,-rvalues-and-temporaries) expression.
2991 Evaluating an assignment expression [either copies or
2992 moves](#moved-and-copied-types) its right-hand operand to its left-hand
3002 #### Compound assignment expressions
3004 The `+`, `-`, `*`, `/`, `%`, `&`, `|`, `^`, `<<`, and `>>` operators may be
3005 composed with the `=` operator. The expression `lval OP= val` is equivalent to
3006 `lval = lval OP val`. For example, `x = x + 1` may be written as `x += 1`.
3008 Any such expression always has the [`unit`](#primitive-types) type.
3010 #### Operator precedence
3012 The precedence of Rust binary operators is ordered as follows, going from
3015 ```{.text .precedence}
3029 Operators at the same precedence level are evaluated left-to-right. [Unary
3030 operators](#unary-operator-expressions) have the same precedence level and are
3031 stronger than any of the binary operators.
3033 ### Grouped expressions
3035 An expression enclosed in parentheses evaluates to the result of the enclosed
3036 expression. Parentheses can be used to explicitly specify evaluation order
3037 within an expression.
3040 paren_expr : '(' expr ')' ;
3043 An example of a parenthesized expression:
3046 let x: i32 = (2 + 3) * 4;
3050 ### Call expressions
3053 expr_list : [ expr [ ',' expr ]* ] ? ;
3054 paren_expr_list : '(' expr_list ')' ;
3055 call_expr : expr paren_expr_list ;
3058 A _call expression_ invokes a function, providing zero or more input variables
3059 and an optional location to move the function's output into. If the function
3060 eventually returns, then the expression completes.
3062 Some examples of call expressions:
3065 # fn add(x: i32, y: i32) -> i32 { 0 }
3067 let x: i32 = add(1i32, 2i32);
3068 let pi: Result<f32, _> = "3.14".parse();
3071 ### Lambda expressions
3074 ident_list : [ ident [ ',' ident ]* ] ? ;
3075 lambda_expr : '|' ident_list '|' expr ;
3078 A _lambda expression_ (sometimes called an "anonymous function expression")
3079 defines a function and denotes it as a value, in a single expression. A lambda
3080 expression is a pipe-symbol-delimited (`|`) list of identifiers followed by an
3083 A lambda expression denotes a function that maps a list of parameters
3084 (`ident_list`) onto the expression that follows the `ident_list`. The
3085 identifiers in the `ident_list` are the parameters to the function. These
3086 parameters' types need not be specified, as the compiler infers them from
3089 Lambda expressions are most useful when passing functions as arguments to other
3090 functions, as an abbreviation for defining and capturing a separate function.
3092 Significantly, lambda expressions _capture their environment_, which regular
3093 [function definitions](#functions) do not. The exact type of capture depends
3094 on the [function type](#function-types) inferred for the lambda expression. In
3095 the simplest and least-expensive form (analogous to a ```|| { }``` expression),
3096 the lambda expression captures its environment by reference, effectively
3097 borrowing pointers to all outer variables mentioned inside the function.
3098 Alternately, the compiler may infer that a lambda expression should copy or
3099 move values (depending on their type) from the environment into the lambda
3100 expression's captured environment.
3102 In this example, we define a function `ten_times` that takes a higher-order
3103 function argument, and call it with a lambda expression as an argument:
3106 fn ten_times<F>(f: F) where F: Fn(i32) {
3114 ten_times(|j| println!("hello, {}", j));
3119 A `loop` expression denotes an infinite loop.
3122 loop_expr : [ lifetime ':' ] "loop" '{' block '}';
3125 A `loop` expression may optionally have a _label_. The label is written as
3126 a lifetime preceding the loop expression, as in `'foo: loop{ }`. If a
3127 label is present, then labeled `break` and `continue` expressions nested
3128 within this loop may exit out of this loop or return control to its head.
3129 See [Break expressions](#break-expressions) and [Continue
3130 expressions](#continue-expressions).
3132 ### Break expressions
3135 break_expr : "break" [ lifetime ];
3138 A `break` expression has an optional _label_. If the label is absent, then
3139 executing a `break` expression immediately terminates the innermost loop
3140 enclosing it. It is only permitted in the body of a loop. If the label is
3141 present, then `break 'foo` terminates the loop with label `'foo`, which need not
3142 be the innermost label enclosing the `break` expression, but must enclose it.
3144 ### Continue expressions
3147 continue_expr : "continue" [ lifetime ];
3150 A `continue` expression has an optional _label_. If the label is absent, then
3151 executing a `continue` expression immediately terminates the current iteration
3152 of the innermost loop enclosing it, returning control to the loop *head*. In
3153 the case of a `while` loop, the head is the conditional expression controlling
3154 the loop. In the case of a `for` loop, the head is the call-expression
3155 controlling the loop. If the label is present, then `continue 'foo` returns
3156 control to the head of the loop with label `'foo`, which need not be the
3157 innermost label enclosing the `break` expression, but must enclose it.
3159 A `continue` expression is only permitted in the body of a loop.
3164 while_expr : [ lifetime ':' ] "while" no_struct_literal_expr '{' block '}' ;
3167 A `while` loop begins by evaluating the boolean loop conditional expression.
3168 If the loop conditional expression evaluates to `true`, the loop body block
3169 executes and control returns to the loop conditional expression. If the loop
3170 conditional expression evaluates to `false`, the `while` expression completes.
3183 Like `loop` expressions, `while` loops can be controlled with `break` or
3184 `continue`, and may optionally have a _label_. See [infinite
3185 loops](#infinite-loops), [break expressions](#break-expressions), and
3186 [continue expressions](#continue-expressions) for more information.
3191 for_expr : [ lifetime ':' ] "for" pat "in" no_struct_literal_expr '{' block '}' ;
3194 A `for` expression is a syntactic construct for looping over elements provided
3195 by an implementation of `std::iter::Iterator`.
3197 An example of a for loop over the contents of an array:
3201 # fn bar(f: Foo) { }
3206 let v: &[Foo] = &[a, b, c];
3213 An example of a for loop over a series of integers:
3216 # fn bar(b:usize) { }
3222 Like `loop` expressions, `for` loops can be controlled with `break` or
3223 `continue`, and may optionally have a _label_. See [infinite
3224 loops](#infinite-loops), [break expressions](#break-expressions), and
3225 [continue expressions](#continue-expressions) for more information.
3230 if_expr : "if" no_struct_literal_expr '{' block '}'
3233 else_tail : "else" [ if_expr | if_let_expr
3237 An `if` expression is a conditional branch in program control. The form of an
3238 `if` expression is a condition expression, followed by a consequent block, any
3239 number of `else if` conditions and blocks, and an optional trailing `else`
3240 block. The condition expressions must have type `bool`. If a condition
3241 expression evaluates to `true`, the consequent block is executed and any
3242 subsequent `else if` or `else` block is skipped. If a condition expression
3243 evaluates to `false`, the consequent block is skipped and any subsequent `else
3244 if` condition is evaluated. If all `if` and `else if` conditions evaluate to
3245 `false` then any `else` block is executed.
3247 ### Match expressions
3250 match_expr : "match" no_struct_literal_expr '{' match_arm * '}' ;
3252 match_arm : attribute * match_pat "=>" [ expr "," | '{' block '}' ] ;
3254 match_pat : pat [ '|' pat ] * [ "if" expr ] ? ;
3257 A `match` expression branches on a *pattern*. The exact form of matching that
3258 occurs depends on the pattern. Patterns consist of some combination of
3259 literals, destructured arrays or enum constructors, structures and tuples,
3260 variable binding specifications, wildcards (`..`), and placeholders (`_`). A
3261 `match` expression has a *head expression*, which is the value to compare to
3262 the patterns. The type of the patterns must equal the type of the head
3265 In a pattern whose head expression has an `enum` type, a placeholder (`_`)
3266 stands for a *single* data field, whereas a wildcard `..` stands for *all* the
3267 fields of a particular variant.
3269 A `match` behaves differently depending on whether or not the head expression
3270 is an [lvalue or an rvalue](#lvalues,-rvalues-and-temporaries). If the head
3271 expression is an rvalue, it is first evaluated into a temporary location, and
3272 the resulting value is sequentially compared to the patterns in the arms until
3273 a match is found. The first arm with a matching pattern is chosen as the branch
3274 target of the `match`, any variables bound by the pattern are assigned to local
3275 variables in the arm's block, and control enters the block.
3277 When the head expression is an lvalue, the match does not allocate a temporary
3278 location (however, a by-value binding may copy or move from the lvalue). When
3279 possible, it is preferable to match on lvalues, as the lifetime of these
3280 matches inherits the lifetime of the lvalue, rather than being restricted to
3281 the inside of the match.
3283 An example of a `match` expression:
3289 1 => println!("one"),
3290 2 => println!("two"),
3291 3 => println!("three"),
3292 4 => println!("four"),
3293 5 => println!("five"),
3294 _ => println!("something else"),
3298 Patterns that bind variables default to binding to a copy or move of the
3299 matched value (depending on the matched value's type). This can be changed to
3300 bind to a reference by using the `ref` keyword, or to a mutable reference using
3303 Subpatterns can also be bound to variables by the use of the syntax `variable @
3304 subpattern`. For example:
3310 e @ 1 ... 5 => println!("got a range element {}", e),
3311 _ => println!("anything"),
3315 Patterns can also dereference pointers by using the `&`, `&mut` and `box`
3316 symbols, as appropriate. For example, these two matches on `x: &i32` are
3321 let y = match *x { 0 => "zero", _ => "some" };
3322 let z = match x { &0 => "zero", _ => "some" };
3327 A pattern that's just an identifier, like `Nil` in the previous example, could
3328 either refer to an enum variant that's in scope, or bind a new variable. The
3329 compiler resolves this ambiguity by forbidding variable bindings that occur in
3330 `match` patterns from shadowing names of variants that are in scope. For
3331 example, wherever `List` is in scope, a `match` pattern would not be able to
3332 bind `Nil` as a new name. The compiler interprets a variable pattern `x` as a
3333 binding _only_ if there is no variant named `x` in scope. A convention you can
3334 use to avoid conflicts is simply to name variants with upper-case letters, and
3335 local variables with lower-case letters.
3337 Multiple match patterns may be joined with the `|` operator. A range of values
3338 may be specified with `...`. For example:
3343 let message = match x {
3344 0 | 1 => "not many",
3350 Range patterns only work on scalar types (like integers and characters; not
3351 like arrays and structs, which have sub-components). A range pattern may not
3352 be a sub-range of another range pattern inside the same `match`.
3354 Finally, match patterns can accept *pattern guards* to further refine the
3355 criteria for matching a case. Pattern guards appear after the pattern and
3356 consist of a bool-typed expression following the `if` keyword. A pattern guard
3357 may refer to the variables bound within the pattern they follow.
3360 # let maybe_digit = Some(0);
3361 # fn process_digit(i: i32) { }
3362 # fn process_other(i: i32) { }
3364 let message = match maybe_digit {
3365 Some(x) if x < 10 => process_digit(x),
3366 Some(x) => process_other(x),
3371 ### If let expressions
3374 if_let_expr : "if" "let" pat '=' expr '{' block '}'
3376 else_tail : "else" [ if_expr | if_let_expr | '{' block '}' ] ;
3379 An `if let` expression is semantically identical to an `if` expression but in place
3380 of a condition expression it expects a refutable let statement. If the value of the
3381 expression on the right hand side of the let statement matches the pattern, the corresponding
3382 block will execute, otherwise flow proceeds to the first `else` block that follows.
3387 while_let_expr : "while" "let" pat '=' expr '{' block '}' ;
3390 A `while let` loop is semantically identical to a `while` loop but in place of a
3391 condition expression it expects a refutable let statement. If the value of the
3392 expression on the right hand side of the let statement matches the pattern, the
3393 loop body block executes and control returns to the pattern matching statement.
3394 Otherwise, the while expression completes.
3396 ### Return expressions
3399 return_expr : "return" expr ? ;
3402 Return expressions are denoted with the keyword `return`. Evaluating a `return`
3403 expression moves its argument into the designated output location for the
3404 current function call, destroys the current function activation frame, and
3405 transfers control to the caller frame.
3407 An example of a `return` expression:
3410 fn max(a: i32, b: i32) -> i32 {
3422 Every variable, item and value in a Rust program has a type. The _type_ of a
3423 *value* defines the interpretation of the memory holding it.
3425 Built-in types and type-constructors are tightly integrated into the language,
3426 in nontrivial ways that are not possible to emulate in user-defined types.
3427 User-defined types have limited capabilities.
3431 The primitive types are the following:
3433 * The "unit" type `()`, having the single "unit" value `()` (occasionally called
3435 * The boolean type `bool` with values `true` and `false`.
3436 * The machine types.
3437 * The machine-dependent integer and floating-point types.
3439 [^unittype]: The "unit" value `()` is *not* a sentinel "null pointer" value for
3440 reference variables; the "unit" type is the implicit return type from functions
3441 otherwise lacking a return type, and can be used in other contexts (such as
3442 message-sending or type-parametric code) as a zero-size type.]
3446 The machine types are the following:
3448 * The unsigned word types `u8`, `u16`, `u32` and `u64`, with values drawn from
3449 the integer intervals [0, 2^8 - 1], [0, 2^16 - 1], [0, 2^32 - 1] and
3450 [0, 2^64 - 1] respectively.
3452 * The signed two's complement word types `i8`, `i16`, `i32` and `i64`, with
3453 values drawn from the integer intervals [-(2^(7)), 2^7 - 1],
3454 [-(2^(15)), 2^15 - 1], [-(2^(31)), 2^31 - 1], [-(2^(63)), 2^63 - 1]
3457 * The IEEE 754-2008 `binary32` and `binary64` floating-point types: `f32` and
3458 `f64`, respectively.
3460 #### Machine-dependent integer types
3462 The `usize` type is an unsigned integer type with the same number of bits as the
3463 platform's pointer type. It can represent every memory address in the process.
3465 The `isize` type is a signed integer type with the same number of bits as the
3466 platform's pointer type. The theoretical upper bound on object and array size
3467 is the maximum `isize` value. This ensures that `isize` can be used to calculate
3468 differences between pointers into an object or array and can address every byte
3469 within an object along with one byte past the end.
3473 The types `char` and `str` hold textual data.
3475 A value of type `char` is a [Unicode scalar value](
3476 http://www.unicode.org/glossary/#unicode_scalar_value) (i.e. a code point that
3477 is not a surrogate), represented as a 32-bit unsigned word in the 0x0000 to
3478 0xD7FF or 0xE000 to 0x10FFFF range. A `[char]` array is effectively an UCS-4 /
3481 A value of type `str` is a Unicode string, represented as an array of 8-bit
3482 unsigned bytes holding a sequence of UTF-8 code points. Since `str` is of
3483 unknown size, it is not a _first-class_ type, but can only be instantiated
3484 through a pointer type, such as `&str` or `String`.
3488 A tuple *type* is a heterogeneous product of other types, called the *elements*
3489 of the tuple. It has no nominal name and is instead structurally typed.
3491 Tuple types and values are denoted by listing the types or values of their
3492 elements, respectively, in a parenthesized, comma-separated list.
3494 Because tuple elements don't have a name, they can only be accessed by
3495 pattern-matching or by using `N` directly as a field to access the
3498 An example of a tuple type and its use:
3501 type Pair<'a> = (i32, &'a str);
3502 let p: Pair<'static> = (10, "hello");
3504 assert!(b != "world");
3508 ### Array, and Slice types
3510 Rust has two different types for a list of items:
3512 * `[T; N]`, an 'array'.
3513 * `&[T]`, a 'slice'.
3515 An array has a fixed size, and can be allocated on either the stack or the
3518 A slice is a 'view' into an array. It doesn't own the data it points
3521 An example of each kind:
3524 let vec: Vec<i32> = vec![1, 2, 3];
3525 let arr: [i32; 3] = [1, 2, 3];
3526 let s: &[i32] = &vec[..];
3529 As you can see, the `vec!` macro allows you to create a `Vec<T>` easily. The
3530 `vec!` macro is also part of the standard library, rather than the language.
3532 All in-bounds elements of arrays, and slices are always initialized, and access
3533 to an array or slice is always bounds-checked.
3537 A `struct` *type* is a heterogeneous product of other types, called the
3538 *fields* of the type.[^structtype]
3540 [^structtype]: `struct` types are analogous `struct` types in C,
3541 the *record* types of the ML family,
3542 or the *structure* types of the Lisp family.
3544 New instances of a `struct` can be constructed with a [struct
3545 expression](#structure-expressions).
3547 The memory layout of a `struct` is undefined by default to allow for compiler
3548 optimizations like field reordering, but it can be fixed with the
3549 `#[repr(...)]` attribute. In either case, fields may be given in any order in
3550 a corresponding struct *expression*; the resulting `struct` value will always
3551 have the same memory layout.
3553 The fields of a `struct` may be qualified by [visibility
3554 modifiers](#re-exporting-and-visibility), to allow access to data in a
3555 structure outside a module.
3557 A _tuple struct_ type is just like a structure type, except that the fields are
3560 A _unit-like struct_ type is like a structure type, except that it has no
3561 fields. The one value constructed by the associated [structure
3562 expression](#structure-expressions) is the only value that inhabits such a
3565 ### Enumerated types
3567 An *enumerated type* is a nominal, heterogeneous disjoint union type, denoted
3568 by the name of an [`enum` item](#enumerations). [^enumtype]
3570 [^enumtype]: The `enum` type is analogous to a `data` constructor declaration in
3571 ML, or a *pick ADT* in Limbo.
3573 An [`enum` item](#enumerations) declares both the type and a number of *variant
3574 constructors*, each of which is independently named and takes an optional tuple
3577 New instances of an `enum` can be constructed by calling one of the variant
3578 constructors, in a [call expression](#call-expressions).
3580 Any `enum` value consumes as much memory as the largest variant constructor for
3581 its corresponding `enum` type.
3583 Enum types cannot be denoted *structurally* as types, but must be denoted by
3584 named reference to an [`enum` item](#enumerations).
3588 Nominal types — [enumerations](#enumerated-types) and
3589 [structures](#structure-types) — may be recursive. That is, each `enum`
3590 constructor or `struct` field may refer, directly or indirectly, to the
3591 enclosing `enum` or `struct` type itself. Such recursion has restrictions:
3593 * Recursive types must include a nominal type in the recursion
3594 (not mere [type definitions](#type-definitions),
3595 or other structural types such as [arrays](#array,-and-slice-types) or [tuples](#tuple-types)).
3596 * A recursive `enum` item must have at least one non-recursive constructor
3597 (in order to give the recursion a basis case).
3598 * The size of a recursive type must be finite;
3599 in other words the recursive fields of the type must be [pointer types](#pointer-types).
3600 * Recursive type definitions can cross module boundaries, but not module *visibility* boundaries,
3601 or crate boundaries (in order to simplify the module system and type checker).
3603 An example of a *recursive* type and its use:
3608 Cons(T, Box<List<T>>)
3611 let a: List<i32> = List::Cons(7, Box::new(List::Cons(13, Box::new(List::Nil))));
3616 All pointers in Rust are explicit first-class values. They can be copied,
3617 stored into data structures, and returned from functions. There are two
3618 varieties of pointer in Rust:
3621 : These point to memory _owned by some other value_.
3622 A reference type is written `&type` for some lifetime-variable `f`,
3623 or just `&'a type` when you need an explicit lifetime.
3624 Copying a reference is a "shallow" operation:
3625 it involves only copying the pointer itself.
3626 Releasing a reference typically has no effect on the value it points to,
3627 with the exception of temporary values, which are released when the last
3628 reference to them is released.
3630 * Raw pointers (`*`)
3631 : Raw pointers are pointers without safety or liveness guarantees.
3632 Raw pointers are written as `*const T` or `*mut T`,
3633 for example `*const int` means a raw pointer to an integer.
3634 Copying or dropping a raw pointer has no effect on the lifecycle of any
3635 other value. Dereferencing a raw pointer or converting it to any other
3636 pointer type is an [`unsafe` operation](#unsafe-functions).
3637 Raw pointers are generally discouraged in Rust code;
3638 they exist to support interoperability with foreign code,
3639 and writing performance-critical or low-level functions.
3641 The standard library contains additional 'smart pointer' types beyond references
3646 The function type constructor `fn` forms new function types. A function type
3647 consists of a possibly-empty set of function-type modifiers (such as `unsafe`
3648 or `extern`), a sequence of input types and an output type.
3650 An example of a `fn` type:
3653 fn add(x: i32, y: i32) -> i32 {
3657 let mut x = add(5,7);
3659 type Binop = fn(i32, i32) -> i32;
3660 let bo: Binop = add;
3666 ```{.ebnf .notation}
3667 closure_type := [ 'unsafe' ] [ '<' lifetime-list '>' ] '|' arg-list '|'
3668 [ ':' bound-list ] [ '->' type ]
3669 lifetime-list := lifetime | lifetime ',' lifetime-list
3670 arg-list := ident ':' type | ident ':' type ',' arg-list
3671 bound-list := bound | bound '+' bound-list
3672 bound := path | lifetime
3675 The type of a closure mapping an input of type `A` to an output of type `B` is
3676 `|A| -> B`. A closure with no arguments or return values has type `||`.
3678 An example of creating and calling a closure:
3681 let captured_var = 10;
3683 let closure_no_args = || println!("captured_var={}", captured_var);
3685 let closure_args = |arg: i32| -> i32 {
3686 println!("captured_var={}, arg={}", captured_var, arg);
3687 arg // Note lack of semicolon after 'arg'
3690 fn call_closure<F: Fn(), G: Fn(i32) -> i32>(c1: F, c2: G) {
3695 call_closure(closure_no_args, closure_args);
3701 Every trait item (see [traits](#traits)) defines a type with the same name as
3702 the trait. This type is called the _trait object_ of the trait. Trait objects
3703 permit "late binding" of methods, dispatched using _virtual method tables_
3704 ("vtables"). Whereas most calls to trait methods are "early bound" (statically
3705 resolved) to specific implementations at compile time, a call to a method on an
3706 trait objects is only resolved to a vtable entry at compile time. The actual
3707 implementation for each vtable entry can vary on an object-by-object basis.
3709 Given a pointer-typed expression `E` of type `&T` or `Box<T>`, where `T`
3710 implements trait `R`, casting `E` to the corresponding pointer type `&R` or
3711 `Box<R>` results in a value of the _trait object_ `R`. This result is
3712 represented as a pair of pointers: the vtable pointer for the `T`
3713 implementation of `R`, and the pointer value of `E`.
3715 An example of a trait object:
3719 fn stringify(&self) -> String;
3722 impl Printable for i32 {
3723 fn stringify(&self) -> String { self.to_string() }
3726 fn print(a: Box<Printable>) {
3727 println!("{}", a.stringify());
3731 print(Box::new(10) as Box<Printable>);
3735 In this example, the trait `Printable` occurs as a trait object in both the
3736 type signature of `print`, and the cast expression in `main`.
3740 Within the body of an item that has type parameter declarations, the names of
3741 its type parameters are types:
3744 fn map<A: Clone, B: Clone>(f: |A| -> B, xs: &[A]) -> Vec<B> {
3748 let first: B = f(xs[0].clone());
3749 let mut rest: Vec<B> = map(f, xs.slice(1, xs.len()));
3750 rest.insert(0, first);
3755 Here, `first` has type `B`, referring to `map`'s `B` type parameter; and `rest`
3756 has type `Vec<B>`, a vector type with element type `B`.
3760 The special type `self` has a meaning within methods inside an impl item. It
3761 refers to the type of the implicit `self` argument. For example, in:
3765 fn make_string(&self) -> String;
3768 impl Printable for String {
3769 fn make_string(&self) -> String {
3775 `self` refers to the value of type `String` that is the receiver for a call to
3776 the method `make_string`.
3780 Several traits define special evaluation behavior.
3784 The `Copy` trait changes the semantics of a type implementing it. Values whose
3785 type implements `Copy` are copied rather than moved upon assignment.
3787 ## The `Sized` trait
3789 The `Sized` trait indicates that the size of this type is known at compile-time.
3793 The `Drop` trait provides a destructor, to be run whenever a value of this type
3798 A Rust program's memory consists of a static set of *items* and a *heap*.
3799 Immutable portions of the heap may be safely shared between threads, mutable
3800 portions may not be safely shared, but several mechanisms for effectively-safe
3801 sharing of mutable values, built on unsafe code but enforcing a safe locking
3802 discipline, exist in the standard library.
3804 Allocations in the stack consist of *variables*, and allocations in the heap
3807 ### Memory allocation and lifetime
3809 The _items_ of a program are those functions, modules and types that have their
3810 value calculated at compile-time and stored uniquely in the memory image of the
3811 rust process. Items are neither dynamically allocated nor freed.
3813 The _heap_ is a general term that describes boxes. The lifetime of an
3814 allocation in the heap depends on the lifetime of the box values pointing to
3815 it. Since box values may themselves be passed in and out of frames, or stored
3816 in the heap, heap allocations may outlive the frame they are allocated within.
3818 ### Memory ownership
3820 When a stack frame is exited, its local allocations are all released, and its
3821 references to boxes are dropped.
3825 A _variable_ is a component of a stack frame, either a named function parameter,
3826 an anonymous [temporary](#lvalues,-rvalues-and-temporaries), or a named local
3829 A _local variable_ (or *stack-local* allocation) holds a value directly,
3830 allocated within the stack's memory. The value is a part of the stack frame.
3832 Local variables are immutable unless declared otherwise like: `let mut x = ...`.
3834 Function parameters are immutable unless declared with `mut`. The `mut` keyword
3835 applies only to the following parameter (so `|mut x, y|` and `fn f(mut x:
3836 Box<i32>, y: Box<i32>)` declare one mutable variable `x` and one immutable
3839 Methods that take either `self` or `Box<Self>` can optionally place them in a
3840 mutable variable by prefixing them with `mut` (similar to regular arguments):
3844 fn change(mut self) -> Self;
3845 fn modify(mut self: Box<Self>) -> Box<Self>;
3849 Local variables are not initialized when allocated; the entire frame worth of
3850 local variables are allocated at once, on frame-entry, in an uninitialized
3851 state. Subsequent statements within a function may or may not initialize the
3852 local variables. Local variables can be used only after they have been
3853 initialized; this is enforced by the compiler.
3857 The Rust compiler supports various methods to link crates together both
3858 statically and dynamically. This section will explore the various methods to
3859 link Rust crates together, and more information about native libraries can be
3860 found in the [ffi section of the book][ffi].
3862 In one session of compilation, the compiler can generate multiple artifacts
3863 through the usage of either command line flags or the `crate_type` attribute.
3864 If one or more command line flag is specified, all `crate_type` attributes will
3865 be ignored in favor of only building the artifacts specified by command line.
3867 * `--crate-type=bin`, `#[crate_type = "bin"]` - A runnable executable will be
3868 produced. This requires that there is a `main` function in the crate which
3869 will be run when the program begins executing. This will link in all Rust and
3870 native dependencies, producing a distributable binary.
3872 * `--crate-type=lib`, `#[crate_type = "lib"]` - A Rust library will be produced.
3873 This is an ambiguous concept as to what exactly is produced because a library
3874 can manifest itself in several forms. The purpose of this generic `lib` option
3875 is to generate the "compiler recommended" style of library. The output library
3876 will always be usable by rustc, but the actual type of library may change from
3877 time-to-time. The remaining output types are all different flavors of
3878 libraries, and the `lib` type can be seen as an alias for one of them (but the
3879 actual one is compiler-defined).
3881 * `--crate-type=dylib`, `#[crate_type = "dylib"]` - A dynamic Rust library will
3882 be produced. This is different from the `lib` output type in that this forces
3883 dynamic library generation. The resulting dynamic library can be used as a
3884 dependency for other libraries and/or executables. This output type will
3885 create `*.so` files on linux, `*.dylib` files on osx, and `*.dll` files on
3888 * `--crate-type=staticlib`, `#[crate_type = "staticlib"]` - A static system
3889 library will be produced. This is different from other library outputs in that
3890 the Rust compiler will never attempt to link to `staticlib` outputs. The
3891 purpose of this output type is to create a static library containing all of
3892 the local crate's code along with all upstream dependencies. The static
3893 library is actually a `*.a` archive on linux and osx and a `*.lib` file on
3894 windows. This format is recommended for use in situations such as linking
3895 Rust code into an existing non-Rust application because it will not have
3896 dynamic dependencies on other Rust code.
3898 * `--crate-type=rlib`, `#[crate_type = "rlib"]` - A "Rust library" file will be
3899 produced. This is used as an intermediate artifact and can be thought of as a
3900 "static Rust library". These `rlib` files, unlike `staticlib` files, are
3901 interpreted by the Rust compiler in future linkage. This essentially means
3902 that `rustc` will look for metadata in `rlib` files like it looks for metadata
3903 in dynamic libraries. This form of output is used to produce statically linked
3904 executables as well as `staticlib` outputs.
3906 Note that these outputs are stackable in the sense that if multiple are
3907 specified, then the compiler will produce each form of output at once without
3908 having to recompile. However, this only applies for outputs specified by the
3909 same method. If only `crate_type` attributes are specified, then they will all
3910 be built, but if one or more `--crate-type` command line flag is specified,
3911 then only those outputs will be built.
3913 With all these different kinds of outputs, if crate A depends on crate B, then
3914 the compiler could find B in various different forms throughout the system. The
3915 only forms looked for by the compiler, however, are the `rlib` format and the
3916 dynamic library format. With these two options for a dependent library, the
3917 compiler must at some point make a choice between these two formats. With this
3918 in mind, the compiler follows these rules when determining what format of
3919 dependencies will be used:
3921 1. If a static library is being produced, all upstream dependencies are
3922 required to be available in `rlib` formats. This requirement stems from the
3923 reason that a dynamic library cannot be converted into a static format.
3925 Note that it is impossible to link in native dynamic dependencies to a static
3926 library, and in this case warnings will be printed about all unlinked native
3927 dynamic dependencies.
3929 2. If an `rlib` file is being produced, then there are no restrictions on what
3930 format the upstream dependencies are available in. It is simply required that
3931 all upstream dependencies be available for reading metadata from.
3933 The reason for this is that `rlib` files do not contain any of their upstream
3934 dependencies. It wouldn't be very efficient for all `rlib` files to contain a
3935 copy of `libstd.rlib`!
3937 3. If an executable is being produced and the `-C prefer-dynamic` flag is not
3938 specified, then dependencies are first attempted to be found in the `rlib`
3939 format. If some dependencies are not available in an rlib format, then
3940 dynamic linking is attempted (see below).
3942 4. If a dynamic library or an executable that is being dynamically linked is
3943 being produced, then the compiler will attempt to reconcile the available
3944 dependencies in either the rlib or dylib format to create a final product.
3946 A major goal of the compiler is to ensure that a library never appears more
3947 than once in any artifact. For example, if dynamic libraries B and C were
3948 each statically linked to library A, then a crate could not link to B and C
3949 together because there would be two copies of A. The compiler allows mixing
3950 the rlib and dylib formats, but this restriction must be satisfied.
3952 The compiler currently implements no method of hinting what format a library
3953 should be linked with. When dynamically linking, the compiler will attempt to
3954 maximize dynamic dependencies while still allowing some dependencies to be
3955 linked in via an rlib.
3957 For most situations, having all libraries available as a dylib is recommended
3958 if dynamically linking. For other situations, the compiler will emit a
3959 warning if it is unable to determine which formats to link each library with.
3961 In general, `--crate-type=bin` or `--crate-type=lib` should be sufficient for
3962 all compilation needs, and the other options are just available if more
3963 fine-grained control is desired over the output format of a Rust crate.
3965 # Appendix: Rationales and design tradeoffs
3969 # Appendix: Influences
3971 Rust is not a particularly original language, with design elements coming from
3972 a wide range of sources. Some of these are listed below (including elements
3973 that have since been removed):
3975 * SML, OCaml: algebraic datatypes, pattern matching, type inference,
3976 semicolon statement separation
3977 * C++: references, RAII, smart pointers, move semantics, monomorphisation,
3979 * ML Kit, Cyclone: region based memory management
3980 * Haskell (GHC): typeclasses, type families
3981 * Newsqueak, Alef, Limbo: channels, concurrency
3982 * Erlang: message passing, task failure, ~~linked task failure~~,
3983 ~~lightweight concurrency~~
3984 * Swift: optional bindings
3985 * Scheme: hygienic macros
3987 * Ruby: ~~block syntax~~
3988 * NIL, Hermes: ~~typestate~~
3989 * [Unicode Annex #31](http://www.unicode.org/reports/tr31/): identifier and
3992 [ffi]: book/ffi.html
3993 [plugin]: book/plugins.html