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 codepoints, 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 codepoint `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 codepoints 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 codepoints. 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 codepoints encoded in UTF-8.
95 Most Rust grammar rules are defined in terms of printable ASCII-range
96 codepoints, but a small number are defined in terms of Unicode properties or
97 explicit codepoint 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, ie `/// Foo` turns into `#[doc="/// Foo"]`).
152 `//!` comments apply to the parent of the comment, rather than the item that
153 follows. `//!` comments are usually used to display information on the crate
156 Non-doc comments are interpreted as a form of whitespace.
161 whitespace_char : '\x20' | '\x09' | '\x0a' | '\x0d' ;
162 whitespace : [ whitespace_char | comment ] + ;
165 The `whitespace_char` production is any nonempty Unicode string consisting of
166 any of the following Unicode characters: `U+0020` (space, `' '`), `U+0009`
167 (tab, `'\t'`), `U+000A` (LF, `'\n'`), `U+000D` (CR, `'\r'`).
169 Rust is a "free-form" language, meaning that all forms of whitespace serve only
170 to separate _tokens_ in the grammar, and have no semantic significance.
172 A Rust program has identical meaning if each whitespace element is replaced
173 with any other legal whitespace element, such as a single space character.
178 simple_token : keyword | unop | binop ;
179 token : simple_token | ident | literal | symbol | whitespace token ;
182 Tokens are primitive productions in the grammar defined by regular
183 (non-recursive) languages. "Simple" tokens are given in [string table
184 production](#string-table-productions) form, and occur in the rest of the
185 grammar as double-quoted strings. Other tokens have exact rules given.
189 <p id="keyword-table-marker"></p>
192 |----------|----------|----------|----------|---------|
193 | abstract | alignof | as | become | box |
194 | break | const | continue | crate | do |
195 | else | enum | extern | false | final |
196 | fn | for | if | impl | in |
197 | let | loop | macro | match | mod |
198 | move | mut | offsetof | override | priv |
199 | pub | pure | ref | return | sizeof |
200 | static | self | struct | super | true |
201 | trait | type | typeof | unsafe | unsized |
202 | use | virtual | where | while | yield |
205 Each of these keywords has special meaning in its grammar, and all of them are
206 excluded from the `ident` rule.
208 Note that some of these keywords are reserved, and do not currently do
213 A literal is an expression consisting of a single token, rather than a sequence
214 of tokens, that immediately and directly denotes the value it evaluates to,
215 rather than referring to it by name or some other evaluation rule. A literal is
216 a form of constant expression, so is evaluated (primarily) at compile time.
220 literal : [ string_lit | char_lit | byte_string_lit | byte_lit | num_lit ] lit_suffix ?;
223 The optional suffix is only used for certain numeric literals, but is
224 reserved for future extension, that is, the above gives the lexical
225 grammar, but a Rust parser will reject everything but the 12 special
226 cases mentioned in [Number literals](#number-literals) below.
230 ##### Characters and strings
232 | | Example | `#` sets | Characters | Escapes |
233 |----------------------------------------------|-----------------|------------|-------------|---------------------|
234 | [Character](#character-literals) | `'H'` | `N/A` | All Unicode | `\'` & [Byte](#byte-escapes) & [Unicode](#unicode-escapes) |
235 | [String](#string-literals) | `"hello"` | `N/A` | All Unicode | `\"` & [Byte](#byte-escapes) & [Unicode](#unicode-escapes) |
236 | [Raw](#raw-string-literals) | `r#"hello"#` | `0...` | All Unicode | `N/A` |
237 | [Byte](#byte-literals) | `b'H'` | `N/A` | All ASCII | `\'` & [Byte](#byte-escapes) |
238 | [Byte string](#byte-string-literals) | `b"hello"` | `N/A` | All ASCII | `\"` & [Byte](#byte-escapes) |
239 | [Raw byte string](#raw-byte-string-literals) | `br#"hello"#` | `0...` | All ASCII | `N/A` |
245 | `\x7F` | 8-bit character code (exactly 2 digits) |
247 | `\r` | Carriage return |
251 ##### Unicode escapes
254 | `\u{7FFF}` | 24-bit Unicode character code (up to 6 digits) |
258 | [Number literals](#number-literals)`*` | Example | Exponentiation | Suffixes |
259 |----------------------------------------|---------|----------------|----------|
260 | Decimal integer | `98_222` | `N/A` | Integer suffixes |
261 | Hex integer | `0xff` | `N/A` | Integer suffixes |
262 | Octal integer | `0o77` | `N/A` | Integer suffixes |
263 | Binary integer | `0b1111_0000` | `N/A` | Integer suffixes |
264 | Floating-point | `123.0E+77` | `Optional` | Floating-point suffixes |
266 `*` All number literals allow `_` as a visual separator: `1_234.0E+18f64`
269 | Integer | Floating-point |
270 |---------|----------------|
271 | `u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`, `i64`, `is` (`isize`), `us` (`usize`) | `f32`, `f64` |
273 #### Character and string literals
276 char_lit : '\x27' char_body '\x27' ;
277 string_lit : '"' string_body * '"' | 'r' raw_string ;
279 char_body : non_single_quote
280 | '\x5c' [ '\x27' | common_escape | unicode_escape ] ;
282 string_body : non_double_quote
283 | '\x5c' [ '\x22' | common_escape | unicode_escape ] ;
284 raw_string : '"' raw_string_body '"' | '#' raw_string '#' ;
286 common_escape : '\x5c'
287 | 'n' | 'r' | 't' | '0'
290 unicode_escape : 'u' '{' hex_digit+ 6 '}';
292 hex_digit : 'a' | 'b' | 'c' | 'd' | 'e' | 'f'
293 | 'A' | 'B' | 'C' | 'D' | 'E' | 'F'
295 oct_digit : '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' ;
296 dec_digit : '0' | nonzero_dec ;
297 nonzero_dec: '1' | '2' | '3' | '4'
298 | '5' | '6' | '7' | '8' | '9' ;
301 ##### Character literals
303 A _character literal_ is a single Unicode character enclosed within two
304 `U+0027` (single-quote) characters, with the exception of `U+0027` itself,
305 which must be _escaped_ by a preceding `U+005C` character (`\`).
307 ##### String literals
309 A _string literal_ is a sequence of any Unicode characters enclosed within two
310 `U+0022` (double-quote) characters, with the exception of `U+0022` itself,
311 which must be _escaped_ by a preceding `U+005C` character (`\`), or a _raw
314 A multi-line string literal may be defined by terminating each line with a
315 `U+005C` character (`\`) immediately before the newline. This causes the
316 `U+005C` character, the newline, and all whitespace at the beginning of the
317 next line to be ignored.
327 ##### Character escapes
329 Some additional _escapes_ are available in either character or non-raw string
330 literals. An escape starts with a `U+005C` (`\`) and continues with one of the
333 * An _8-bit codepoint escape_ escape starts with `U+0078` (`x`) and is
334 followed by exactly two _hex digits_. It denotes the Unicode codepoint
335 equal to the provided hex value.
336 * A _24-bit codepoint escape_ starts with `U+0075` (`u`) and is followed
337 by up to six _hex digits_ surrounded by braces `U+007B` (`{`) and `U+007D`
338 (`}`). It denotes the Unicode codepoint equal to the provided hex value.
339 * A _whitespace escape_ is one of the characters `U+006E` (`n`), `U+0072`
340 (`r`), or `U+0074` (`t`), denoting the unicode values `U+000A` (LF),
341 `U+000D` (CR) or `U+0009` (HT) respectively.
342 * The _backslash escape_ is the character `U+005C` (`\`) which must be
343 escaped in order to denote *itself*.
345 ##### Raw string literals
347 Raw string literals do not process any escapes. They start with the character
348 `U+0072` (`r`), followed by zero or more of the character `U+0023` (`#`) and a
349 `U+0022` (double-quote) character. The _raw string body_ is not defined in the
350 EBNF grammar above: it can contain any sequence of Unicode characters and is
351 terminated only by another `U+0022` (double-quote) character, followed by the
352 same number of `U+0023` (`#`) characters that preceded the opening `U+0022`
353 (double-quote) character.
355 All Unicode characters contained in the raw string body represent themselves,
356 the characters `U+0022` (double-quote) (except when followed by at least as
357 many `U+0023` (`#`) characters as were used to start the raw string literal) or
358 `U+005C` (`\`) do not have any special meaning.
360 Examples for string literals:
363 "foo"; r"foo"; // foo
364 "\"foo\""; r#""foo""#; // "foo"
367 r##"foo #"# bar"##; // foo #"# bar
369 "\x52"; "R"; r"R"; // R
370 "\\x52"; r"\x52"; // \x52
373 #### Byte and byte string literals
376 byte_lit : "b\x27" byte_body '\x27' ;
377 byte_string_lit : "b\x22" string_body * '\x22' | "br" raw_byte_string ;
379 byte_body : ascii_non_single_quote
380 | '\x5c' [ '\x27' | common_escape ] ;
382 byte_string_body : ascii_non_double_quote
383 | '\x5c' [ '\x22' | common_escape ] ;
384 raw_byte_string : '"' raw_byte_string_body '"' | '#' raw_byte_string '#' ;
390 A _byte literal_ is a single ASCII character (in the `U+0000` to `U+007F`
391 range) enclosed within two `U+0027` (single-quote) characters, with the
392 exception of `U+0027` itself, which must be _escaped_ by a preceding U+005C
393 character (`\`), or a single _escape_. It is equivalent to a `u8` unsigned
394 8-bit integer _number literal_.
396 ##### Byte string literals
398 A non-raw _byte string literal_ is a sequence of ASCII characters and _escapes_,
399 preceded by the characters `U+0062` (`b`) and `U+0022` (double-quote), and
400 followed by the character `U+0022`. If the character `U+0022` is present within
401 the literal, it must be _escaped_ by a preceding `U+005C` (`\`) character.
402 Alternatively, a byte string literal can be a _raw byte string literal_, defined
403 below. A byte string literal is equivalent to a `&'static [u8]` borrowed array
404 of unsigned 8-bit integers.
406 Some additional _escapes_ are available in either byte or non-raw byte string
407 literals. An escape starts with a `U+005C` (`\`) and continues with one of the
410 * An _byte escape_ escape starts with `U+0078` (`x`) and is
411 followed by exactly two _hex digits_. It denotes the byte
412 equal to the provided hex value.
413 * A _whitespace escape_ is one of the characters `U+006E` (`n`), `U+0072`
414 (`r`), or `U+0074` (`t`), denoting the bytes values `0x0A` (ASCII LF),
415 `0x0D` (ASCII CR) or `0x09` (ASCII HT) respectively.
416 * The _backslash escape_ is the character `U+005C` (`\`) which must be
417 escaped in order to denote its ASCII encoding `0x5C`.
419 ##### Raw byte string literals
421 Raw byte string literals do not process any escapes. They start with the
422 character `U+0062` (`b`), followed by `U+0072` (`r`), followed by zero or more
423 of the character `U+0023` (`#`), and a `U+0022` (double-quote) character. The
424 _raw string body_ is not defined in the EBNF grammar above: it can contain any
425 sequence of ASCII characters and is terminated only by another `U+0022`
426 (double-quote) character, followed by the same number of `U+0023` (`#`)
427 characters that preceded the opening `U+0022` (double-quote) character. A raw
428 byte string literal can not contain any non-ASCII byte.
430 All characters contained in the raw string body represent their ASCII encoding,
431 the characters `U+0022` (double-quote) (except when followed by at least as
432 many `U+0023` (`#`) characters as were used to start the raw string literal) or
433 `U+005C` (`\`) do not have any special meaning.
435 Examples for byte string literals:
438 b"foo"; br"foo"; // foo
439 b"\"foo\""; br#""foo""#; // "foo"
442 br##"foo #"# bar"##; // foo #"# bar
444 b"\x52"; b"R"; br"R"; // R
445 b"\\x52"; br"\x52"; // \x52
451 num_lit : nonzero_dec [ dec_digit | '_' ] * float_suffix ?
452 | '0' [ [ dec_digit | '_' ] * float_suffix ?
453 | 'b' [ '1' | '0' | '_' ] +
454 | 'o' [ oct_digit | '_' ] +
455 | 'x' [ hex_digit | '_' ] + ] ;
457 float_suffix : [ exponent | '.' dec_lit exponent ? ] ? ;
459 exponent : ['E' | 'e'] ['-' | '+' ] ? dec_lit ;
460 dec_lit : [ dec_digit | '_' ] + ;
463 A _number literal_ is either an _integer literal_ or a _floating-point
464 literal_. The grammar for recognizing the two kinds of literals is mixed.
466 ##### Integer literals
468 An _integer literal_ has one of four forms:
470 * A _decimal literal_ starts with a *decimal digit* and continues with any
471 mixture of *decimal digits* and _underscores_.
472 * A _hex literal_ starts with the character sequence `U+0030` `U+0078`
473 (`0x`) and continues as any mixture of hex digits and underscores.
474 * An _octal literal_ starts with the character sequence `U+0030` `U+006F`
475 (`0o`) and continues as any mixture of octal digits and underscores.
476 * A _binary literal_ starts with the character sequence `U+0030` `U+0062`
477 (`0b`) and continues as any mixture of binary digits and underscores.
479 Like any literal, an integer literal may be followed (immediately,
480 without any spaces) by an _integer suffix_, which forcibly sets the
481 type of the literal. The integer suffix must be the name of one of the
482 integral types: `u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`, `i64`,
485 The type of an _unsuffixed_ integer literal is determined by type inference.
486 If an integer type can be _uniquely_ determined from the surrounding program
487 context, the unsuffixed integer literal has that type. If the program context
488 underconstrains the type, it defaults to the signed 32-bit integer `i32`; if
489 the program context overconstrains the type, it is considered a static type
492 Examples of integer literals of various forms:
499 0o70_i16; // type i16
500 0b1111_1111_1001_0000_i32; // type i32
501 0usize; // type usize
504 ##### Floating-point literals
506 A _floating-point literal_ has one of two forms:
508 * A _decimal literal_ followed by a period character `U+002E` (`.`). This is
509 optionally followed by another decimal literal, with an optional _exponent_.
510 * A single _decimal literal_ followed by an _exponent_.
512 By default, a floating-point literal has a generic type, and, like integer
513 literals, the type must be uniquely determined from the context. There are two valid
514 _floating-point suffixes_, `f32` and `f64` (the 32-bit and 64-bit floating point
515 types), which explicitly determine the type of the literal.
517 Examples of floating-point literals of various forms:
520 123.0f64; // type f64
523 12E+99_f64; // type f64
524 let x: f64 = 2.; // type f64
527 This last example is different because it is not possible to use the suffix
528 syntax with a floating point literal ending in a period. `2.f64` would attempt
529 to call a method named `f64` on `2`.
531 The representation semantics of floating-point numbers are described in
532 ["Machine Types"](#machine-types).
534 #### Boolean literals
536 The two values of the boolean type are written `true` and `false`.
542 | '#' | '[' | ']' | '(' | ')' | '{' | '}'
546 Symbols are a general class of printable [token](#tokens) that play structural
547 roles in a variety of grammar productions. They are catalogued here for
548 completeness as the set of remaining miscellaneous printable tokens that do not
549 otherwise appear as [unary operators](#unary-operator-expressions), [binary
550 operators](#binary-operator-expressions), or [keywords](#keywords).
556 expr_path : [ "::" ] ident [ "::" expr_path_tail ] + ;
557 expr_path_tail : '<' type_expr [ ',' type_expr ] + '>'
560 type_path : ident [ type_path_tail ] + ;
561 type_path_tail : '<' type_expr [ ',' type_expr ] + '>'
565 A _path_ is a sequence of one or more path components _logically_ separated by
566 a namespace qualifier (`::`). If a path consists of only one component, it may
567 refer to either an [item](#items) or a [variable](#variables) in a local control
568 scope. If a path has multiple components, it refers to an item.
570 Every item has a _canonical path_ within its crate, but the path naming an item
571 is only meaningful within a given crate. There is no global namespace across
572 crates; an item's canonical path merely identifies it within the crate.
574 Two examples of simple paths consisting of only identifier components:
581 Path components are usually [identifiers](#identifiers), but the trailing
582 component of a path may be an angle-bracket-enclosed list of type arguments. In
583 [expression](#expressions) context, the type argument list is given after a
584 final (`::`) namespace qualifier in order to disambiguate it from a relational
585 expression involving the less-than symbol (`<`). In type expression context,
586 the final namespace qualifier is omitted.
588 Two examples of paths with type arguments:
591 # struct HashMap<K, V>(K,V);
593 # fn id<T>(t: T) -> T { t }
594 type T = HashMap<i32,String>; // Type arguments used in a type expression
595 let x = id::<i32>(10); // Type arguments used in a call expression
599 Paths can be denoted with various leading qualifiers to change the meaning of
602 * Paths starting with `::` are considered to be global paths where the
603 components of the path start being resolved from the crate root. Each
604 identifier in the path must resolve to an item.
612 ::a::foo(); // call a's foo function
618 * Paths starting with the keyword `super` begin resolution relative to the
619 parent module. Each further identifier must resolve to an item.
627 super::a::foo(); // call a's foo function
633 * Paths starting with the keyword `self` begin resolution relative to the
634 current module. Each further identifier must resolve to an item.
646 A number of minor features of Rust are not central enough to have their own
647 syntax, and yet are not implementable as functions. Instead, they are given
648 names, and invoked through a consistent syntax: `some_extension!(...)`.
650 Users of `rustc` can define new syntax extensions in two ways:
652 * [Compiler plugins][plugin] can include arbitrary
653 Rust code that manipulates syntax trees at compile time.
655 * [Macros](book/macros.html) define new syntax in a higher-level,
661 expr_macro_rules : "macro_rules" '!' ident '(' macro_rule * ')' ;
662 macro_rule : '(' matcher * ')' "=>" '(' transcriber * ')' ';' ;
663 matcher : '(' matcher * ')' | '[' matcher * ']'
664 | '{' matcher * '}' | '$' ident ':' ident
665 | '$' '(' matcher * ')' sep_token? [ '*' | '+' ]
666 | non_special_token ;
667 transcriber : '(' transcriber * ')' | '[' transcriber * ']'
668 | '{' transcriber * '}' | '$' ident
669 | '$' '(' transcriber * ')' sep_token? [ '*' | '+' ]
670 | non_special_token ;
673 `macro_rules` allows users to define syntax extension in a declarative way. We
674 call such extensions "macros by example" or simply "macros" — to be distinguished
675 from the "procedural macros" defined in [compiler plugins][plugin].
677 Currently, macros can expand to expressions, statements, items, or patterns.
679 (A `sep_token` is any token other than `*` and `+`. A `non_special_token` is
680 any token other than a delimiter or `$`.)
682 The macro expander looks up macro invocations by name, and tries each macro
683 rule in turn. It transcribes the first successful match. Matching and
684 transcription are closely related to each other, and we will describe them
689 The macro expander matches and transcribes every token that does not begin with
690 a `$` literally, including delimiters. For parsing reasons, delimiters must be
691 balanced, but they are otherwise not special.
693 In the matcher, `$` _name_ `:` _designator_ matches the nonterminal in the Rust
694 syntax named by _designator_. Valid designators are `item`, `block`, `stmt`,
695 `pat`, `expr`, `ty` (type), `ident`, `path`, `tt` (either side of the `=>`
696 in macro rules). In the transcriber, the designator is already known, and so
697 only the name of a matched nonterminal comes after the dollar sign.
699 In both the matcher and transcriber, the Kleene star-like operator indicates
700 repetition. The Kleene star operator consists of `$` and parens, optionally
701 followed by a separator token, followed by `*` or `+`. `*` means zero or more
702 repetitions, `+` means at least one repetition. The parens are not matched or
703 transcribed. On the matcher side, a name is bound to _all_ of the names it
704 matches, in a structure that mimics the structure of the repetition encountered
705 on a successful match. The job of the transcriber is to sort that structure
708 The rules for transcription of these repetitions are called "Macro By Example".
709 Essentially, one "layer" of repetition is discharged at a time, and all of them
710 must be discharged by the time a name is transcribed. Therefore, `( $( $i:ident
711 ),* ) => ( $i )` is an invalid macro, but `( $( $i:ident ),* ) => ( $( $i:ident
712 ),* )` is acceptable (if trivial).
714 When Macro By Example encounters a repetition, it examines all of the `$`
715 _name_ s that occur in its body. At the "current layer", they all must repeat
716 the same number of times, so ` ( $( $i:ident ),* ; $( $j:ident ),* ) => ( $(
717 ($i,$j) ),* )` is valid if given the argument `(a,b,c ; d,e,f)`, but not
718 `(a,b,c ; d,e)`. The repetition walks through the choices at that layer in
719 lockstep, so the former input transcribes to `( (a,d), (b,e), (c,f) )`.
721 Nested repetitions are allowed.
723 ### Parsing limitations
725 The parser used by the macro system is reasonably powerful, but the parsing of
726 Rust syntax is restricted in two ways:
728 1. The parser will always parse as much as possible. If it attempts to match
729 `$i:expr [ , ]` against `8 [ , ]`, it will attempt to parse `i` as an array
730 index operation and fail. Adding a separator can solve this problem.
731 2. The parser must have eliminated all ambiguity by the time it reaches a `$`
732 _name_ `:` _designator_. This requirement most often affects name-designator
733 pairs when they occur at the beginning of, or immediately after, a `$(...)*`;
734 requiring a distinctive token in front can solve the problem.
736 # Crates and source files
738 Rust is a *compiled* language. Its semantics obey a *phase distinction* between
739 compile-time and run-time. Those semantic rules that have a *static
740 interpretation* govern the success or failure of compilation. Those semantics
741 that have a *dynamic interpretation* govern the behavior of the program at
744 The compilation model centers on artifacts called _crates_. Each compilation
745 processes a single crate in source form, and if successful, produces a single
746 crate in binary form: either an executable or a library.[^cratesourcefile]
748 [^cratesourcefile]: A crate is somewhat analogous to an *assembly* in the
749 ECMA-335 CLI model, a *library* in the SML/NJ Compilation Manager, a *unit*
750 in the Owens and Flatt module system, or a *configuration* in Mesa.
752 A _crate_ is a unit of compilation and linking, as well as versioning,
753 distribution and runtime loading. A crate contains a _tree_ of nested
754 [module](#modules) scopes. The top level of this tree is a module that is
755 anonymous (from the point of view of paths within the module) and any item
756 within a crate has a canonical [module path](#paths) denoting its location
757 within the crate's module tree.
759 The Rust compiler is always invoked with a single source file as input, and
760 always produces a single output crate. The processing of that source file may
761 result in other source files being loaded as modules. Source files have the
764 A Rust source file describes a module, the name and location of which —
765 in the module tree of the current crate — are defined from outside the
766 source file: either by an explicit `mod_item` in a referencing source file, or
767 by the name of the crate itself.
769 Each source file contains a sequence of zero or more `item` definitions, and
770 may optionally begin with any number of `attributes` that apply to the
771 containing module. Attributes on the anonymous crate module define important
772 metadata that influences the behavior of the compiler.
776 #![crate_name = "projx"]
778 // Specify the output type
779 #![crate_type = "lib"]
782 #![warn(non_camel_case_types)]
785 A crate that contains a `main` function can be compiled to an executable. If a
786 `main` function is present, its return type must be [`unit`](#primitive-types)
787 and it must take no arguments.
789 # Items and attributes
791 Crates contain [items](#items), each of which may have some number of
792 [attributes](#attributes) attached to it.
797 item : extern_crate_decl | use_decl | mod_item | fn_item | type_item
798 | struct_item | enum_item | static_item | trait_item | impl_item
802 An _item_ is a component of a crate. Items are organized within a crate by a
803 nested set of [modules](#modules). Every crate has a single "outermost"
804 anonymous module; all further items within the crate have [paths](#paths)
805 within the module tree of the crate.
807 Items are entirely determined at compile-time, generally remain fixed during
808 execution, and may reside in read-only memory.
810 There are several kinds of item:
812 * [`extern crate` declarations](#extern-crate-declarations)
813 * [`use` declarations](#use-declarations)
814 * [modules](#modules)
815 * [functions](#functions)
816 * [type definitions](#type-definitions)
817 * [structures](#structures)
818 * [enumerations](#enumerations)
819 * [static items](#static-items)
821 * [implementations](#implementations)
823 Some items form an implicit scope for the declaration of sub-items. In other
824 words, within a function or module, declarations of items can (in many cases)
825 be mixed with the statements, control blocks, and similar artifacts that
826 otherwise compose the item body. The meaning of these scoped items is the same
827 as if the item was declared outside the scope — it is still a static item
828 — except that the item's *path name* within the module namespace is
829 qualified by the name of the enclosing item, or is private to the enclosing
830 item (in the case of functions). The grammar specifies the exact locations in
831 which sub-item declarations may appear.
835 All items except modules may be *parameterized* by type. Type parameters are
836 given as a comma-separated list of identifiers enclosed in angle brackets
837 (`<...>`), after the name of the item and before its definition. The type
838 parameters of an item are considered "part of the name", not part of the type
839 of the item. A referencing [path](#paths) must (in principle) provide type
840 arguments as a list of comma-separated types enclosed within angle brackets, in
841 order to refer to the type-parameterized item. In practice, the type-inference
842 system can usually infer such argument types from context. There are no
843 general type-parametric types, only type-parametric items. That is, Rust has
844 no notion of type abstraction: there are no first-class "forall" types.
849 mod_item : "mod" ident ( ';' | '{' mod '}' );
853 A module is a container for zero or more [items](#items).
855 A _module item_ is a module, surrounded in braces, named, and prefixed with the
856 keyword `mod`. A module item introduces a new, named module into the tree of
857 modules making up a crate. Modules can nest arbitrarily.
859 An example of a module:
863 type Complex = (f64, f64);
864 fn sin(f: f64) -> f64 {
868 fn cos(f: f64) -> f64 {
872 fn tan(f: f64) -> f64 {
879 Modules and types share the same namespace. Declaring a named type with
880 the same name as a module in scope is forbidden: that is, a type definition,
881 trait, struct, enumeration, or type parameter can't shadow the name of a module
882 in scope, or vice versa.
884 A module without a body is loaded from an external file, by default with the
885 same name as the module, plus the `.rs` extension. When a nested submodule is
886 loaded from an external file, it is loaded from a subdirectory path that
887 mirrors the module hierarchy.
890 // Load the `vec` module from `vec.rs`
894 // Load the `local_data` module from `thread/local_data.rs`
899 The directories and files used for loading external file modules can be
900 influenced with the `path` attribute.
903 #[path = "thread_files"]
905 // Load the `local_data` module from `thread_files/tls.rs`
911 ##### Extern crate declarations
914 extern_crate_decl : "extern" "crate" crate_name
915 crate_name: ident | ( string_lit "as" ident )
918 An _`extern crate` declaration_ specifies a dependency on an external crate.
919 The external crate is then bound into the declaring scope as the `ident`
920 provided in the `extern_crate_decl`.
922 The external crate is resolved to a specific `soname` at compile time, and a
923 runtime linkage requirement to that `soname` is passed to the linker for
924 loading at runtime. The `soname` is resolved at compile time by scanning the
925 compiler's library path and matching the optional `crateid` provided as a
926 string literal against the `crateid` attributes that were declared on the
927 external crate when it was compiled. If no `crateid` is provided, a default
928 `name` attribute is assumed, equal to the `ident` given in the
931 Three examples of `extern crate` declarations:
936 extern crate std; // equivalent to: extern crate std as std;
938 extern crate std as ruststd; // linking to 'std' under another name
941 ##### Use declarations
944 use_decl : "pub" ? "use" [ path "as" ident
947 path_glob : ident [ "::" [ path_glob
949 | '{' path_item [ ',' path_item ] * '}' ;
951 path_item : ident | "self" ;
954 A _use declaration_ creates one or more local name bindings synonymous with
955 some other [path](#paths). Usually a `use` declaration is used to shorten the
956 path required to refer to a module item. These declarations may appear at the
957 top of [modules](#modules) and [blocks](#blocks).
959 > **Note**: Unlike in many languages,
960 > `use` declarations in Rust do *not* declare linkage dependency with external crates.
961 > Rather, [`extern crate` declarations](#extern-crate-declarations) declare linkage dependencies.
963 Use declarations support a number of convenient shortcuts:
965 * Rebinding the target name as a new local name, using the syntax `use p::q::r as x;`
966 * Simultaneously binding a list of paths differing only in their final element,
967 using the glob-like brace syntax `use a::b::{c,d,e,f};`
968 * Binding all paths matching a given prefix, using the asterisk wildcard syntax
970 * Simultaneously binding a list of paths differing only in their final element
971 and their immediate parent module, using the `self` keyword, such as
972 `use a::b::{self, c, d};`
974 An example of `use` declarations:
978 use std::option::Option::{Some, None};
979 use std::collections::hash_map::{self, HashMap};
982 fn bar(map1: HashMap<String, usize>, map2: hash_map::HashMap<String, usize>){}
985 // Equivalent to 'foo(vec![std::option::Option::Some(1.0f64),
986 // std::option::Option::None]);'
987 foo(vec![Some(1.0f64), None]);
989 // Both `hash_map` and `HashMap` are in scope.
990 let map1 = HashMap::new();
991 let map2 = hash_map::HashMap::new();
996 Like items, `use` declarations are private to the containing module, by
997 default. Also like items, a `use` declaration can be public, if qualified by
998 the `pub` keyword. Such a `use` declaration serves to _re-export_ a name. A
999 public `use` declaration can therefore _redirect_ some public name to a
1000 different target definition: even a definition with a private canonical path,
1001 inside a different module. If a sequence of such redirections form a cycle or
1002 cannot be resolved unambiguously, they represent a compile-time error.
1004 An example of re-exporting:
1009 pub use quux::foo::{bar, baz};
1018 In this example, the module `quux` re-exports two public names defined in
1021 Also note that the paths contained in `use` items are relative to the crate
1022 root. So, in the previous example, the `use` refers to `quux::foo::{bar,
1023 baz}`, and not simply to `foo::{bar, baz}`. This also means that top-level
1024 module declarations should be at the crate root if direct usage of the declared
1025 modules within `use` items is desired. It is also possible to use `self` and
1026 `super` at the beginning of a `use` item to refer to the current and direct
1027 parent modules respectively. All rules regarding accessing declared modules in
1028 `use` declarations applies to both module declarations and `extern crate`
1031 An example of what will and will not work for `use` items:
1035 # #![allow(unused_imports)]
1036 use foo::core::iter; // good: foo is at the root of the crate
1037 use foo::baz::foobaz; // good: foo is at the root of the crate
1042 use foo::core::iter; // good: foo is at crate root
1043 // use core::iter; // bad: core is not at the crate root
1044 use self::baz::foobaz; // good: self refers to module 'foo'
1045 use foo::bar::foobar; // good: foo is at crate root
1052 use super::bar::foobar; // good: super refers to module 'foo'
1062 A _function item_ defines a sequence of [statements](#statements) and an
1063 optional final [expression](#expressions), along with a name and a set of
1064 parameters. Functions are declared with the keyword `fn`. Functions declare a
1065 set of *input* [*variables*](#variables) as parameters, through which the caller
1066 passes arguments into the function, and the *output* [*type*](#types)
1067 of the value the function will return to its caller on completion.
1069 A function may also be copied into a first-class *value*, in which case the
1070 value has the corresponding [*function type*](#function-types), and can be used
1071 otherwise exactly as a function item (with a minor additional cost of calling
1072 the function indirectly).
1074 Every control path in a function logically ends with a `return` expression or a
1075 diverging expression. If the outermost block of a function has a
1076 value-producing expression in its final-expression position, that expression is
1077 interpreted as an implicit `return` expression applied to the final-expression.
1079 An example of a function:
1082 fn add(x: i32, y: i32) -> i32 {
1087 As with `let` bindings, function arguments are irrefutable patterns, so any
1088 pattern that is valid in a let binding is also valid as an argument.
1091 fn first((value, _): (i32, i32)) -> i32 { value }
1095 #### Generic functions
1097 A _generic function_ allows one or more _parameterized types_ to appear in its
1098 signature. Each type parameter must be explicitly declared, in an
1099 angle-bracket-enclosed, comma-separated list following the function name.
1102 fn iter<T>(seq: &[T], f: |T|) {
1103 for elt in seq.iter() { f(elt); }
1105 fn map<T, U>(seq: &[T], f: |T| -> U) -> Vec<U> {
1106 let mut acc = vec![];
1107 for elt in seq.iter() { acc.push(f(elt)); }
1112 Inside the function signature and body, the name of the type parameter can be
1113 used as a type name.
1115 When a generic function is referenced, its type is instantiated based on the
1116 context of the reference. For example, calling the `iter` function defined
1117 above on `[1, 2]` will instantiate type parameter `T` with `i32`, and require
1118 the closure parameter to have type `fn(i32)`.
1120 The type parameters can also be explicitly supplied in a trailing
1121 [path](#paths) component after the function name. This might be necessary if
1122 there is not sufficient context to determine the type parameters. For example,
1123 `mem::size_of::<u32>() == 4`.
1125 Since a parameter type is opaque to the generic function, the set of operations
1126 that can be performed on it is limited. Values of parameter type can only be
1130 fn id<T>(x: T) -> T { x }
1133 Similarly, [trait](#traits) bounds can be specified for type parameters to
1134 allow methods with that trait to be called on values of that type.
1138 Unsafe operations are those that potentially violate the memory-safety
1139 guarantees of Rust's static semantics.
1141 The following language level features cannot be used in the safe subset of
1144 - Dereferencing a [raw pointer](#pointer-types).
1145 - Reading or writing a [mutable static variable](#mutable-statics).
1146 - Calling an unsafe function (including an intrinsic or foreign function).
1148 ##### Unsafe functions
1150 Unsafe functions are functions that are not safe in all contexts and/or for all
1151 possible inputs. Such a function must be prefixed with the keyword `unsafe` and
1152 can only be called from an `unsafe` block or another `unsafe` function.
1156 A block of code can be prefixed with the `unsafe` keyword, to permit calling
1157 `unsafe` functions or dereferencing raw pointers within a safe function.
1159 When a programmer has sufficient conviction that a sequence of potentially
1160 unsafe operations is actually safe, they can encapsulate that sequence (taken
1161 as a whole) within an `unsafe` block. The compiler will consider uses of such
1162 code safe, in the surrounding context.
1164 Unsafe blocks are used to wrap foreign libraries, make direct use of hardware
1165 or implement features not directly present in the language. For example, Rust
1166 provides the language features necessary to implement memory-safe concurrency
1167 in the language but the implementation of threads and message passing is in the
1170 Rust's type system is a conservative approximation of the dynamic safety
1171 requirements, so in some cases there is a performance cost to using safe code.
1172 For example, a doubly-linked list is not a tree structure and can only be
1173 represented with reference-counted pointers in safe code. By using `unsafe`
1174 blocks to represent the reverse links as raw pointers, it can be implemented
1177 ##### Behavior considered undefined
1179 The following is a list of behavior which is forbidden in all Rust code,
1180 including within `unsafe` blocks and `unsafe` functions. Type checking provides
1181 the guarantee that these issues are never caused by safe code.
1184 * Dereferencing a null/dangling raw pointer
1185 * Reads of [undef](http://llvm.org/docs/LangRef.html#undefined-values)
1186 (uninitialized) memory
1187 * Breaking the [pointer aliasing
1188 rules](http://llvm.org/docs/LangRef.html#pointer-aliasing-rules)
1189 with raw pointers (a subset of the rules used by C)
1190 * `&mut` and `&` follow LLVM’s scoped [noalias] model, except if the `&T`
1191 contains an `UnsafeCell<U>`. Unsafe code must not violate these aliasing
1193 * Mutating an immutable value/reference without `UnsafeCell<U>`
1194 * Invoking undefined behavior via compiler intrinsics:
1195 * Indexing outside of the bounds of an object with `std::ptr::offset`
1196 (`offset` intrinsic), with
1197 the exception of one byte past the end which is permitted.
1198 * Using `std::ptr::copy_nonoverlapping_memory` (`memcpy32`/`memcpy64`
1199 intrinsics) on overlapping buffers
1200 * Invalid values in primitive types, even in private fields/locals:
1201 * Dangling/null references or boxes
1202 * A value other than `false` (0) or `true` (1) in a `bool`
1203 * A discriminant in an `enum` not included in the type definition
1204 * A value in a `char` which is a surrogate or above `char::MAX`
1205 * Non-UTF-8 byte sequences in a `str`
1206 * Unwinding into Rust from foreign code or unwinding from Rust into foreign
1207 code. Rust's failure system is not compatible with exception handling in
1208 other languages. Unwinding must be caught and handled at FFI boundaries.
1210 [noalias]: http://llvm.org/docs/LangRef.html#noalias
1212 ##### Behaviour not considered unsafe
1214 This is a list of behaviour not considered *unsafe* in Rust terms, but that may
1218 * Reading data from private fields (`std::repr`)
1219 * Leaks due to reference count cycles, even in the global heap
1220 * Exiting without calling destructors
1222 * Accessing/modifying the file system
1223 * Unsigned integer overflow (well-defined as wrapping)
1224 * Signed integer overflow (well-defined as two’s complement representation
1227 #### Diverging functions
1229 A special kind of function can be declared with a `!` character where the
1230 output type would normally be. For example:
1233 fn my_err(s: &str) -> ! {
1239 We call such functions "diverging" because they never return a value to the
1240 caller. Every control path in a diverging function must end with a `panic!()` or
1241 a call to another diverging function on every control path. The `!` annotation
1242 does *not* denote a type.
1244 It might be necessary to declare a diverging function because as mentioned
1245 previously, the typechecker checks that every control path in a function ends
1246 with a [`return`](#return-expressions) or diverging expression. So, if `my_err`
1247 were declared without the `!` annotation, the following code would not
1251 # fn my_err(s: &str) -> ! { panic!() }
1253 fn f(i: i32) -> i32 {
1258 my_err("Bad number!");
1263 This will not compile without the `!` annotation on `my_err`, since the `else`
1264 branch of the conditional in `f` does not return an `i32`, as required by the
1265 signature of `f`. Adding the `!` annotation to `my_err` informs the
1266 typechecker that, should control ever enter `my_err`, no further type judgments
1267 about `f` need to hold, since control will never resume in any context that
1268 relies on those judgments. Thus the return type on `f` only needs to reflect
1269 the `if` branch of the conditional.
1271 #### Extern functions
1273 Extern functions are part of Rust's foreign function interface, providing the
1274 opposite functionality to [external blocks](#external-blocks). Whereas
1275 external blocks allow Rust code to call foreign code, extern functions with
1276 bodies defined in Rust code _can be called by foreign code_. They are defined
1277 in the same way as any other Rust function, except that they have the `extern`
1281 // Declares an extern fn, the ABI defaults to "C"
1282 extern fn new_i32() -> i32 { 0 }
1284 // Declares an extern fn with "stdcall" ABI
1285 extern "stdcall" fn new_i32_stdcall() -> i32 { 0 }
1288 Unlike normal functions, extern fns have an `extern "ABI" fn()`. This is the
1289 same type as the functions declared in an extern block.
1292 # extern fn new_i32() -> i32 { 0 }
1293 let fptr: extern "C" fn() -> i32 = new_i32;
1296 Extern functions may be called directly from Rust code as Rust uses large,
1297 contiguous stack segments like C.
1301 A _type alias_ defines a new name for an existing [type](#types). Type
1302 aliases are declared with the keyword `type`. Every value has a single,
1303 specific type, but may implement several different traits, or be compatible with
1304 several different type constraints.
1306 For example, the following defines the type `Point` as a synonym for the type
1307 `(u8, u8)`, the type of pairs of unsigned 8 bit integers.:
1310 type Point = (u8, u8);
1311 let p: Point = (41, 68);
1316 A _structure_ is a nominal [structure type](#structure-types) defined with the
1319 An example of a `struct` item and its use:
1322 struct Point {x: i32, y: i32}
1323 let p = Point {x: 10, y: 11};
1327 A _tuple structure_ is a nominal [tuple type](#tuple-types), also defined with
1328 the keyword `struct`. For example:
1331 struct Point(i32, i32);
1332 let p = Point(10, 11);
1333 let px: i32 = match p { Point(x, _) => x };
1336 A _unit-like struct_ is a structure without any fields, defined by leaving off
1337 the list of fields entirely. Such types will have a single value, just like
1338 the [unit value `()`](#unit-and-boolean-literals) of the unit type. For
1343 let c = [Cookie, Cookie, Cookie, Cookie];
1346 The precise memory layout of a structure is not specified. One can specify a
1347 particular layout using the [`repr` attribute](#ffi-attributes).
1351 An _enumeration_ is a simultaneous definition of a nominal [enumerated
1352 type](#enumerated-types) as well as a set of *constructors*, that can be used
1353 to create or pattern-match values of the corresponding enumerated type.
1355 Enumerations are declared with the keyword `enum`.
1357 An example of an `enum` item and its use:
1365 let mut a: Animal = Animal::Dog;
1369 Enumeration constructors can have either named or unnamed fields:
1372 # #![feature(struct_variant)]
1376 Cat { name: String, weight: f64 }
1379 let mut a: Animal = Animal::Dog("Cocoa".to_string(), 37.2);
1380 a = Animal::Cat { name: "Spotty".to_string(), weight: 2.7 };
1384 In this example, `Cat` is a _struct-like enum variant_,
1385 whereas `Dog` is simply called an enum variant.
1387 Enums have a discriminant. You can assign them explicitly:
1395 If a discriminant isn't assigned, they start at zero, and add one for each
1398 You can cast an enum to get this value:
1401 # enum Foo { Bar = 123 }
1402 let x = Foo::Bar as u32; // x is now 123u32
1405 This only works as long as none of the variants have data attached. If
1406 it were `Bar(i32)`, this is disallowed.
1411 const_item : "const" ident ':' type '=' expr ';' ;
1414 A *constant item* is a named _constant value_ which is not associated with a
1415 specific memory location in the program. Constants are essentially inlined
1416 wherever they are used, meaning that they are copied directly into the relevant
1417 context when used. References to the same constant are not necessarily
1418 guaranteed to refer to the same memory address.
1420 Constant values must not have destructors, and otherwise permit most forms of
1421 data. Constants may refer to the address of other constants, in which case the
1422 address will have the `static` lifetime. The compiler is, however, still at
1423 liberty to translate the constant many times, so the address referred to may not
1426 Constants must be explicitly typed. The type may be `bool`, `char`, a number, or
1427 a type derived from those primitive types. The derived types are references with
1428 the `static` lifetime, fixed-size arrays, tuples, enum variants, and structs.
1431 const BIT1: u32 = 1 << 0;
1432 const BIT2: u32 = 1 << 1;
1434 const BITS: [u32; 2] = [BIT1, BIT2];
1435 const STRING: &'static str = "bitstring";
1437 struct BitsNStrings<'a> {
1442 const BITS_N_STRINGS: BitsNStrings<'static> = BitsNStrings {
1451 static_item : "static" ident ':' type '=' expr ';' ;
1454 A *static item* is similar to a *constant*, except that it represents a precise
1455 memory location in the program. A static is never "inlined" at the usage site,
1456 and all references to it refer to the same memory location. Static items have
1457 the `static` lifetime, which outlives all other lifetimes in a Rust program.
1458 Static items may be placed in read-only memory if they do not contain any
1459 interior mutability.
1461 Statics may contain interior mutability through the `UnsafeCell` language item.
1462 All access to a static is safe, but there are a number of restrictions on
1465 * Statics may not contain any destructors.
1466 * The types of static values must ascribe to `Sync` to allow threadsafe access.
1467 * Statics may not refer to other statics by value, only by reference.
1468 * Constants cannot refer to statics.
1470 Constants should in general be preferred over statics, unless large amounts of
1471 data are being stored, or single-address and mutability properties are required.
1473 #### Mutable statics
1475 If a static item is declared with the `mut` keyword, then it is allowed to
1476 be modified by the program. One of Rust's goals is to make concurrency bugs
1477 hard to run into, and this is obviously a very large source of race conditions
1478 or other bugs. For this reason, an `unsafe` block is required when either
1479 reading or writing a mutable static variable. Care should be taken to ensure
1480 that modifications to a mutable static are safe with respect to other threads
1481 running in the same process.
1483 Mutable statics are still very useful, however. They can be used with C
1484 libraries and can also be bound from C libraries (in an `extern` block).
1487 # fn atomic_add(_: &mut u32, _: u32) -> u32 { 2 }
1489 static mut LEVELS: u32 = 0;
1491 // This violates the idea of no shared state, and this doesn't internally
1492 // protect against races, so this function is `unsafe`
1493 unsafe fn bump_levels_unsafe1() -> u32 {
1499 // Assuming that we have an atomic_add function which returns the old value,
1500 // this function is "safe" but the meaning of the return value may not be what
1501 // callers expect, so it's still marked as `unsafe`
1502 unsafe fn bump_levels_unsafe2() -> u32 {
1503 return atomic_add(&mut LEVELS, 1);
1507 Mutable statics have the same restrictions as normal statics, except that the
1508 type of the value is not required to ascribe to `Sync`.
1512 A _trait_ describes a set of method types.
1514 Traits can include default implementations of methods, written in terms of some
1515 unknown [`self` type](#self-types); the `self` type may either be completely
1516 unspecified, or constrained by some other trait.
1518 Traits are implemented for specific types through separate
1519 [implementations](#implementations).
1522 # type Surface = i32;
1523 # type BoundingBox = i32;
1525 fn draw(&self, Surface);
1526 fn bounding_box(&self) -> BoundingBox;
1530 This defines a trait with two methods. All values that have
1531 [implementations](#implementations) of this trait in scope can have their
1532 `draw` and `bounding_box` methods called, using `value.bounding_box()`
1533 [syntax](#method-call-expressions).
1535 Type parameters can be specified for a trait to make it generic. These appear
1536 after the trait name, using the same syntax used in [generic
1537 functions](#generic-functions).
1541 fn len(&self) -> u32;
1542 fn elt_at(&self, n: u32) -> T;
1543 fn iter<F>(&self, F) where F: Fn(T);
1547 Generic functions may use traits as _bounds_ on their type parameters. This
1548 will have two effects: only types that have the trait may instantiate the
1549 parameter, and within the generic function, the methods of the trait can be
1550 called on values that have the parameter's type. For example:
1553 # type Surface = i32;
1554 # trait Shape { fn draw(&self, Surface); }
1555 fn draw_twice<T: Shape>(surface: Surface, sh: T) {
1561 Traits also define an [object type](#object-types) with the same name as the
1562 trait. Values of this type are created by [casting](#type-cast-expressions)
1563 pointer values (pointing to a type for which an implementation of the given
1564 trait is in scope) to pointers to the trait name, used as a type.
1567 # trait Shape { fn dummy(&self) { } }
1568 # impl Shape for i32 { }
1569 # let mycircle = 0i32;
1570 let myshape: Box<Shape> = Box::new(mycircle) as Box<Shape>;
1573 The resulting value is a box containing the value that was cast, along with
1574 information that identifies the methods of the implementation that was used.
1575 Values with a trait type can have [methods called](#method-call-expressions) on
1576 them, for any method in the trait, and can be used to instantiate type
1577 parameters that are bounded by the trait.
1579 Trait methods may be static, which means that they lack a `self` argument.
1580 This means that they can only be called with function call syntax (`f(x)`) and
1581 not method call syntax (`obj.f()`). The way to refer to the name of a static
1582 method is to qualify it with the trait name, treating the trait name like a
1583 module. For example:
1587 fn from_i32(n: i32) -> Self;
1590 fn from_i32(n: i32) -> f64 { n as f64 }
1592 let x: f64 = Num::from_i32(42);
1595 Traits may inherit from other traits. For example, in
1598 trait Shape { fn area(&self) -> f64; }
1599 trait Circle : Shape { fn radius(&self) -> f64; }
1602 the syntax `Circle : Shape` means that types that implement `Circle` must also
1603 have an implementation for `Shape`. Multiple supertraits are separated by `+`,
1604 `trait Circle : Shape + PartialEq { }`. In an implementation of `Circle` for a
1605 given type `T`, methods can refer to `Shape` methods, since the typechecker
1606 checks that any type with an implementation of `Circle` also has an
1607 implementation of `Shape`.
1609 In type-parameterized functions, methods of the supertrait may be called on
1610 values of subtrait-bound type parameters. Referring to the previous example of
1611 `trait Circle : Shape`:
1614 # trait Shape { fn area(&self) -> f64; }
1615 # trait Circle : Shape { fn radius(&self) -> f64; }
1616 fn radius_times_area<T: Circle>(c: T) -> f64 {
1617 // `c` is both a Circle and a Shape
1618 c.radius() * c.area()
1622 Likewise, supertrait methods may also be called on trait objects.
1625 # trait Shape { fn area(&self) -> f64; }
1626 # trait Circle : Shape { fn radius(&self) -> f64; }
1627 # impl Shape for i32 { fn area(&self) -> f64 { 0.0 } }
1628 # impl Circle for i32 { fn radius(&self) -> f64 { 0.0 } }
1629 # let mycircle = 0i32;
1630 let mycircle = Box::new(mycircle) as Box<Circle>;
1631 let nonsense = mycircle.radius() * mycircle.area();
1636 An _implementation_ is an item that implements a [trait](#traits) for a
1639 Implementations are defined with the keyword `impl`.
1642 # #[derive(Copy, Clone)]
1643 # struct Point {x: f64, y: f64};
1644 # type Surface = i32;
1645 # struct BoundingBox {x: f64, y: f64, width: f64, height: f64};
1646 # trait Shape { fn draw(&self, Surface); fn bounding_box(&self) -> BoundingBox; }
1647 # fn do_draw_circle(s: Surface, c: Circle) { }
1653 impl Copy for Circle {}
1655 impl Clone for Circle {
1656 fn clone(&self) -> Circle { *self }
1659 impl Shape for Circle {
1660 fn draw(&self, s: Surface) { do_draw_circle(s, *self); }
1661 fn bounding_box(&self) -> BoundingBox {
1662 let r = self.radius;
1663 BoundingBox{x: self.center.x - r, y: self.center.y - r,
1664 width: 2.0 * r, height: 2.0 * r}
1669 It is possible to define an implementation without referring to a trait. The
1670 methods in such an implementation can only be used as direct calls on the
1671 values of the type that the implementation targets. In such an implementation,
1672 the trait type and `for` after `impl` are omitted. Such implementations are
1673 limited to nominal types (enums, structs), and the implementation must appear
1674 in the same module or a sub-module as the `self` type:
1677 struct Point {x: i32, y: i32}
1681 println!("Point is at ({}, {})", self.x, self.y);
1685 let my_point = Point {x: 10, y:11};
1689 When a trait _is_ specified in an `impl`, all methods declared as part of the
1690 trait must be implemented, with matching types and type parameter counts.
1692 An implementation can take type parameters, which can be different from the
1693 type parameters taken by the trait it implements. Implementation parameters
1694 are written after the `impl` keyword.
1697 # trait Seq<T> { fn dummy(&self, _: T) { } }
1698 impl<T> Seq<T> for Vec<T> {
1701 impl Seq<bool> for u32 {
1702 /* Treat the integer as a sequence of bits */
1709 extern_block_item : "extern" '{' extern_block '}' ;
1710 extern_block : [ foreign_fn ] * ;
1713 External blocks form the basis for Rust's foreign function interface.
1714 Declarations in an external block describe symbols in external, non-Rust
1717 Functions within external blocks are declared in the same way as other Rust
1718 functions, with the exception that they may not have a body and are instead
1719 terminated by a semicolon.
1724 use libc::{c_char, FILE};
1727 fn fopen(filename: *const c_char, mode: *const c_char) -> *mut FILE;
1732 Functions within external blocks may be called by Rust code, just like
1733 functions defined in Rust. The Rust compiler automatically translates between
1734 the Rust ABI and the foreign ABI.
1736 A number of [attributes](#attributes) control the behavior of external blocks.
1738 By default external blocks assume that the library they are calling uses the
1739 standard C "cdecl" ABI. Other ABIs may be specified using an `abi` string, as
1743 // Interface to the Windows API
1744 extern "stdcall" { }
1747 The `link` attribute allows the name of the library to be specified. When
1748 specified the compiler will attempt to link against the native library of the
1752 #[link(name = "crypto")]
1756 The type of a function declared in an extern block is `extern "abi" fn(A1, ...,
1757 An) -> R`, where `A1...An` are the declared types of its arguments and `R` is
1758 the declared return type.
1760 ## Visibility and Privacy
1762 These two terms are often used interchangeably, and what they are attempting to
1763 convey is the answer to the question "Can this item be used at this location?"
1765 Rust's name resolution operates on a global hierarchy of namespaces. Each level
1766 in the hierarchy can be thought of as some item. The items are one of those
1767 mentioned above, but also include external crates. Declaring or defining a new
1768 module can be thought of as inserting a new tree into the hierarchy at the
1769 location of the definition.
1771 To control whether interfaces can be used across modules, Rust checks each use
1772 of an item to see whether it should be allowed or not. This is where privacy
1773 warnings are generated, or otherwise "you used a private item of another module
1774 and weren't allowed to."
1776 By default, everything in Rust is *private*, with one exception. Enum variants
1777 in a `pub` enum are also public by default. You are allowed to alter this
1778 default visibility with the `priv` keyword. When an item is declared as `pub`,
1779 it can be thought of as being accessible to the outside world. For example:
1783 // Declare a private struct
1786 // Declare a public struct with a private field
1791 // Declare a public enum with two public variants
1793 PubliclyAccessibleState,
1794 PubliclyAccessibleState2,
1798 With the notion of an item being either public or private, Rust allows item
1799 accesses in two cases:
1801 1. If an item is public, then it can be used externally through any of its
1803 2. If an item is private, it may be accessed by the current module and its
1806 These two cases are surprisingly powerful for creating module hierarchies
1807 exposing public APIs while hiding internal implementation details. To help
1808 explain, here's a few use cases and what they would entail:
1810 * A library developer needs to expose functionality to crates which link
1811 against their library. As a consequence of the first case, this means that
1812 anything which is usable externally must be `pub` from the root down to the
1813 destination item. Any private item in the chain will disallow external
1816 * A crate needs a global available "helper module" to itself, but it doesn't
1817 want to expose the helper module as a public API. To accomplish this, the
1818 root of the crate's hierarchy would have a private module which then
1819 internally has a "public api". Because the entire crate is a descendant of
1820 the root, then the entire local crate can access this private module through
1823 * When writing unit tests for a module, it's often a common idiom to have an
1824 immediate child of the module to-be-tested named `mod test`. This module
1825 could access any items of the parent module through the second case, meaning
1826 that internal implementation details could also be seamlessly tested from the
1829 In the second case, it mentions that a private item "can be accessed" by the
1830 current module and its descendants, but the exact meaning of accessing an item
1831 depends on what the item is. Accessing a module, for example, would mean
1832 looking inside of it (to import more items). On the other hand, accessing a
1833 function would mean that it is invoked. Additionally, path expressions and
1834 import statements are considered to access an item in the sense that the
1835 import/expression is only valid if the destination is in the current visibility
1838 Here's an example of a program which exemplifies the three cases outlined
1842 // This module is private, meaning that no external crate can access this
1843 // module. Because it is private at the root of this current crate, however, any
1844 // module in the crate may access any publicly visible item in this module.
1845 mod crate_helper_module {
1847 // This function can be used by anything in the current crate
1848 pub fn crate_helper() {}
1850 // This function *cannot* be used by anything else in the crate. It is not
1851 // publicly visible outside of the `crate_helper_module`, so only this
1852 // current module and its descendants may access it.
1853 fn implementation_detail() {}
1856 // This function is "public to the root" meaning that it's available to external
1857 // crates linking against this one.
1858 pub fn public_api() {}
1860 // Similarly to 'public_api', this module is public so external crates may look
1863 use crate_helper_module;
1865 pub fn my_method() {
1866 // Any item in the local crate may invoke the helper module's public
1867 // interface through a combination of the two rules above.
1868 crate_helper_module::crate_helper();
1871 // This function is hidden to any module which is not a descendant of
1873 fn my_implementation() {}
1879 fn test_my_implementation() {
1880 // Because this module is a descendant of `submodule`, it's allowed
1881 // to access private items inside of `submodule` without a privacy
1883 super::my_implementation();
1891 For a rust program to pass the privacy checking pass, all paths must be valid
1892 accesses given the two rules above. This includes all use statements,
1893 expressions, types, etc.
1895 ### Re-exporting and Visibility
1897 Rust allows publicly re-exporting items through a `pub use` directive. Because
1898 this is a public directive, this allows the item to be used in the current
1899 module through the rules above. It essentially allows public access into the
1900 re-exported item. For example, this program is valid:
1903 pub use self::implementation::api;
1905 mod implementation {
1914 This means that any external crate referencing `implementation::api::f` would
1915 receive a privacy violation, while the path `api::f` would be allowed.
1917 When re-exporting a private item, it can be thought of as allowing the "privacy
1918 chain" being short-circuited through the reexport instead of passing through
1919 the namespace hierarchy as it normally would.
1924 attribute : '#' '!' ? '[' meta_item ']' ;
1925 meta_item : ident [ '=' literal
1926 | '(' meta_seq ')' ] ? ;
1927 meta_seq : meta_item [ ',' meta_seq ] ? ;
1930 Any item declaration may have an _attribute_ applied to it. Attributes in Rust
1931 are modeled on Attributes in ECMA-335, with the syntax coming from ECMA-334
1932 (C#). An attribute is a general, free-form metadatum that is interpreted
1933 according to name, convention, and language and compiler version. Attributes
1934 may appear as any of:
1936 * A single identifier, the attribute name
1937 * An identifier followed by the equals sign '=' and a literal, providing a
1939 * An identifier followed by a parenthesized list of sub-attribute arguments
1941 Attributes with a bang ("!") after the hash ("#") apply to the item that the
1942 attribute is declared within. Attributes that do not have a bang after the hash
1943 apply to the item that follows the attribute.
1945 An example of attributes:
1948 // General metadata applied to the enclosing module or crate.
1949 #![crate_type = "lib"]
1951 // A function marked as a unit test
1957 // A conditionally-compiled module
1958 #[cfg(target_os="linux")]
1963 // A lint attribute used to suppress a warning/error
1964 #[allow(non_camel_case_types)]
1968 > **Note:** At some point in the future, the compiler will distinguish between
1969 > language-reserved and user-available attributes. Until then, there is
1970 > effectively no difference between an attribute handled by a loadable syntax
1971 > extension and the compiler.
1973 ### Crate-only attributes
1975 - `crate_name` - specify the this crate's crate name.
1976 - `crate_type` - see [linkage](#linkage).
1977 - `feature` - see [compiler features](#compiler-features).
1978 - `no_builtins` - disable optimizing certain code patterns to invocations of
1979 library functions that are assumed to exist
1980 - `no_main` - disable emitting the `main` symbol. Useful when some other
1981 object being linked to defines `main`.
1982 - `no_start` - disable linking to the `native` crate, which specifies the
1983 "start" language item.
1984 - `no_std` - disable linking to the `std` crate.
1985 - `plugin` — load a list of named crates as compiler plugins, e.g.
1986 `#![plugin(foo, bar)]`. Optional arguments for each plugin,
1987 i.e. `#![plugin(foo(... args ...))]`, are provided to the plugin's
1988 registrar function. The `plugin` feature gate is required to use
1991 ### Module-only attributes
1993 - `no_implicit_prelude` - disable injecting `use std::prelude::*` in this
1995 - `path` - specifies the file to load the module from. `#[path="foo.rs"] mod
1996 bar;` is equivalent to `mod bar { /* contents of foo.rs */ }`. The path is
1997 taken relative to the directory that the current module is in.
1999 ### Function-only attributes
2001 - `main` - indicates that this function should be passed to the entry point,
2002 rather than the function in the crate root named `main`.
2003 - `plugin_registrar` - mark this function as the registration point for
2004 [compiler plugins][plugin], such as loadable syntax extensions.
2005 - `start` - indicates that this function should be used as the entry point,
2006 overriding the "start" language item. See the "start" [language
2007 item](#language-items) for more details.
2008 - `test` - indicates that this function is a test function, to only be compiled
2009 in case of `--test`.
2010 - `should_panic` - indicates that this test function should panic, inverting the success condition.
2011 - `cold` - The function is unlikely to be executed, so optimize it (and calls
2014 ### Static-only attributes
2016 - `thread_local` - on a `static mut`, this signals that the value of this
2017 static may change depending on the current thread. The exact consequences of
2018 this are implementation-defined.
2022 On an `extern` block, the following attributes are interpreted:
2024 - `link_args` - specify arguments to the linker, rather than just the library
2025 name and type. This is feature gated and the exact behavior is
2026 implementation-defined (due to variety of linker invocation syntax).
2027 - `link` - indicate that a native library should be linked to for the
2028 declarations in this block to be linked correctly. `link` supports an optional `kind`
2029 key with three possible values: `dylib`, `static`, and `framework`. See [external blocks](#external-blocks) for more about external blocks. Two
2030 examples: `#[link(name = "readline")]` and
2031 `#[link(name = "CoreFoundation", kind = "framework")]`.
2033 On declarations inside an `extern` block, the following attributes are
2036 - `link_name` - the name of the symbol that this function or static should be
2038 - `linkage` - on a static, this specifies the [linkage
2039 type](http://llvm.org/docs/LangRef.html#linkage-types).
2043 - `repr` - on C-like enums, this sets the underlying type used for
2044 representation. Takes one argument, which is the primitive
2045 type this enum should be represented for, or `C`, which specifies that it
2046 should be the default `enum` size of the C ABI for that platform. Note that
2047 enum representation in C is undefined, and this may be incorrect when the C
2048 code is compiled with certain flags.
2052 - `repr` - specifies the representation to use for this struct. Takes a list
2053 of options. The currently accepted ones are `C` and `packed`, which may be
2054 combined. `C` will use a C ABI compatible struct layout, and `packed` will
2055 remove any padding between fields (note that this is very fragile and may
2056 break platforms which require aligned access).
2058 ### Macro-related attributes
2060 - `macro_use` on a `mod` — macros defined in this module will be visible in the
2061 module's parent, after this module has been included.
2063 - `macro_use` on an `extern crate` — load macros from this crate. An optional
2064 list of names `#[macro_use(foo, bar)]` restricts the import to just those
2065 macros named. The `extern crate` must appear at the crate root, not inside
2066 `mod`, which ensures proper function of the [`$crate` macro
2067 variable](book/macros.html#the-variable-$crate).
2069 - `macro_reexport` on an `extern crate` — re-export the named macros.
2071 - `macro_export` - export a macro for cross-crate usage.
2073 - `no_link` on an `extern crate` — even if we load this crate for macros, don't
2074 link it into the output.
2076 See the [macros section of the
2077 book](book/macros.html#scoping-and-macro-import/export) for more information on
2081 ### Miscellaneous attributes
2083 - `export_name` - on statics and functions, this determines the name of the
2085 - `link_section` - on statics and functions, this specifies the section of the
2086 object file that this item's contents will be placed into.
2087 - `no_mangle` - on any item, do not apply the standard name mangling. Set the
2088 symbol for this item to its identifier.
2089 - `packed` - on structs or enums, eliminate any padding that would be used to
2091 - `simd` - on certain tuple structs, derive the arithmetic operators, which
2092 lower to the target's SIMD instructions, if any; the `simd` feature gate
2093 is necessary to use this attribute.
2094 - `static_assert` - on statics whose type is `bool`, terminates compilation
2095 with an error if it is not initialized to `true`.
2096 - `unsafe_destructor` - allow implementations of the "drop" language item
2097 where the type it is implemented for does not implement the "send" language
2098 item; the `unsafe_destructor` feature gate is needed to use this attribute
2099 - `unsafe_no_drop_flag` - on structs, remove the flag that prevents
2100 destructors from being run twice. Destructors might be run multiple times on
2101 the same object with this attribute.
2102 - `doc` - Doc comments such as `/// foo` are equivalent to `#[doc = "foo"]`.
2103 - `rustc_on_unimplemented` - Write a custom note to be shown along with the error
2104 when the trait is found to be unimplemented on a type.
2105 You may use format arguments like `{T}`, `{A}` to correspond to the
2106 types at the point of use corresponding to the type parameters of the
2107 trait of the same name. `{Self}` will be replaced with the type that is supposed
2108 to implement the trait but doesn't. To use this, the `on_unimplemented` feature gate
2111 ### Conditional compilation
2113 Sometimes one wants to have different compiler outputs from the same code,
2114 depending on build target, such as targeted operating system, or to enable
2117 There are two kinds of configuration options, one that is either defined or not
2118 (`#[cfg(foo)]`), and the other that contains a string that can be checked
2119 against (`#[cfg(bar = "baz")]` (currently only compiler-defined configuration
2120 options can have the latter form).
2123 // The function is only included in the build when compiling for OSX
2124 #[cfg(target_os = "macos")]
2129 // This function is only included when either foo or bar is defined
2130 #[cfg(any(foo, bar))]
2131 fn needs_foo_or_bar() {
2135 // This function is only included when compiling for a unixish OS with a 32-bit
2137 #[cfg(all(unix, target_pointer_width = "32"))]
2138 fn on_32bit_unix() {
2142 // This function is only included when foo is not defined
2144 fn needs_not_foo() {
2149 This illustrates some conditional compilation can be achieved using the
2150 `#[cfg(...)]` attribute. `any`, `all` and `not` can be used to assemble
2151 arbitrarily complex configurations through nesting.
2153 The following configurations must be defined by the implementation:
2155 * `target_arch = "..."`. Target CPU architecture, such as `"x86"`, `"x86_64"`
2156 `"mips"`, `"powerpc"`, `"arm"`, or `"aarch64"`.
2157 * `target_endian = "..."`. Endianness of the target CPU, either `"little"` or
2159 * `target_family = "..."`. Operating system family of the target, e. g.
2160 `"unix"` or `"windows"`. The value of this configuration option is defined
2161 as a configuration itself, like `unix` or `windows`.
2162 * `target_os = "..."`. Operating system of the target, examples include
2163 `"win32"`, `"macos"`, `"linux"`, `"android"`, `"freebsd"`, `"dragonfly"`,
2164 `"bitrig"` or `"openbsd"`.
2165 * `target_pointer_width = "..."`. Target pointer width in bits. This is set
2166 to `"32"` for targets with 32-bit pointers, and likewise set to `"64"` for
2168 * `unix`. See `target_family`.
2169 * `windows`. See `target_family`.
2171 You can also set another attribute based on a `cfg` variable with `cfg_attr`:
2177 Will be the same as `#[b]` if `a` is set by `cfg`, and nothing otherwise.
2179 ### Lint check attributes
2181 A lint check names a potentially undesirable coding pattern, such as
2182 unreachable code or omitted documentation, for the static entity to which the
2185 For any lint check `C`:
2187 * `allow(C)` overrides the check for `C` so that violations will go
2189 * `deny(C)` signals an error after encountering a violation of `C`,
2190 * `forbid(C)` is the same as `deny(C)`, but also forbids changing the lint
2192 * `warn(C)` warns about violations of `C` but continues compilation.
2194 The lint checks supported by the compiler can be found via `rustc -W help`,
2195 along with their default settings. [Compiler
2196 plugins](book/plugins.html#lint-plugins) can provide additional lint checks.
2200 // Missing documentation is ignored here
2201 #[allow(missing_docs)]
2202 pub fn undocumented_one() -> i32 { 1 }
2204 // Missing documentation signals a warning here
2205 #[warn(missing_docs)]
2206 pub fn undocumented_too() -> i32 { 2 }
2208 // Missing documentation signals an error here
2209 #[deny(missing_docs)]
2210 pub fn undocumented_end() -> i32 { 3 }
2214 This example shows how one can use `allow` and `warn` to toggle a particular
2218 #[warn(missing_docs)]
2220 #[allow(missing_docs)]
2222 // Missing documentation is ignored here
2223 pub fn undocumented_one() -> i32 { 1 }
2225 // Missing documentation signals a warning here,
2226 // despite the allow above.
2227 #[warn(missing_docs)]
2228 pub fn undocumented_two() -> i32 { 2 }
2231 // Missing documentation signals a warning here
2232 pub fn undocumented_too() -> i32 { 3 }
2236 This example shows how one can use `forbid` to disallow uses of `allow` for
2240 #[forbid(missing_docs)]
2242 // Attempting to toggle warning signals an error here
2243 #[allow(missing_docs)]
2245 pub fn undocumented_too() -> i32 { 2 }
2251 Some primitive Rust operations are defined in Rust code, rather than being
2252 implemented directly in C or assembly language. The definitions of these
2253 operations have to be easy for the compiler to find. The `lang` attribute
2254 makes it possible to declare these operations. For example, the `str` module
2255 in the Rust standard library defines the string equality function:
2259 pub fn eq_slice(a: &str, b: &str) -> bool {
2264 The name `str_eq` has a special meaning to the Rust compiler, and the presence
2265 of this definition means that it will use this definition when generating calls
2266 to the string equality function.
2268 A complete list of the built-in language items will be added in the future.
2270 ### Inline attributes
2272 The inline attribute is used to suggest to the compiler to perform an inline
2273 expansion and place a copy of the function or static in the caller rather than
2274 generating code to call the function or access the static where it is defined.
2276 The compiler automatically inlines functions based on internal heuristics.
2277 Incorrectly inlining functions can actually making the program slower, so it
2278 should be used with care.
2280 Immutable statics are always considered inlineable unless marked with
2281 `#[inline(never)]`. It is undefined whether two different inlineable statics
2282 have the same memory address. In other words, the compiler is free to collapse
2283 duplicate inlineable statics together.
2285 `#[inline]` and `#[inline(always)]` always causes the function to be serialized
2286 into crate metadata to allow cross-crate inlining.
2288 There are three different types of inline attributes:
2290 * `#[inline]` hints the compiler to perform an inline expansion.
2291 * `#[inline(always)]` asks the compiler to always perform an inline expansion.
2292 * `#[inline(never)]` asks the compiler to never perform an inline expansion.
2296 The `derive` attribute allows certain traits to be automatically implemented
2297 for data structures. For example, the following will create an `impl` for the
2298 `PartialEq` and `Clone` traits for `Foo`, the type parameter `T` will be given
2299 the `PartialEq` or `Clone` constraints for the appropriate `impl`:
2302 #[derive(PartialEq, Clone)]
2309 The generated `impl` for `PartialEq` is equivalent to
2312 # struct Foo<T> { a: i32, b: T }
2313 impl<T: PartialEq> PartialEq for Foo<T> {
2314 fn eq(&self, other: &Foo<T>) -> bool {
2315 self.a == other.a && self.b == other.b
2318 fn ne(&self, other: &Foo<T>) -> bool {
2319 self.a != other.a || self.b != other.b
2324 ### Compiler Features
2326 Certain aspects of Rust may be implemented in the compiler, but they're not
2327 necessarily ready for every-day use. These features are often of "prototype
2328 quality" or "almost production ready", but may not be stable enough to be
2329 considered a full-fledged language feature.
2331 For this reason, Rust recognizes a special crate-level attribute of the form:
2334 #![feature(feature1, feature2, feature3)]
2337 This directive informs the compiler that the feature list: `feature1`,
2338 `feature2`, and `feature3` should all be enabled. This is only recognized at a
2339 crate-level, not at a module-level. Without this directive, all features are
2340 considered off, and using the features will result in a compiler error.
2342 The currently implemented features of the reference compiler are:
2344 * `advanced_slice_patterns` - See the [match expressions](#match-expressions)
2345 section for discussion; the exact semantics of
2346 slice patterns are subject to change, so some types
2349 * `slice_patterns` - OK, actually, slice patterns are just scary and
2350 completely unstable.
2352 * `asm` - The `asm!` macro provides a means for inline assembly. This is often
2353 useful, but the exact syntax for this feature along with its
2354 semantics are likely to change, so this macro usage must be opted
2357 * `associated_types` - Allows type aliases in traits. Experimental.
2359 * `box_patterns` - Allows `box` patterns, the exact semantics of which
2360 is subject to change.
2362 * `box_syntax` - Allows use of `box` expressions, the exact semantics of which
2363 is subject to change.
2365 * `concat_idents` - Allows use of the `concat_idents` macro, which is in many
2366 ways insufficient for concatenating identifiers, and may be
2367 removed entirely for something more wholesome.
2369 * `custom_attribute` - Allows the usage of attributes unknown to the compiler
2370 so that new attributes can be added in a backwards compatible
2373 * `custom_derive` - Allows the use of `#[derive(Foo,Bar)]` as sugar for
2374 `#[derive_Foo] #[derive_Bar]`, which can be user-defined syntax
2377 * `intrinsics` - Allows use of the "rust-intrinsics" ABI. Compiler intrinsics
2378 are inherently unstable and no promise about them is made.
2380 * `lang_items` - Allows use of the `#[lang]` attribute. Like `intrinsics`,
2381 lang items are inherently unstable and no promise about them
2384 * `link_args` - This attribute is used to specify custom flags to the linker,
2385 but usage is strongly discouraged. The compiler's usage of the
2386 system linker is not guaranteed to continue in the future, and
2387 if the system linker is not used then specifying custom flags
2388 doesn't have much meaning.
2390 * `link_llvm_intrinsics` – Allows linking to LLVM intrinsics via
2391 `#[link_name="llvm.*"]`.
2393 * `linkage` - Allows use of the `linkage` attribute, which is not portable.
2395 * `log_syntax` - Allows use of the `log_syntax` macro attribute, which is a
2396 nasty hack that will certainly be removed.
2398 * `main` - Allows use of the `#[main]` attribute, which changes the entry point
2399 into a Rust program. This capability is subject to change.
2401 * `macro_reexport` - Allows macros to be re-exported from one crate after being imported
2402 from another. This feature was originally designed with the sole
2403 use case of the Rust standard library in mind, and is subject to
2406 * `non_ascii_idents` - The compiler supports the use of non-ascii identifiers,
2407 but the implementation is a little rough around the
2408 edges, so this can be seen as an experimental feature
2409 for now until the specification of identifiers is fully
2412 * `no_std` - Allows the `#![no_std]` crate attribute, which disables the implicit
2413 `extern crate std`. This typically requires use of the unstable APIs
2414 behind the libstd "facade", such as libcore and libcollections. It
2415 may also cause problems when using syntax extensions, including
2418 * `on_unimplemented` - Allows the `#[rustc_on_unimplemented]` attribute, which allows
2419 trait definitions to add specialized notes to error messages
2420 when an implementation was expected but not found.
2422 * `optin_builtin_traits` - Allows the definition of default and negative trait
2423 implementations. Experimental.
2425 * `plugin` - Usage of [compiler plugins][plugin] for custom lints or syntax extensions.
2426 These depend on compiler internals and are subject to change.
2428 * `plugin_registrar` - Indicates that a crate provides [compiler plugins][plugin].
2430 * `quote` - Allows use of the `quote_*!` family of macros, which are
2431 implemented very poorly and will likely change significantly
2432 with a proper implementation.
2434 * `rustc_attrs` - Gates internal `#[rustc_*]` attributes which may be
2435 for internal use only or have meaning added to them in the future.
2437 * `rustc_diagnostic_macros`- A mysterious feature, used in the implementation
2438 of rustc, not meant for mortals.
2440 * `simd` - Allows use of the `#[simd]` attribute, which is overly simple and
2441 not the SIMD interface we want to expose in the long term.
2443 * `simd_ffi` - Allows use of SIMD vectors in signatures for foreign functions.
2444 The SIMD interface is subject to change.
2446 * `staged_api` - Allows usage of stability markers and `#![staged_api]` in a
2447 crate. Stability markers are also attributes: `#[stable]`,
2448 `#[unstable]`, and `#[deprecated]` are the three levels.
2450 * `static_assert` - The `#[static_assert]` functionality is experimental and
2451 unstable. The attribute can be attached to a `static` of
2452 type `bool` and the compiler will error if the `bool` is
2453 `false` at compile time. This version of this functionality
2454 is unintuitive and suboptimal.
2456 * `start` - Allows use of the `#[start]` attribute, which changes the entry point
2457 into a Rust program. This capability, especially the signature for the
2458 annotated function, is subject to change.
2460 * `struct_inherit` - Allows using struct inheritance, which is barely
2461 implemented and will probably be removed. Don't use this.
2463 * `struct_variant` - Structural enum variants (those with named fields). It is
2464 currently unknown whether this style of enum variant is as
2465 fully supported as the tuple-forms, and it's not certain
2466 that this style of variant should remain in the language.
2467 For now this style of variant is hidden behind a feature
2470 * `thread_local` - The usage of the `#[thread_local]` attribute is experimental
2471 and should be seen as unstable. This attribute is used to
2472 declare a `static` as being unique per-thread leveraging
2473 LLVM's implementation which works in concert with the kernel
2474 loader and dynamic linker. This is not necessarily available
2475 on all platforms, and usage of it is discouraged.
2477 * `trace_macros` - Allows use of the `trace_macros` macro, which is a nasty
2478 hack that will certainly be removed.
2480 * `unboxed_closures` - Rust's new closure design, which is currently a work in
2481 progress feature with many known bugs.
2483 * `unsafe_destructor` - Allows use of the `#[unsafe_destructor]` attribute,
2484 which is considered wildly unsafe and will be
2485 obsoleted by language improvements.
2487 * `unsafe_no_drop_flag` - Allows use of the `#[unsafe_no_drop_flag]` attribute,
2488 which removes hidden flag added to a type that
2489 implements the `Drop` trait. The design for the
2490 `Drop` flag is subject to change, and this feature
2491 may be removed in the future.
2493 * `unmarked_api` - Allows use of items within a `#![staged_api]` crate
2494 which have not been marked with a stability marker.
2495 Such items should not be allowed by the compiler to exist,
2496 so if you need this there probably is a compiler bug.
2498 * `visible_private_types` - Allows public APIs to expose otherwise private
2499 types, e.g. as the return type of a public function.
2500 This capability may be removed in the future.
2502 * `allow_internal_unstable` - Allows `macro_rules!` macros to be tagged with the
2503 `#[allow_internal_unstable]` attribute, designed
2504 to allow `std` macros to call
2505 `#[unstable]`/feature-gated functionality
2506 internally without imposing on callers
2507 (i.e. making them behave like function calls in
2508 terms of encapsulation).
2510 If a feature is promoted to a language feature, then all existing programs will
2511 start to receive compilation warnings about #[feature] directives which enabled
2512 the new feature (because the directive is no longer necessary). However, if a
2513 feature is decided to be removed from the language, errors will be issued (if
2514 there isn't a parser error first). The directive in this case is no longer
2515 necessary, and it's likely that existing code will break if the feature isn't
2518 If an unknown feature is found in a directive, it results in a compiler error.
2519 An unknown feature is one which has never been recognized by the compiler.
2521 # Statements and expressions
2523 Rust is _primarily_ an expression language. This means that most forms of
2524 value-producing or effect-causing evaluation are directed by the uniform syntax
2525 category of _expressions_. Each kind of expression can typically _nest_ within
2526 each other kind of expression, and rules for evaluation of expressions involve
2527 specifying both the value produced by the expression and the order in which its
2528 sub-expressions are themselves evaluated.
2530 In contrast, statements in Rust serve _mostly_ to contain and explicitly
2531 sequence expression evaluation.
2535 A _statement_ is a component of a block, which is in turn a component of an
2536 outer [expression](#expressions) or [function](#functions).
2538 Rust has two kinds of statement: [declaration
2539 statements](#declaration-statements) and [expression
2540 statements](#expression-statements).
2542 ### Declaration statements
2544 A _declaration statement_ is one that introduces one or more *names* into the
2545 enclosing statement block. The declared names may denote new variables or new
2548 #### Item declarations
2550 An _item declaration statement_ has a syntactic form identical to an
2551 [item](#items) declaration within a module. Declaring an item — a
2552 function, enumeration, structure, type, static, trait, implementation or module
2553 — locally within a statement block is simply a way of restricting its
2554 scope to a narrow region containing all of its uses; it is otherwise identical
2555 in meaning to declaring the item outside the statement block.
2557 > **Note**: there is no implicit capture of the function's dynamic environment when
2558 > declaring a function-local item.
2560 #### Variable declarations
2563 let_decl : "let" pat [':' type ] ? [ init ] ? ';' ;
2564 init : [ '=' ] expr ;
2567 A _variable declaration_ introduces a new set of variable, given by a pattern. The
2568 pattern may be followed by a type annotation, and/or an initializer expression.
2569 When no type annotation is given, the compiler will infer the type, or signal
2570 an error if insufficient type information is available for definite inference.
2571 Any variables introduced by a variable declaration are visible from the point of
2572 declaration until the end of the enclosing block scope.
2574 ### Expression statements
2576 An _expression statement_ is one that evaluates an [expression](#expressions)
2577 and ignores its result. The type of an expression statement `e;` is always
2578 `()`, regardless of the type of `e`. As a rule, an expression statement's
2579 purpose is to trigger the effects of evaluating its expression.
2583 An expression may have two roles: it always produces a *value*, and it may have
2584 *effects* (otherwise known as "side effects"). An expression *evaluates to* a
2585 value, and has effects during *evaluation*. Many expressions contain
2586 sub-expressions (operands). The meaning of each kind of expression dictates
2589 * Whether or not to evaluate the sub-expressions when evaluating the expression
2590 * The order in which to evaluate the sub-expressions
2591 * How to combine the sub-expressions' values to obtain the value of the expression
2593 In this way, the structure of expressions dictates the structure of execution.
2594 Blocks are just another kind of expression, so blocks, statements, expressions,
2595 and blocks again can recursively nest inside each other to an arbitrary depth.
2597 #### Lvalues, rvalues and temporaries
2599 Expressions are divided into two main categories: _lvalues_ and _rvalues_.
2600 Likewise within each expression, sub-expressions may occur in _lvalue context_
2601 or _rvalue context_. The evaluation of an expression depends both on its own
2602 category and the context it occurs within.
2604 An lvalue is an expression that represents a memory location. These expressions
2605 are [paths](#path-expressions) (which refer to local variables, function and
2606 method arguments, or static variables), dereferences (`*expr`), [indexing
2607 expressions](#index-expressions) (`expr[expr]`), and [field
2608 references](#field-expressions) (`expr.f`). All other expressions are rvalues.
2610 The left operand of an [assignment](#assignment-expressions) or
2611 [compound-assignment](#compound-assignment-expressions) expression is an lvalue
2612 context, as is the single operand of a unary
2613 [borrow](#unary-operator-expressions). All other expression contexts are
2616 When an lvalue is evaluated in an _lvalue context_, it denotes a memory
2617 location; when evaluated in an _rvalue context_, it denotes the value held _in_
2618 that memory location.
2620 When an rvalue is used in an lvalue context, a temporary un-named lvalue is
2621 created and used instead. A temporary's lifetime equals the largest lifetime
2622 of any reference that points to it.
2624 #### Moved and copied types
2626 When a [local variable](#variables) is used as an
2627 [rvalue](#lvalues,-rvalues-and-temporaries) the variable will either be moved
2628 or copied, depending on its type. All values whose type implements `Copy` are
2629 copied, all others are moved.
2631 ### Literal expressions
2633 A _literal expression_ consists of one of the [literal](#literals) forms
2634 described earlier. It directly describes a number, character, string, boolean
2635 value, or the unit value.
2639 "hello"; // string type
2640 '5'; // character type
2644 ### Path expressions
2646 A [path](#paths) used as an expression context denotes either a local variable
2647 or an item. Path expressions are [lvalues](#lvalues,-rvalues-and-temporaries).
2649 ### Tuple expressions
2651 Tuples are written by enclosing zero or more comma-separated expressions in
2652 parentheses. They are used to create [tuple-typed](#tuple-types) values.
2660 ### Unit expressions
2662 The expression `()` denotes the _unit value_, the only value of the type with
2665 ### Structure expressions
2668 struct_expr : expr_path '{' ident ':' expr
2669 [ ',' ident ':' expr ] *
2672 [ ',' expr ] * ')' |
2676 There are several forms of structure expressions. A _structure expression_
2677 consists of the [path](#paths) of a [structure item](#structures), followed by
2678 a brace-enclosed list of one or more comma-separated name-value pairs,
2679 providing the field values of a new instance of the structure. A field name
2680 can be any identifier, and is separated from its value expression by a colon.
2681 The location denoted by a structure field is mutable if and only if the
2682 enclosing structure is mutable.
2684 A _tuple structure expression_ consists of the [path](#paths) of a [structure
2685 item](#structures), followed by a parenthesized list of one or more
2686 comma-separated expressions (in other words, the path of a structure item
2687 followed by a tuple expression). The structure item must be a tuple structure
2690 A _unit-like structure expression_ consists only of the [path](#paths) of a
2691 [structure item](#structures).
2693 The following are examples of structure expressions:
2696 # struct Point { x: f64, y: f64 }
2697 # struct TuplePoint(f64, f64);
2698 # mod game { pub struct User<'a> { pub name: &'a str, pub age: u32, pub score: usize } }
2699 # struct Cookie; fn some_fn<T>(t: T) {}
2700 Point {x: 10.0, y: 20.0};
2701 TuplePoint(10.0, 20.0);
2702 let u = game::User {name: "Joe", age: 35, score: 100_000};
2703 some_fn::<Cookie>(Cookie);
2706 A structure expression forms a new value of the named structure type. Note
2707 that for a given *unit-like* structure type, this will always be the same
2710 A structure expression can terminate with the syntax `..` followed by an
2711 expression to denote a functional update. The expression following `..` (the
2712 base) must have the same structure type as the new structure type being formed.
2713 The entire expression denotes the result of constructing a new structure (with
2714 the same type as the base expression) with the given values for the fields that
2715 were explicitly specified and the values in the base expression for all other
2719 # struct Point3d { x: i32, y: i32, z: i32 }
2720 let base = Point3d {x: 1, y: 2, z: 3};
2721 Point3d {y: 0, z: 10, .. base};
2724 ### Block expressions
2727 block_expr : '{' [ stmt ';' | item ] *
2731 A _block expression_ is similar to a module in terms of the declarations that
2732 are possible. Each block conceptually introduces a new namespace scope. Use
2733 items can bring new names into scopes and declared items are in scope for only
2736 A block will execute each statement sequentially, and then execute the
2737 expression (if given). If the block ends in a statement, its value is `()`:
2740 let x: () = { println!("Hello."); };
2743 If it ends in an expression, its value and type are that of the expression:
2746 let x: i32 = { println!("Hello."); 5 };
2751 ### Method-call expressions
2754 method_call_expr : expr '.' ident paren_expr_list ;
2757 A _method call_ consists of an expression followed by a single dot, an
2758 identifier, and a parenthesized expression-list. Method calls are resolved to
2759 methods on specific traits, either statically dispatching to a method if the
2760 exact `self`-type of the left-hand-side is known, or dynamically dispatching if
2761 the left-hand-side expression is an indirect [object type](#object-types).
2763 ### Field expressions
2766 field_expr : expr '.' ident ;
2769 A _field expression_ consists of an expression followed by a single dot and an
2770 identifier, when not immediately followed by a parenthesized expression-list
2771 (the latter is a [method call expression](#method-call-expressions)). A field
2772 expression denotes a field of a [structure](#structure-types).
2777 (Struct {a: 10, b: 20}).a;
2780 A field access is an [lvalue](#lvalues,-rvalues-and-temporaries) referring to
2781 the value of that field. When the type providing the field inherits mutability,
2782 it can be [assigned](#assignment-expressions) to.
2784 Also, if the type of the expression to the left of the dot is a pointer, it is
2785 automatically dereferenced to make the field access possible.
2787 ### Array expressions
2790 array_expr : '[' "mut" ? array_elems? ']' ;
2792 array_elems : [expr [',' expr]*] | [expr ';' expr] ;
2795 An [array](#array,-and-slice-types) _expression_ is written by enclosing zero
2796 or more comma-separated expressions of uniform type in square brackets.
2798 In the `[expr ';' expr]` form, the expression after the `';'` must be a
2799 constant expression that can be evaluated at compile time, such as a
2800 [literal](#literals) or a [static item](#static-items).
2804 ["a", "b", "c", "d"];
2805 [0; 128]; // array with 128 zeros
2806 [0u8, 0u8, 0u8, 0u8];
2809 ### Index expressions
2812 idx_expr : expr '[' expr ']' ;
2815 [Array](#array,-and-slice-types)-typed expressions can be indexed by
2816 writing a square-bracket-enclosed expression (the index) after them. When the
2817 array is mutable, the resulting [lvalue](#lvalues,-rvalues-and-temporaries) can
2820 Indices are zero-based, and may be of any integral type. Vector access is
2821 bounds-checked at run-time. When the check fails, it will put the thread in a
2826 (["a", "b"])[10]; // panics
2829 ### Unary operator expressions
2831 Rust defines three unary operators. They are all written as prefix operators,
2832 before the expression they apply to.
2835 : Negation. May only be applied to numeric types.
2837 : Dereference. When applied to a [pointer](#pointer-types) it denotes the
2838 pointed-to location. For pointers to mutable locations, the resulting
2839 [lvalue](#lvalues,-rvalues-and-temporaries) can be assigned to.
2840 On non-pointer types, it calls the `deref` method of the `std::ops::Deref`
2841 trait, or the `deref_mut` method of the `std::ops::DerefMut` trait (if
2842 implemented by the type and required for an outer expression that will or
2843 could mutate the dereference), and produces the result of dereferencing the
2844 `&` or `&mut` borrowed pointer returned from the overload method.
2847 : Logical negation. On the boolean type, this flips between `true` and
2848 `false`. On integer types, this inverts the individual bits in the
2849 two's complement representation of the value.
2851 ### Binary operator expressions
2854 binop_expr : expr binop expr ;
2857 Binary operators expressions are given in terms of [operator
2858 precedence](#operator-precedence).
2860 #### Arithmetic operators
2862 Binary arithmetic expressions are syntactic sugar for calls to built-in traits,
2863 defined in the `std::ops` module of the `std` library. This means that
2864 arithmetic operators can be overridden for user-defined types. The default
2865 meaning of the operators on standard types is given here.
2868 : Addition and array/string concatenation.
2869 Calls the `add` method on the `std::ops::Add` trait.
2872 Calls the `sub` method on the `std::ops::Sub` trait.
2875 Calls the `mul` method on the `std::ops::Mul` trait.
2878 Calls the `div` method on the `std::ops::Div` trait.
2881 Calls the `rem` method on the `std::ops::Rem` trait.
2883 #### Bitwise operators
2885 Like the [arithmetic operators](#arithmetic-operators), bitwise operators are
2886 syntactic sugar for calls to methods of built-in traits. This means that
2887 bitwise operators can be overridden for user-defined types. The default
2888 meaning of the operators on standard types is given here.
2892 Calls the `bitand` method of the `std::ops::BitAnd` trait.
2895 Calls the `bitor` method of the `std::ops::BitOr` trait.
2898 Calls the `bitxor` method of the `std::ops::BitXor` trait.
2901 Calls the `shl` method of the `std::ops::Shl` trait.
2904 Calls the `shr` method of the `std::ops::Shr` trait.
2906 #### Lazy boolean operators
2908 The operators `||` and `&&` may be applied to operands of boolean type. The
2909 `||` operator denotes logical 'or', and the `&&` operator denotes logical
2910 'and'. They differ from `|` and `&` in that the right-hand operand is only
2911 evaluated when the left-hand operand does not already determine the result of
2912 the expression. That is, `||` only evaluates its right-hand operand when the
2913 left-hand operand evaluates to `false`, and `&&` only when it evaluates to
2916 #### Comparison operators
2918 Comparison operators are, like the [arithmetic
2919 operators](#arithmetic-operators), and [bitwise operators](#bitwise-operators),
2920 syntactic sugar for calls to built-in traits. This means that comparison
2921 operators can be overridden for user-defined types. The default meaning of the
2922 operators on standard types is given here.
2926 Calls the `eq` method on the `std::cmp::PartialEq` trait.
2929 Calls the `ne` method on the `std::cmp::PartialEq` trait.
2932 Calls the `lt` method on the `std::cmp::PartialOrd` trait.
2935 Calls the `gt` method on the `std::cmp::PartialOrd` trait.
2937 : Less than or equal.
2938 Calls the `le` method on the `std::cmp::PartialOrd` trait.
2940 : Greater than or equal.
2941 Calls the `ge` method on the `std::cmp::PartialOrd` trait.
2943 #### Type cast expressions
2945 A type cast expression is denoted with the binary operator `as`.
2947 Executing an `as` expression casts the value on the left-hand side to the type
2948 on the right-hand side.
2950 An example of an `as` expression:
2953 # fn sum(v: &[f64]) -> f64 { 0.0 }
2954 # fn len(v: &[f64]) -> i32 { 0 }
2956 fn avg(v: &[f64]) -> f64 {
2957 let sum: f64 = sum(v);
2958 let sz: f64 = len(v) as f64;
2963 #### Assignment expressions
2965 An _assignment expression_ consists of an
2966 [lvalue](#lvalues,-rvalues-and-temporaries) expression followed by an equals
2967 sign (`=`) and an [rvalue](#lvalues,-rvalues-and-temporaries) expression.
2969 Evaluating an assignment expression [either copies or
2970 moves](#moved-and-copied-types) its right-hand operand to its left-hand
2980 #### Compound assignment expressions
2982 The `+`, `-`, `*`, `/`, `%`, `&`, `|`, `^`, `<<`, and `>>` operators may be
2983 composed with the `=` operator. The expression `lval OP= val` is equivalent to
2984 `lval = lval OP val`. For example, `x = x + 1` may be written as `x += 1`.
2986 Any such expression always has the [`unit`](#primitive-types) type.
2988 #### Operator precedence
2990 The precedence of Rust binary operators is ordered as follows, going from
2993 ```{.text .precedence}
3007 Operators at the same precedence level are evaluated left-to-right. [Unary
3008 operators](#unary-operator-expressions) have the same precedence level and are
3009 stronger than any of the binary operators.
3011 ### Grouped expressions
3013 An expression enclosed in parentheses evaluates to the result of the enclosed
3014 expression. Parentheses can be used to explicitly specify evaluation order
3015 within an expression.
3018 paren_expr : '(' expr ')' ;
3021 An example of a parenthesized expression:
3024 let x: i32 = (2 + 3) * 4;
3028 ### Call expressions
3031 expr_list : [ expr [ ',' expr ]* ] ? ;
3032 paren_expr_list : '(' expr_list ')' ;
3033 call_expr : expr paren_expr_list ;
3036 A _call expression_ invokes a function, providing zero or more input variables
3037 and an optional location to move the function's output into. If the function
3038 eventually returns, then the expression completes.
3040 Some examples of call expressions:
3043 # fn add(x: i32, y: i32) -> i32 { 0 }
3045 let x: i32 = add(1i32, 2i32);
3046 let pi: Result<f32, _> = "3.14".parse();
3049 ### Lambda expressions
3052 ident_list : [ ident [ ',' ident ]* ] ? ;
3053 lambda_expr : '|' ident_list '|' expr ;
3056 A _lambda expression_ (sometimes called an "anonymous function expression")
3057 defines a function and denotes it as a value, in a single expression. A lambda
3058 expression is a pipe-symbol-delimited (`|`) list of identifiers followed by an
3061 A lambda expression denotes a function that maps a list of parameters
3062 (`ident_list`) onto the expression that follows the `ident_list`. The
3063 identifiers in the `ident_list` are the parameters to the function. These
3064 parameters' types need not be specified, as the compiler infers them from
3067 Lambda expressions are most useful when passing functions as arguments to other
3068 functions, as an abbreviation for defining and capturing a separate function.
3070 Significantly, lambda expressions _capture their environment_, which regular
3071 [function definitions](#functions) do not. The exact type of capture depends
3072 on the [function type](#function-types) inferred for the lambda expression. In
3073 the simplest and least-expensive form (analogous to a ```|| { }``` expression),
3074 the lambda expression captures its environment by reference, effectively
3075 borrowing pointers to all outer variables mentioned inside the function.
3076 Alternately, the compiler may infer that a lambda expression should copy or
3077 move values (depending on their type) from the environment into the lambda
3078 expression's captured environment.
3080 In this example, we define a function `ten_times` that takes a higher-order
3081 function argument, and call it with a lambda expression as an argument:
3084 fn ten_times<F>(f: F) where F: Fn(i32) {
3092 ten_times(|j| println!("hello, {}", j));
3098 while_expr : [ lifetime ':' ] "while" no_struct_literal_expr '{' block '}' ;
3101 A `while` loop begins by evaluating the boolean loop conditional expression.
3102 If the loop conditional expression evaluates to `true`, the loop body block
3103 executes and control returns to the loop conditional expression. If the loop
3104 conditional expression evaluates to `false`, the `while` expression completes.
3119 A `loop` expression denotes an infinite loop.
3122 loop_expr : [ lifetime ':' ] "loop" '{' block '}';
3125 A `loop` expression may optionally have a _label_. If a label is present, then
3126 labeled `break` and `continue` expressions nested within this loop may exit out
3127 of this loop or return control to its head. See [Break
3128 expressions](#break-expressions) and [Continue
3129 expressions](#continue-expressions).
3131 ### Break expressions
3134 break_expr : "break" [ lifetime ];
3137 A `break` expression has an optional _label_. If the label is absent, then
3138 executing a `break` expression immediately terminates the innermost loop
3139 enclosing it. It is only permitted in the body of a loop. If the label is
3140 present, then `break foo` terminates the loop with label `foo`, which need not
3141 be the innermost label enclosing the `break` expression, but must enclose it.
3143 ### Continue expressions
3146 continue_expr : "continue" [ lifetime ];
3149 A `continue` expression has an optional _label_. If the label is absent, then
3150 executing a `continue` expression immediately terminates the current iteration
3151 of the innermost loop enclosing it, returning control to the loop *head*. In
3152 the case of a `while` loop, the head is the conditional expression controlling
3153 the loop. In the case of a `for` loop, the head is the call-expression
3154 controlling the loop. If the label is present, then `continue foo` returns
3155 control to the head of the loop with label `foo`, which need not be the
3156 innermost label enclosing the `break` expression, but must enclose it.
3158 A `continue` expression is only permitted in the body of a loop.
3163 for_expr : [ lifetime ':' ] "for" pat "in" no_struct_literal_expr '{' block '}' ;
3166 A `for` expression is a syntactic construct for looping over elements provided
3167 by an implementation of `std::iter::Iterator`.
3169 An example of a for loop over the contents of an array:
3173 # fn bar(f: Foo) { }
3178 let v: &[Foo] = &[a, b, c];
3185 An example of a for loop over a series of integers:
3188 # fn bar(b:usize) { }
3197 if_expr : "if" no_struct_literal_expr '{' block '}'
3200 else_tail : "else" [ if_expr | if_let_expr
3204 An `if` expression is a conditional branch in program control. The form of an
3205 `if` expression is a condition expression, followed by a consequent block, any
3206 number of `else if` conditions and blocks, and an optional trailing `else`
3207 block. The condition expressions must have type `bool`. If a condition
3208 expression evaluates to `true`, the consequent block is executed and any
3209 subsequent `else if` or `else` block is skipped. If a condition expression
3210 evaluates to `false`, the consequent block is skipped and any subsequent `else
3211 if` condition is evaluated. If all `if` and `else if` conditions evaluate to
3212 `false` then any `else` block is executed.
3214 ### Match expressions
3217 match_expr : "match" no_struct_literal_expr '{' match_arm * '}' ;
3219 match_arm : attribute * match_pat "=>" [ expr "," | '{' block '}' ] ;
3221 match_pat : pat [ '|' pat ] * [ "if" expr ] ? ;
3224 A `match` expression branches on a *pattern*. The exact form of matching that
3225 occurs depends on the pattern. Patterns consist of some combination of
3226 literals, destructured arrays or enum constructors, structures and tuples,
3227 variable binding specifications, wildcards (`..`), and placeholders (`_`). A
3228 `match` expression has a *head expression*, which is the value to compare to
3229 the patterns. The type of the patterns must equal the type of the head
3232 In a pattern whose head expression has an `enum` type, a placeholder (`_`)
3233 stands for a *single* data field, whereas a wildcard `..` stands for *all* the
3234 fields of a particular variant. For example:
3237 #![feature(box_patterns)]
3238 #![feature(box_syntax)]
3239 enum List<X> { Nil, Cons(X, Box<List<X>>) }
3242 let x: List<i32> = List::Cons(10, box List::Cons(11, box List::Nil));
3245 List::Cons(_, box List::Nil) => panic!("singleton list"),
3246 List::Cons(..) => return,
3247 List::Nil => panic!("empty list")
3252 The first pattern matches lists constructed by applying `Cons` to any head
3253 value, and a tail value of `box Nil`. The second pattern matches _any_ list
3254 constructed with `Cons`, ignoring the values of its arguments. The difference
3255 between `_` and `..` is that the pattern `C(_)` is only type-correct if `C` has
3256 exactly one argument, while the pattern `C(..)` is type-correct for any enum
3257 variant `C`, regardless of how many arguments `C` has.
3259 Used inside an array pattern, `..` stands for any number of elements, when the
3260 `advanced_slice_patterns` feature gate is turned on. This wildcard can be used
3261 at most once for a given array, which implies that it cannot be used to
3262 specifically match elements that are at an unknown distance from both ends of a
3263 array, like `[.., 42, ..]`. If preceded by a variable name, it will bind the
3264 corresponding slice to the variable. Example:
3267 # #![feature(advanced_slice_patterns, slice_patterns)]
3268 fn is_symmetric(list: &[u32]) -> bool {
3271 [x, inside.., y] if x == y => is_symmetric(inside),
3277 let sym = &[0, 1, 4, 2, 4, 1, 0];
3278 let not_sym = &[0, 1, 7, 2, 4, 1, 0];
3279 assert!(is_symmetric(sym));
3280 assert!(!is_symmetric(not_sym));
3284 A `match` behaves differently depending on whether or not the head expression
3285 is an [lvalue or an rvalue](#lvalues,-rvalues-and-temporaries). If the head
3286 expression is an rvalue, it is first evaluated into a temporary location, and
3287 the resulting value is sequentially compared to the patterns in the arms until
3288 a match is found. The first arm with a matching pattern is chosen as the branch
3289 target of the `match`, any variables bound by the pattern are assigned to local
3290 variables in the arm's block, and control enters the block.
3292 When the head expression is an lvalue, the match does not allocate a temporary
3293 location (however, a by-value binding may copy or move from the lvalue). When
3294 possible, it is preferable to match on lvalues, as the lifetime of these
3295 matches inherits the lifetime of the lvalue, rather than being restricted to
3296 the inside of the match.
3298 An example of a `match` expression:
3301 #![feature(box_patterns)]
3302 #![feature(box_syntax)]
3303 # fn process_pair(a: i32, b: i32) { }
3304 # fn process_ten() { }
3306 enum List<X> { Nil, Cons(X, Box<List<X>>) }
3309 let x: List<i32> = List::Cons(10, box List::Cons(11, box List::Nil));
3312 List::Cons(a, box List::Cons(b, _)) => {
3315 List::Cons(10, _) => {
3328 Patterns that bind variables default to binding to a copy or move of the
3329 matched value (depending on the matched value's type). This can be changed to
3330 bind to a reference by using the `ref` keyword, or to a mutable reference using
3333 Subpatterns can also be bound to variables by the use of the syntax `variable @
3334 subpattern`. For example:
3337 #![feature(box_patterns)]
3338 #![feature(box_syntax)]
3340 enum List { Nil, Cons(u32, Box<List>) }
3342 fn is_sorted(list: &List) -> bool {
3344 List::Nil | List::Cons(_, box List::Nil) => true,
3345 List::Cons(x, ref r @ box List::Cons(_, _)) => {
3347 box List::Cons(y, _) => (x <= y) && is_sorted(&**r),
3355 let a = List::Cons(6, box List::Cons(7, box List::Cons(42, box List::Nil)));
3356 assert!(is_sorted(&a));
3361 Patterns can also dereference pointers by using the `&`, `&mut` and `box`
3362 symbols, as appropriate. For example, these two matches on `x: &i32` are
3367 let y = match *x { 0 => "zero", _ => "some" };
3368 let z = match x { &0 => "zero", _ => "some" };
3373 A pattern that's just an identifier, like `Nil` in the previous example, could
3374 either refer to an enum variant that's in scope, or bind a new variable. The
3375 compiler resolves this ambiguity by forbidding variable bindings that occur in
3376 `match` patterns from shadowing names of variants that are in scope. For
3377 example, wherever `List` is in scope, a `match` pattern would not be able to
3378 bind `Nil` as a new name. The compiler interprets a variable pattern `x` as a
3379 binding _only_ if there is no variant named `x` in scope. A convention you can
3380 use to avoid conflicts is simply to name variants with upper-case letters, and
3381 local variables with lower-case letters.
3383 Multiple match patterns may be joined with the `|` operator. A range of values
3384 may be specified with `...`. For example:
3389 let message = match x {
3390 0 | 1 => "not many",
3396 Range patterns only work on scalar types (like integers and characters; not
3397 like arrays and structs, which have sub-components). A range pattern may not
3398 be a sub-range of another range pattern inside the same `match`.
3400 Finally, match patterns can accept *pattern guards* to further refine the
3401 criteria for matching a case. Pattern guards appear after the pattern and
3402 consist of a bool-typed expression following the `if` keyword. A pattern guard
3403 may refer to the variables bound within the pattern they follow.
3406 # let maybe_digit = Some(0);
3407 # fn process_digit(i: i32) { }
3408 # fn process_other(i: i32) { }
3410 let message = match maybe_digit {
3411 Some(x) if x < 10 => process_digit(x),
3412 Some(x) => process_other(x),
3417 ### If let expressions
3420 if_let_expr : "if" "let" pat '=' expr '{' block '}'
3422 else_tail : "else" [ if_expr | if_let_expr | '{' block '}' ] ;
3425 An `if let` expression is semantically identical to an `if` expression but in place
3426 of a condition expression it expects a refutable let statement. If the value of the
3427 expression on the right hand side of the let statement matches the pattern, the corresponding
3428 block will execute, otherwise flow proceeds to the first `else` block that follows.
3433 while_let_expr : "while" "let" pat '=' expr '{' block '}' ;
3436 A `while let` loop is semantically identical to a `while` loop but in place of a
3437 condition expression it expects a refutable let statement. If the value of the
3438 expression on the right hand side of the let statement matches the pattern, the
3439 loop body block executes and control returns to the pattern matching statement.
3440 Otherwise, the while expression completes.
3442 ### Return expressions
3445 return_expr : "return" expr ? ;
3448 Return expressions are denoted with the keyword `return`. Evaluating a `return`
3449 expression moves its argument into the designated output location for the
3450 current function call, destroys the current function activation frame, and
3451 transfers control to the caller frame.
3453 An example of a `return` expression:
3456 fn max(a: i32, b: i32) -> i32 {
3468 Every variable, item and value in a Rust program has a type. The _type_ of a
3469 *value* defines the interpretation of the memory holding it.
3471 Built-in types and type-constructors are tightly integrated into the language,
3472 in nontrivial ways that are not possible to emulate in user-defined types.
3473 User-defined types have limited capabilities.
3477 The primitive types are the following:
3479 * The "unit" type `()`, having the single "unit" value `()` (occasionally called
3481 * The boolean type `bool` with values `true` and `false`.
3482 * The machine types.
3483 * The machine-dependent integer and floating-point types.
3485 [^unittype]: The "unit" value `()` is *not* a sentinel "null pointer" value for
3486 reference variables; the "unit" type is the implicit return type from functions
3487 otherwise lacking a return type, and can be used in other contexts (such as
3488 message-sending or type-parametric code) as a zero-size type.]
3492 The machine types are the following:
3494 * The unsigned word types `u8`, `u16`, `u32` and `u64`, with values drawn from
3495 the integer intervals [0, 2^8 - 1], [0, 2^16 - 1], [0, 2^32 - 1] and
3496 [0, 2^64 - 1] respectively.
3498 * The signed two's complement word types `i8`, `i16`, `i32` and `i64`, with
3499 values drawn from the integer intervals [-(2^(7)), 2^7 - 1],
3500 [-(2^(15)), 2^15 - 1], [-(2^(31)), 2^31 - 1], [-(2^(63)), 2^63 - 1]
3503 * The IEEE 754-2008 `binary32` and `binary64` floating-point types: `f32` and
3504 `f64`, respectively.
3506 #### Machine-dependent integer types
3508 The `usize` type is an unsigned integer type with the same number of bits as the
3509 platform's pointer type. It can represent every memory address in the process.
3511 The `isize` type is a signed integer type with the same number of bits as the
3512 platform's pointer type. The theoretical upper bound on object and array size
3513 is the maximum `isize` value. This ensures that `isize` can be used to calculate
3514 differences between pointers into an object or array and can address every byte
3515 within an object along with one byte past the end.
3519 The types `char` and `str` hold textual data.
3521 A value of type `char` is a [Unicode scalar value](
3522 http://www.unicode.org/glossary/#unicode_scalar_value) (i.e. a code point that
3523 is not a surrogate), represented as a 32-bit unsigned word in the 0x0000 to
3524 0xD7FF or 0xE000 to 0x10FFFF range. A `[char]` array is effectively an UCS-4 /
3527 A value of type `str` is a Unicode string, represented as an array of 8-bit
3528 unsigned bytes holding a sequence of UTF-8 codepoints. Since `str` is of
3529 unknown size, it is not a _first-class_ type, but can only be instantiated
3530 through a pointer type, such as `&str` or `String`.
3534 A tuple *type* is a heterogeneous product of other types, called the *elements*
3535 of the tuple. It has no nominal name and is instead structurally typed.
3537 Tuple types and values are denoted by listing the types or values of their
3538 elements, respectively, in a parenthesized, comma-separated list.
3540 Because tuple elements don't have a name, they can only be accessed by
3541 pattern-matching or by using `N` directly as a field to access the
3544 An example of a tuple type and its use:
3547 type Pair<'a> = (i32, &'a str);
3548 let p: Pair<'static> = (10, "hello");
3550 assert!(b != "world");
3554 ### Array, and Slice types
3556 Rust has two different types for a list of items:
3558 * `[T; N]`, an 'array'.
3559 * `&[T]`, a 'slice'.
3561 An array has a fixed size, and can be allocated on either the stack or the
3564 A slice is a 'view' into an array. It doesn't own the data it points
3567 An example of each kind:
3570 let vec: Vec<i32> = vec![1, 2, 3];
3571 let arr: [i32; 3] = [1, 2, 3];
3572 let s: &[i32] = &vec[..];
3575 As you can see, the `vec!` macro allows you to create a `Vec<T>` easily. The
3576 `vec!` macro is also part of the standard library, rather than the language.
3578 All in-bounds elements of arrays, and slices are always initialized, and access
3579 to an array or slice is always bounds-checked.
3583 A `struct` *type* is a heterogeneous product of other types, called the
3584 *fields* of the type.[^structtype]
3586 [^structtype]: `struct` types are analogous `struct` types in C,
3587 the *record* types of the ML family,
3588 or the *structure* types of the Lisp family.
3590 New instances of a `struct` can be constructed with a [struct
3591 expression](#structure-expressions).
3593 The memory layout of a `struct` is undefined by default to allow for compiler
3594 optimizations like field reordering, but it can be fixed with the
3595 `#[repr(...)]` attribute. In either case, fields may be given in any order in
3596 a corresponding struct *expression*; the resulting `struct` value will always
3597 have the same memory layout.
3599 The fields of a `struct` may be qualified by [visibility
3600 modifiers](#re-exporting-and-visibility), to allow access to data in a
3601 structure outside a module.
3603 A _tuple struct_ type is just like a structure type, except that the fields are
3606 A _unit-like struct_ type is like a structure type, except that it has no
3607 fields. The one value constructed by the associated [structure
3608 expression](#structure-expressions) is the only value that inhabits such a
3611 ### Enumerated types
3613 An *enumerated type* is a nominal, heterogeneous disjoint union type, denoted
3614 by the name of an [`enum` item](#enumerations). [^enumtype]
3616 [^enumtype]: The `enum` type is analogous to a `data` constructor declaration in
3617 ML, or a *pick ADT* in Limbo.
3619 An [`enum` item](#enumerations) declares both the type and a number of *variant
3620 constructors*, each of which is independently named and takes an optional tuple
3623 New instances of an `enum` can be constructed by calling one of the variant
3624 constructors, in a [call expression](#call-expressions).
3626 Any `enum` value consumes as much memory as the largest variant constructor for
3627 its corresponding `enum` type.
3629 Enum types cannot be denoted *structurally* as types, but must be denoted by
3630 named reference to an [`enum` item](#enumerations).
3634 Nominal types — [enumerations](#enumerated-types) and
3635 [structures](#structure-types) — may be recursive. That is, each `enum`
3636 constructor or `struct` field may refer, directly or indirectly, to the
3637 enclosing `enum` or `struct` type itself. Such recursion has restrictions:
3639 * Recursive types must include a nominal type in the recursion
3640 (not mere [type definitions](#type-definitions),
3641 or other structural types such as [arrays](#array,-and-slice-types) or [tuples](#tuple-types)).
3642 * A recursive `enum` item must have at least one non-recursive constructor
3643 (in order to give the recursion a basis case).
3644 * The size of a recursive type must be finite;
3645 in other words the recursive fields of the type must be [pointer types](#pointer-types).
3646 * Recursive type definitions can cross module boundaries, but not module *visibility* boundaries,
3647 or crate boundaries (in order to simplify the module system and type checker).
3649 An example of a *recursive* type and its use:
3654 Cons(T, Box<List<T>>)
3657 let a: List<i32> = List::Cons(7, Box::new(List::Cons(13, Box::new(List::Nil))));
3662 All pointers in Rust are explicit first-class values. They can be copied,
3663 stored into data structures, and returned from functions. There are two
3664 varieties of pointer in Rust:
3667 : These point to memory _owned by some other value_.
3668 A reference type is written `&type` for some lifetime-variable `f`,
3669 or just `&'a type` when you need an explicit lifetime.
3670 Copying a reference is a "shallow" operation:
3671 it involves only copying the pointer itself.
3672 Releasing a reference typically has no effect on the value it points to,
3673 with the exception of temporary values, which are released when the last
3674 reference to them is released.
3676 * Raw pointers (`*`)
3677 : Raw pointers are pointers without safety or liveness guarantees.
3678 Raw pointers are written as `*const T` or `*mut T`,
3679 for example `*const int` means a raw pointer to an integer.
3680 Copying or dropping a raw pointer has no effect on the lifecycle of any
3681 other value. Dereferencing a raw pointer or converting it to any other
3682 pointer type is an [`unsafe` operation](#unsafe-functions).
3683 Raw pointers are generally discouraged in Rust code;
3684 they exist to support interoperability with foreign code,
3685 and writing performance-critical or low-level functions.
3687 The standard library contains additional 'smart pointer' types beyond references
3692 The function type constructor `fn` forms new function types. A function type
3693 consists of a possibly-empty set of function-type modifiers (such as `unsafe`
3694 or `extern`), a sequence of input types and an output type.
3696 An example of a `fn` type:
3699 fn add(x: i32, y: i32) -> i32 {
3703 let mut x = add(5,7);
3705 type Binop = fn(i32, i32) -> i32;
3706 let bo: Binop = add;
3712 ```{.ebnf .notation}
3713 closure_type := [ 'unsafe' ] [ '<' lifetime-list '>' ] '|' arg-list '|'
3714 [ ':' bound-list ] [ '->' type ]
3715 lifetime-list := lifetime | lifetime ',' lifetime-list
3716 arg-list := ident ':' type | ident ':' type ',' arg-list
3717 bound-list := bound | bound '+' bound-list
3718 bound := path | lifetime
3721 The type of a closure mapping an input of type `A` to an output of type `B` is
3722 `|A| -> B`. A closure with no arguments or return values has type `||`.
3724 An example of creating and calling a closure:
3727 let captured_var = 10;
3729 let closure_no_args = || println!("captured_var={}", captured_var);
3731 let closure_args = |arg: i32| -> i32 {
3732 println!("captured_var={}, arg={}", captured_var, arg);
3733 arg // Note lack of semicolon after 'arg'
3736 fn call_closure<F: Fn(), G: Fn(i32) -> i32>(c1: F, c2: G) {
3741 call_closure(closure_no_args, closure_args);
3747 Every trait item (see [traits](#traits)) defines a type with the same name as
3748 the trait. This type is called the _object type_ of the trait. Object types
3749 permit "late binding" of methods, dispatched using _virtual method tables_
3750 ("vtables"). Whereas most calls to trait methods are "early bound" (statically
3751 resolved) to specific implementations at compile time, a call to a method on an
3752 object type is only resolved to a vtable entry at compile time. The actual
3753 implementation for each vtable entry can vary on an object-by-object basis.
3755 Given a pointer-typed expression `E` of type `&T` or `Box<T>`, where `T`
3756 implements trait `R`, casting `E` to the corresponding pointer type `&R` or
3757 `Box<R>` results in a value of the _object type_ `R`. This result is
3758 represented as a pair of pointers: the vtable pointer for the `T`
3759 implementation of `R`, and the pointer value of `E`.
3761 An example of an object type:
3765 fn stringify(&self) -> String;
3768 impl Printable for i32 {
3769 fn stringify(&self) -> String { self.to_string() }
3772 fn print(a: Box<Printable>) {
3773 println!("{}", a.stringify());
3777 print(Box::new(10) as Box<Printable>);
3781 In this example, the trait `Printable` occurs as an object type in both the
3782 type signature of `print`, and the cast expression in `main`.
3786 Within the body of an item that has type parameter declarations, the names of
3787 its type parameters are types:
3790 fn map<A: Clone, B: Clone>(f: |A| -> B, xs: &[A]) -> Vec<B> {
3794 let first: B = f(xs[0].clone());
3795 let mut rest: Vec<B> = map(f, xs.slice(1, xs.len()));
3796 rest.insert(0, first);
3801 Here, `first` has type `B`, referring to `map`'s `B` type parameter; and `rest`
3802 has type `Vec<B>`, a vector type with element type `B`.
3806 The special type `self` has a meaning within methods inside an impl item. It
3807 refers to the type of the implicit `self` argument. For example, in:
3811 fn make_string(&self) -> String;
3814 impl Printable for String {
3815 fn make_string(&self) -> String {
3821 `self` refers to the value of type `String` that is the receiver for a call to
3822 the method `make_string`.
3826 Several traits define special evaluation behavior.
3830 The `Copy` trait changes the semantics of a type implementing it. Values whose
3831 type implements `Copy` are copied rather than moved upon assignment.
3833 ## The `Sized` trait
3835 The `Sized` trait indicates that the size of this type is known at compile-time.
3839 The `Drop` trait provides a destructor, to be run whenever a value of this type
3844 A Rust program's memory consists of a static set of *items* and a *heap*.
3845 Immutable portions of the heap may be safely shared between threads, mutable
3846 portions may not be safely shared, but several mechanisms for effectively-safe
3847 sharing of mutable values, built on unsafe code but enforcing a safe locking
3848 discipline, exist in the standard library.
3850 Allocations in the stack consist of *variables*, and allocations in the heap
3853 ### Memory allocation and lifetime
3855 The _items_ of a program are those functions, modules and types that have their
3856 value calculated at compile-time and stored uniquely in the memory image of the
3857 rust process. Items are neither dynamically allocated nor freed.
3859 The _heap_ is a general term that describes boxes. The lifetime of an
3860 allocation in the heap depends on the lifetime of the box values pointing to
3861 it. Since box values may themselves be passed in and out of frames, or stored
3862 in the heap, heap allocations may outlive the frame they are allocated within.
3864 ### Memory ownership
3866 When a stack frame is exited, its local allocations are all released, and its
3867 references to boxes are dropped.
3871 A _variable_ is a component of a stack frame, either a named function parameter,
3872 an anonymous [temporary](#lvalues,-rvalues-and-temporaries), or a named local
3875 A _local variable_ (or *stack-local* allocation) holds a value directly,
3876 allocated within the stack's memory. The value is a part of the stack frame.
3878 Local variables are immutable unless declared otherwise like: `let mut x = ...`.
3880 Function parameters are immutable unless declared with `mut`. The `mut` keyword
3881 applies only to the following parameter (so `|mut x, y|` and `fn f(mut x:
3882 Box<i32>, y: Box<i32>)` declare one mutable variable `x` and one immutable
3885 Methods that take either `self` or `Box<Self>` can optionally place them in a
3886 mutable variable by prefixing them with `mut` (similar to regular arguments):
3890 fn change(mut self) -> Self;
3891 fn modify(mut self: Box<Self>) -> Box<Self>;
3895 Local variables are not initialized when allocated; the entire frame worth of
3896 local variables are allocated at once, on frame-entry, in an uninitialized
3897 state. Subsequent statements within a function may or may not initialize the
3898 local variables. Local variables can be used only after they have been
3899 initialized; this is enforced by the compiler.
3901 # Runtime services, linkage and debugging
3903 The Rust _runtime_ is a relatively compact collection of Rust code that
3904 provides fundamental services and datatypes to all Rust threads at run-time. It
3905 is smaller and simpler than many modern language runtimes. It is tightly
3906 integrated into the language's execution model of memory, threads, communication
3909 ### Memory allocation
3911 The runtime memory-management system is based on a _service-provider
3912 interface_, through which the runtime requests blocks of memory from its
3913 environment and releases them back to its environment when they are no longer
3914 needed. The default implementation of the service-provider interface consists
3915 of the C runtime functions `malloc` and `free`.
3917 The runtime memory-management system, in turn, supplies Rust threads with
3918 facilities for allocating releasing stacks, as well as allocating and freeing
3923 The runtime provides C and Rust code to assist with various built-in types,
3924 such as arrays, strings, and the low level communication system (ports,
3927 Support for other built-in types such as simple types, tuples and enums is
3928 open-coded by the Rust compiler.
3930 ### Thread scheduling and communication
3932 The runtime provides code to manage inter-thread communication. This includes
3933 the system of thread-lifecycle state transitions depending on the contents of
3934 queues, as well as code to copy values between queues and their recipients and
3935 to serialize values for transmission over operating-system inter-process
3936 communication facilities.
3940 The Rust compiler supports various methods to link crates together both
3941 statically and dynamically. This section will explore the various methods to
3942 link Rust crates together, and more information about native libraries can be
3943 found in the [ffi section of the book][ffi].
3945 In one session of compilation, the compiler can generate multiple artifacts
3946 through the usage of either command line flags or the `crate_type` attribute.
3947 If one or more command line flag is specified, all `crate_type` attributes will
3948 be ignored in favor of only building the artifacts specified by command line.
3950 * `--crate-type=bin`, `#[crate_type = "bin"]` - A runnable executable will be
3951 produced. This requires that there is a `main` function in the crate which
3952 will be run when the program begins executing. This will link in all Rust and
3953 native dependencies, producing a distributable binary.
3955 * `--crate-type=lib`, `#[crate_type = "lib"]` - A Rust library will be produced.
3956 This is an ambiguous concept as to what exactly is produced because a library
3957 can manifest itself in several forms. The purpose of this generic `lib` option
3958 is to generate the "compiler recommended" style of library. The output library
3959 will always be usable by rustc, but the actual type of library may change from
3960 time-to-time. The remaining output types are all different flavors of
3961 libraries, and the `lib` type can be seen as an alias for one of them (but the
3962 actual one is compiler-defined).
3964 * `--crate-type=dylib`, `#[crate_type = "dylib"]` - A dynamic Rust library will
3965 be produced. This is different from the `lib` output type in that this forces
3966 dynamic library generation. The resulting dynamic library can be used as a
3967 dependency for other libraries and/or executables. This output type will
3968 create `*.so` files on linux, `*.dylib` files on osx, and `*.dll` files on
3971 * `--crate-type=staticlib`, `#[crate_type = "staticlib"]` - A static system
3972 library will be produced. This is different from other library outputs in that
3973 the Rust compiler will never attempt to link to `staticlib` outputs. The
3974 purpose of this output type is to create a static library containing all of
3975 the local crate's code along with all upstream dependencies. The static
3976 library is actually a `*.a` archive on linux and osx and a `*.lib` file on
3977 windows. This format is recommended for use in situations such as linking
3978 Rust code into an existing non-Rust application because it will not have
3979 dynamic dependencies on other Rust code.
3981 * `--crate-type=rlib`, `#[crate_type = "rlib"]` - A "Rust library" file will be
3982 produced. This is used as an intermediate artifact and can be thought of as a
3983 "static Rust library". These `rlib` files, unlike `staticlib` files, are
3984 interpreted by the Rust compiler in future linkage. This essentially means
3985 that `rustc` will look for metadata in `rlib` files like it looks for metadata
3986 in dynamic libraries. This form of output is used to produce statically linked
3987 executables as well as `staticlib` outputs.
3989 Note that these outputs are stackable in the sense that if multiple are
3990 specified, then the compiler will produce each form of output at once without
3991 having to recompile. However, this only applies for outputs specified by the
3992 same method. If only `crate_type` attributes are specified, then they will all
3993 be built, but if one or more `--crate-type` command line flag is specified,
3994 then only those outputs will be built.
3996 With all these different kinds of outputs, if crate A depends on crate B, then
3997 the compiler could find B in various different forms throughout the system. The
3998 only forms looked for by the compiler, however, are the `rlib` format and the
3999 dynamic library format. With these two options for a dependent library, the
4000 compiler must at some point make a choice between these two formats. With this
4001 in mind, the compiler follows these rules when determining what format of
4002 dependencies will be used:
4004 1. If a static library is being produced, all upstream dependencies are
4005 required to be available in `rlib` formats. This requirement stems from the
4006 reason that a dynamic library cannot be converted into a static format.
4008 Note that it is impossible to link in native dynamic dependencies to a static
4009 library, and in this case warnings will be printed about all unlinked native
4010 dynamic dependencies.
4012 2. If an `rlib` file is being produced, then there are no restrictions on what
4013 format the upstream dependencies are available in. It is simply required that
4014 all upstream dependencies be available for reading metadata from.
4016 The reason for this is that `rlib` files do not contain any of their upstream
4017 dependencies. It wouldn't be very efficient for all `rlib` files to contain a
4018 copy of `libstd.rlib`!
4020 3. If an executable is being produced and the `-C prefer-dynamic` flag is not
4021 specified, then dependencies are first attempted to be found in the `rlib`
4022 format. If some dependencies are not available in an rlib format, then
4023 dynamic linking is attempted (see below).
4025 4. If a dynamic library or an executable that is being dynamically linked is
4026 being produced, then the compiler will attempt to reconcile the available
4027 dependencies in either the rlib or dylib format to create a final product.
4029 A major goal of the compiler is to ensure that a library never appears more
4030 than once in any artifact. For example, if dynamic libraries B and C were
4031 each statically linked to library A, then a crate could not link to B and C
4032 together because there would be two copies of A. The compiler allows mixing
4033 the rlib and dylib formats, but this restriction must be satisfied.
4035 The compiler currently implements no method of hinting what format a library
4036 should be linked with. When dynamically linking, the compiler will attempt to
4037 maximize dynamic dependencies while still allowing some dependencies to be
4038 linked in via an rlib.
4040 For most situations, having all libraries available as a dylib is recommended
4041 if dynamically linking. For other situations, the compiler will emit a
4042 warning if it is unable to determine which formats to link each library with.
4044 In general, `--crate-type=bin` or `--crate-type=lib` should be sufficient for
4045 all compilation needs, and the other options are just available if more
4046 fine-grained control is desired over the output format of a Rust crate.
4048 # Appendix: Rationales and design tradeoffs
4052 # Appendix: Influences
4054 Rust is not a particularly original language, with design elements coming from
4055 a wide range of sources. Some of these are listed below (including elements
4056 that have since been removed):
4058 * SML, OCaml: algebraic datatypes, pattern matching, type inference,
4059 semicolon statement separation
4060 * C++: references, RAII, smart pointers, move semantics, monomorphisation,
4062 * ML Kit, Cyclone: region based memory management
4063 * Haskell (GHC): typeclasses, type families
4064 * Newsqueak, Alef, Limbo: channels, concurrency
4065 * Erlang: message passing, task failure, ~~linked task failure~~,
4066 ~~lightweight concurrency~~
4067 * Swift: optional bindings
4068 * Scheme: hygienic macros
4070 * Ruby: ~~block syntax~~
4071 * NIL, Hermes: ~~typestate~~
4072 * [Unicode Annex #31](http://www.unicode.org/reports/tr31/): identifier and
4075 [ffi]: book/ffi.html
4076 [plugin]: book/plugins.html