5 This document is the primary reference for the Rust programming language. It
6 provides three kinds of material:
8 - Chapters that formally define the language grammar and, for each
9 construct, informally describe its semantics and give examples of its
11 - Chapters that informally describe the memory model, concurrency model,
12 runtime services, linkage model and debugging facilities.
13 - Appendix chapters providing rationale and references to languages that
14 influenced the design.
16 This document does not serve as an introduction to the language. Background
17 familiarity with the language is assumed. A separate [book] is available to
18 help acquire such background familiarity.
20 This document also does not serve as a reference to the [standard] library
21 included in the language distribution. Those libraries are documented
22 separately by extracting documentation attributes from their source code. Many
23 of the features that one might expect to be language features are library
24 features in Rust, so what you're looking for may be there, not here.
26 [book]: book/index.html
27 [standard]: std/index.html
31 Rust's grammar is defined over Unicode codepoints, each conventionally denoted
32 `U+XXXX`, for 4 or more hexadecimal digits `X`. _Most_ of Rust's grammar is
33 confined to the ASCII range of Unicode, and is described in this document by a
34 dialect of Extended Backus-Naur Form (EBNF), specifically a dialect of EBNF
35 supported by common automated LL(k) parsing tools such as `llgen`, rather than
36 the dialect given in ISO 14977. The dialect can be defined self-referentially
41 rule : nonterminal ':' productionrule ';' ;
42 productionrule : production [ '|' production ] * ;
44 term : element repeats ;
45 element : LITERAL | IDENTIFIER | '[' productionrule ']' ;
46 repeats : [ '*' | '+' ] NUMBER ? | NUMBER ? | '?' ;
51 - Whitespace in the grammar is ignored.
52 - Square brackets are used to group rules.
53 - `LITERAL` is a single printable ASCII character, or an escaped hexadecimal
54 ASCII code of the form `\xQQ`, in single quotes, denoting the corresponding
55 Unicode codepoint `U+00QQ`.
56 - `IDENTIFIER` is a nonempty string of ASCII letters and underscores.
57 - The `repeat` forms apply to the adjacent `element`, and are as follows:
58 - `?` means zero or one repetition
59 - `*` means zero or more repetitions
60 - `+` means one or more repetitions
61 - NUMBER trailing a repeat symbol gives a maximum repetition count
62 - NUMBER on its own gives an exact repetition count
64 This EBNF dialect should hopefully be familiar to many readers.
66 ## Unicode productions
68 A few productions in Rust's grammar permit Unicode codepoints outside the ASCII
69 range. We define these productions in terms of character properties specified
70 in the Unicode standard, rather than in terms of ASCII-range codepoints. The
71 section [Special Unicode Productions](#special-unicode-productions) lists these
74 ## String table productions
76 Some rules in the grammar — notably [unary
77 operators](#unary-operator-expressions), [binary
78 operators](#binary-operator-expressions), and [keywords](#keywords) — are
79 given in a simplified form: as a listing of a table of unquoted, printable
80 whitespace-separated strings. These cases form a subset of the rules regarding
81 the [token](#tokens) rule, and are assumed to be the result of a
82 lexical-analysis phase feeding the parser, driven by a DFA, operating over the
83 disjunction of all such string table entries.
85 When such a string enclosed in double-quotes (`"`) occurs inside the grammar,
86 it is an implicit reference to a single member of such a string table
87 production. See [tokens](#tokens) for more information.
93 Rust input is interpreted as a sequence of Unicode codepoints encoded in UTF-8.
94 Most Rust grammar rules are defined in terms of printable ASCII-range
95 codepoints, but a small number are defined in terms of Unicode properties or
96 explicit codepoint lists. [^inputformat]
98 [^inputformat]: Substitute definitions for the special Unicode productions are
99 provided to the grammar verifier, restricted to ASCII range, when verifying the
100 grammar in this document.
102 ## Special Unicode Productions
104 The following productions in the Rust grammar are defined in terms of Unicode
105 properties: `ident`, `non_null`, `non_star`, `non_eol`, `non_slash_or_star`,
106 `non_single_quote` and `non_double_quote`.
110 The `ident` production is any nonempty Unicode string of the following form:
112 - The first character has property `XID_start`
113 - The remaining characters have property `XID_continue`
115 that does _not_ occur in the set of [keywords](#keywords).
117 > **Note**: `XID_start` and `XID_continue` as character properties cover the
118 > character ranges used to form the more familiar C and Java language-family
121 ### Delimiter-restricted productions
123 Some productions are defined by exclusion of particular Unicode characters:
125 - `non_null` is any single Unicode character aside from `U+0000` (null)
126 - `non_eol` is `non_null` restricted to exclude `U+000A` (`'\n'`)
127 - `non_star` is `non_null` restricted to exclude `U+002A` (`*`)
128 - `non_slash_or_star` is `non_null` restricted to exclude `U+002F` (`/`) and `U+002A` (`*`)
129 - `non_single_quote` is `non_null` restricted to exclude `U+0027` (`'`)
130 - `non_double_quote` is `non_null` restricted to exclude `U+0022` (`"`)
135 comment : block_comment | line_comment ;
136 block_comment : "/*" block_comment_body * "*/" ;
137 block_comment_body : [block_comment | character] * ;
138 line_comment : "//" non_eol * ;
141 Comments in Rust code follow the general C++ style of line and block-comment
142 forms. Nested block comments are supported.
144 Line comments beginning with exactly _three_ slashes (`///`), and block
145 comments beginning with exactly one repeated asterisk in the block-open
146 sequence (`/**`), are interpreted as a special syntax for `doc`
147 [attributes](#attributes). That is, they are equivalent to writing
148 `#[doc="..."]` around the body of the comment (this includes the comment
149 characters themselves, ie `/// Foo` turns into `#[doc="/// Foo"]`).
151 `//!` comments apply to the parent of the comment, rather than the item that
152 follows. `//!` comments are usually used to display information on the crate
155 Non-doc comments are interpreted as a form of whitespace.
160 whitespace_char : '\x20' | '\x09' | '\x0a' | '\x0d' ;
161 whitespace : [ whitespace_char | comment ] + ;
164 The `whitespace_char` production is any nonempty Unicode string consisting of
165 any of the following Unicode characters: `U+0020` (space, `' '`), `U+0009`
166 (tab, `'\t'`), `U+000A` (LF, `'\n'`), `U+000D` (CR, `'\r'`).
168 Rust is a "free-form" language, meaning that all forms of whitespace serve only
169 to separate _tokens_ in the grammar, and have no semantic significance.
171 A Rust program has identical meaning if each whitespace element is replaced
172 with any other legal whitespace element, such as a single space character.
177 simple_token : keyword | unop | binop ;
178 token : simple_token | ident | literal | symbol | whitespace token ;
181 Tokens are primitive productions in the grammar defined by regular
182 (non-recursive) languages. "Simple" tokens are given in [string table
183 production](#string-table-productions) form, and occur in the rest of the
184 grammar as double-quoted strings. Other tokens have exact rules given.
188 <p id="keyword-table-marker"></p>
191 |----------|----------|----------|----------|---------|
192 | abstract | alignof | as | be | box |
193 | break | const | continue | crate | do |
194 | else | enum | extern | false | final |
195 | fn | for | if | impl | in |
196 | let | loop | macro | match | mod |
197 | move | mut | offsetof | override | priv |
198 | pub | pure | ref | return | sizeof |
199 | static | self | struct | super | true |
200 | trait | type | typeof | unsafe | unsized |
201 | use | virtual | where | while | yield |
204 Each of these keywords has special meaning in its grammar, and all of them are
205 excluded from the `ident` rule.
207 Note that some of these keywords are reserved, and do not currently do
212 A literal is an expression consisting of a single token, rather than a sequence
213 of tokens, that immediately and directly denotes the value it evaluates to,
214 rather than referring to it by name or some other evaluation rule. A literal is
215 a form of constant expression, so is evaluated (primarily) at compile time.
219 literal : [ string_lit | char_lit | byte_string_lit | byte_lit | num_lit ] lit_suffix ?;
222 The optional suffix is only used for certain numeric literals, but is
223 reserved for future extension, that is, the above gives the lexical
224 grammar, but a Rust parser will reject everything but the 12 special
225 cases mentioned in [Number literals](#number-literals) below.
229 ##### Characters and strings
231 | | Example | Number of `#` pairs allowed | Available characters | Escapes | Equivalent to |
232 |---|---------|-----------------------------|----------------------|---------|---------------|
233 | [Character](#character-literals) | `'H'` | `N/A` | All unicode | `\'` & [Byte escapes](#byte-escapes) & [Unicode escapes](#unicode-escapes) | `N/A` |
234 | [String](#string-literals) | `"hello"` | `N/A` | All unicode | `\"` & [Byte escapes](#byte-escapes) & [Unicode escapes](#unicode-escapes) | `N/A` |
235 | [Raw](#raw-string-literals) | `r##"hello"##` | `0...` | All unicode | `N/A` | `N/A` |
236 | [Byte](#byte-literals) | `b'H'` | `N/A` | All ASCII | `\'` & [Byte escapes](#byte-escapes) | `u8` |
237 | [Byte string](#byte-string-literals) | `b"hello"` | `N/A` | All ASCII | `\"` & [Byte escapes](#byte-escapes) | `&'static [u8]` |
238 | [Raw byte string](#raw-byte-string-literals) | `br##"hello"##` | `0...` | All ASCII | `N/A` | `&'static [u8]` (unsure...not stated) |
244 | `\x7F` | 8-bit character code (exactly 2 digits) |
246 | `\r` | Carriage return |
250 ##### Unicode escapes
253 | `\u7FFF` | 16-bit character code (exactly 4 digits) |
254 | `\U7EEEFFFF` | 32-bit character code (exactly 8 digits) |
258 | [Number literals](#number-literals)`*` | Example | Exponentiation | Suffixes |
259 |----------------------------------------|---------|----------------|----------|
260 | Decimal integer | `98_222is` | `N/A` | Integer suffixes |
261 | Hex integer | `0xffis` | `N/A` | Integer suffixes |
262 | Octal integer | `0o77is` | `N/A` | Integer suffixes |
263 | Binary integer | `0b1111_0000is` | `N/A` | Integer suffixes |
264 | Floating-point | `123.0E+77f64` | `Optional` | Floating-point suffixes |
266 `*` All number literals allow `_` as a visual separator: `1_234.0E+18f64`
269 | Integer | Floating-point |
270 |---------|----------------|
271 | `is` (`isize`), `us` (`usize`), `u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`, `i64` | `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'
289 unicode_escape : 'u' hex_digit 4
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 ##### Character escapes
316 Some additional _escapes_ are available in either character or non-raw string
317 literals. An escape starts with a `U+005C` (`\`) and continues with one of the
320 * An _8-bit codepoint escape_ escape starts with `U+0078` (`x`) and is
321 followed by exactly two _hex digits_. It denotes the Unicode codepoint
322 equal to the provided hex value.
323 * A _16-bit codepoint escape_ starts with `U+0075` (`u`) and is followed
324 by exactly four _hex digits_. It denotes the Unicode codepoint equal to
325 the provided hex value.
326 * A _32-bit codepoint escape_ starts with `U+0055` (`U`) and is followed
327 by exactly eight _hex digits_. It denotes the Unicode codepoint equal to
328 the provided hex value.
329 * A _whitespace escape_ is one of the characters `U+006E` (`n`), `U+0072`
330 (`r`), or `U+0074` (`t`), denoting the unicode values `U+000A` (LF),
331 `U+000D` (CR) or `U+0009` (HT) respectively.
332 * The _backslash escape_ is the character `U+005C` (`\`) which must be
333 escaped in order to denote *itself*.
335 ##### Raw string literals
337 Raw string literals do not process any escapes. They start with the character
338 `U+0072` (`r`), followed by zero or more of the character `U+0023` (`#`) and a
339 `U+0022` (double-quote) character. The _raw string body_ is not defined in the
340 EBNF grammar above: it can contain any sequence of Unicode characters and is
341 terminated only by another `U+0022` (double-quote) character, followed by the
342 same number of `U+0023` (`#`) characters that preceded the opening `U+0022`
343 (double-quote) character.
345 All Unicode characters contained in the raw string body represent themselves,
346 the characters `U+0022` (double-quote) (except when followed by at least as
347 many `U+0023` (`#`) characters as were used to start the raw string literal) or
348 `U+005C` (`\`) do not have any special meaning.
350 Examples for string literals:
353 "foo"; r"foo"; // foo
354 "\"foo\""; r#""foo""#; // "foo"
357 r##"foo #"# bar"##; // foo #"# bar
359 "\x52"; "R"; r"R"; // R
360 "\\x52"; r"\x52"; // \x52
363 #### Byte and byte string literals
366 byte_lit : "b\x27" byte_body '\x27' ;
367 byte_string_lit : "b\x22" string_body * '\x22' | "br" raw_byte_string ;
369 byte_body : ascii_non_single_quote
370 | '\x5c' [ '\x27' | common_escape ] ;
372 byte_string_body : ascii_non_double_quote
373 | '\x5c' [ '\x22' | common_escape ] ;
374 raw_byte_string : '"' raw_byte_string_body '"' | '#' raw_byte_string '#' ;
380 A _byte literal_ is a single ASCII character (in the `U+0000` to `U+007F`
381 range) enclosed within two `U+0027` (single-quote) characters, with the
382 exception of `U+0027` itself, which must be _escaped_ by a preceding U+005C
383 character (`\`), or a single _escape_. It is equivalent to a `u8` unsigned
384 8-bit integer _number literal_.
386 ##### Byte string literals
388 A _byte string literal_ is a sequence of ASCII characters and _escapes_
389 enclosed within two `U+0022` (double-quote) characters, with the exception of
390 `U+0022` itself, which must be _escaped_ by a preceding `U+005C` character
391 (`\`), or a _raw byte string literal_. It is equivalent to a `&'static [u8]`
392 borrowed array of unsigned 8-bit integers.
394 Some additional _escapes_ are available in either byte or non-raw byte string
395 literals. An escape starts with a `U+005C` (`\`) and continues with one of the
398 * An _byte escape_ escape starts with `U+0078` (`x`) and is
399 followed by exactly two _hex digits_. It denotes the byte
400 equal to the provided hex value.
401 * A _whitespace escape_ is one of the characters `U+006E` (`n`), `U+0072`
402 (`r`), or `U+0074` (`t`), denoting the bytes values `0x0A` (ASCII LF),
403 `0x0D` (ASCII CR) or `0x09` (ASCII HT) respectively.
404 * The _backslash escape_ is the character `U+005C` (`\`) which must be
405 escaped in order to denote its ASCII encoding `0x5C`.
407 ##### Raw byte string literals
409 Raw byte string literals do not process any escapes. They start with the
410 character `U+0062` (`b`), followed by `U+0072` (`r`), followed by zero or more
411 of the character `U+0023` (`#`), and a `U+0022` (double-quote) character. The
412 _raw string body_ is not defined in the EBNF grammar above: it can contain any
413 sequence of ASCII characters and is terminated only by another `U+0022`
414 (double-quote) character, followed by the same number of `U+0023` (`#`)
415 characters that preceded the opening `U+0022` (double-quote) character. A raw
416 byte string literal can not contain any non-ASCII byte.
418 All characters contained in the raw string body represent their ASCII encoding,
419 the characters `U+0022` (double-quote) (except when followed by at least as
420 many `U+0023` (`#`) characters as were used to start the raw string literal) or
421 `U+005C` (`\`) do not have any special meaning.
423 Examples for byte string literals:
426 b"foo"; br"foo"; // foo
427 b"\"foo\""; br#""foo""#; // "foo"
430 br##"foo #"# bar"##; // foo #"# bar
432 b"\x52"; b"R"; br"R"; // R
433 b"\\x52"; br"\x52"; // \x52
439 num_lit : nonzero_dec [ dec_digit | '_' ] * float_suffix ?
440 | '0' [ [ dec_digit | '_' ] * float_suffix ?
441 | 'b' [ '1' | '0' | '_' ] +
442 | 'o' [ oct_digit | '_' ] +
443 | 'x' [ hex_digit | '_' ] + ] ;
445 float_suffix : [ exponent | '.' dec_lit exponent ? ] ? ;
447 exponent : ['E' | 'e'] ['-' | '+' ] ? dec_lit ;
448 dec_lit : [ dec_digit | '_' ] + ;
451 A _number literal_ is either an _integer literal_ or a _floating-point
452 literal_. The grammar for recognizing the two kinds of literals is mixed.
454 ##### Integer literals
456 An _integer literal_ has one of four forms:
458 * A _decimal literal_ starts with a *decimal digit* and continues with any
459 mixture of *decimal digits* and _underscores_.
460 * A _hex literal_ starts with the character sequence `U+0030` `U+0078`
461 (`0x`) and continues as any mixture of hex digits and underscores.
462 * An _octal literal_ starts with the character sequence `U+0030` `U+006F`
463 (`0o`) and continues as any mixture of octal digits and underscores.
464 * A _binary literal_ starts with the character sequence `U+0030` `U+0062`
465 (`0b`) and continues as any mixture of binary digits and underscores.
467 Like any literal, an integer literal may be followed (immediately,
468 without any spaces) by an _integer suffix_, which forcibly sets the
469 type of the literal. There are 10 valid values for an integer suffix:
471 * The `is` and `us` suffixes give the literal type `isize` or `usize`,
473 * Each of the signed and unsigned machine types `u8`, `i8`,
474 `u16`, `i16`, `u32`, `i32`, `u64` and `i64`
475 give the literal the corresponding machine type.
477 The type of an _unsuffixed_ integer literal is determined by type inference.
478 If an integer type can be _uniquely_ determined from the surrounding program
479 context, the unsuffixed integer literal has that type. If the program context
480 underconstrains the type, it is considered a static type error; if the program
481 context overconstrains the type, it is also considered a static type error.
483 Examples of integer literals of various forms:
488 123_us; // type usize
490 0o70_i16; // type i16
491 0b1111_1111_1001_0000_i32; // type i32
494 ##### Floating-point literals
496 A _floating-point literal_ has one of two forms:
498 * A _decimal literal_ followed by a period character `U+002E` (`.`). This is
499 optionally followed by another decimal literal, with an optional _exponent_.
500 * A single _decimal literal_ followed by an _exponent_.
502 By default, a floating-point literal has a generic type, and, like integer
503 literals, the type must be uniquely determined from the context. There are two valid
504 _floating-point suffixes_, `f32` and `f64` (the 32-bit and 64-bit floating point
505 types), which explicitly determine the type of the literal.
507 Examples of floating-point literals of various forms:
510 123.0f64; // type f64
513 12E+99_f64; // type f64
514 let x: f64 = 2.; // type f64
517 This last example is different because it is not possible to use the suffix
518 syntax with a floating point literal ending in a period. `2.f64` would attempt
519 to call a method named `f64` on `2`.
521 #### Boolean literals
523 The two values of the boolean type are written `true` and `false`.
529 | '#' | '[' | ']' | '(' | ')' | '{' | '}'
533 Symbols are a general class of printable [token](#tokens) that play structural
534 roles in a variety of grammar productions. They are catalogued here for
535 completeness as the set of remaining miscellaneous printable tokens that do not
536 otherwise appear as [unary operators](#unary-operator-expressions), [binary
537 operators](#binary-operator-expressions), or [keywords](#keywords).
543 expr_path : [ "::" ] ident [ "::" expr_path_tail ] + ;
544 expr_path_tail : '<' type_expr [ ',' type_expr ] + '>'
547 type_path : ident [ type_path_tail ] + ;
548 type_path_tail : '<' type_expr [ ',' type_expr ] + '>'
552 A _path_ is a sequence of one or more path components _logically_ separated by
553 a namespace qualifier (`::`). If a path consists of only one component, it may
554 refer to either an [item](#items) or a [slot](#memory-slots) in a local control
555 scope. If a path has multiple components, it refers to an item.
557 Every item has a _canonical path_ within its crate, but the path naming an item
558 is only meaningful within a given crate. There is no global namespace across
559 crates; an item's canonical path merely identifies it within the crate.
561 Two examples of simple paths consisting of only identifier components:
568 Path components are usually [identifiers](#identifiers), but the trailing
569 component of a path may be an angle-bracket-enclosed list of type arguments. In
570 [expression](#expressions) context, the type argument list is given after a
571 final (`::`) namespace qualifier in order to disambiguate it from a relational
572 expression involving the less-than symbol (`<`). In type expression context,
573 the final namespace qualifier is omitted.
575 Two examples of paths with type arguments:
578 # struct HashMap<K, V>;
580 # fn id<T>(t: T) -> T { t }
581 type T = HashMap<i32,String>; // Type arguments used in a type expression
582 let x = id::<i32>(10); // Type arguments used in a call expression
586 Paths can be denoted with various leading qualifiers to change the meaning of
589 * Paths starting with `::` are considered to be global paths where the
590 components of the path start being resolved from the crate root. Each
591 identifier in the path must resolve to an item.
599 ::a::foo(); // call a's foo function
605 * Paths starting with the keyword `super` begin resolution relative to the
606 parent module. Each further identifier must resolve to an item.
614 super::a::foo(); // call a's foo function
620 * Paths starting with the keyword `self` begin resolution relative to the
621 current module. Each further identifier must resolve to an item.
633 A number of minor features of Rust are not central enough to have their own
634 syntax, and yet are not implementable as functions. Instead, they are given
635 names, and invoked through a consistent syntax: `name!(...)`. Examples include:
637 * `format!` : format data into a string
638 * `env!` : look up an environment variable's value at compile time
639 * `file!`: return the path to the file being compiled
640 * `stringify!` : pretty-print the Rust expression given as an argument
641 * `include!` : include the Rust expression in the given file
642 * `include_str!` : include the contents of the given file as a string
643 * `include_bytes!` : include the contents of the given file as a binary blob
644 * `error!`, `warn!`, `info!`, `debug!` : provide diagnostic information.
646 All of the above extensions are expressions with values.
648 Users of `rustc` can define new syntax extensions in two ways:
650 * [Compiler plugins](book/syntax-extensions.html) can include arbitrary
651 Rust code that manipulates syntax trees at compile time.
653 * [Macros](book/macros.html) define new syntax in a higher-level,
659 expr_macro_rules : "macro_rules" '!' ident '(' macro_rule * ')' ;
660 macro_rule : '(' matcher * ')' "=>" '(' transcriber * ')' ';' ;
661 matcher : '(' matcher * ')' | '[' matcher * ']'
662 | '{' matcher * '}' | '$' ident ':' ident
663 | '$' '(' matcher * ')' sep_token? [ '*' | '+' ]
664 | non_special_token ;
665 transcriber : '(' transcriber * ')' | '[' transcriber * ']'
666 | '{' transcriber * '}' | '$' ident
667 | '$' '(' transcriber * ')' sep_token? [ '*' | '+' ]
668 | non_special_token ;
671 `macro_rules` allows users to define syntax extension in a declarative way. We
672 call such extensions "macros by example" or simply "macros" — to be distinguished
673 from the "procedural macros" defined in [compiler plugins][plugin].
675 Currently, macros can expand to expressions, statements, items, or patterns.
677 (A `sep_token` is any token other than `*` and `+`. A `non_special_token` is
678 any token other than a delimiter or `$`.)
680 The macro expander looks up macro invocations by name, and tries each macro
681 rule in turn. It transcribes the first successful match. Matching and
682 transcription are closely related to each other, and we will describe them
687 The macro expander matches and transcribes every token that does not begin with
688 a `$` literally, including delimiters. For parsing reasons, delimiters must be
689 balanced, but they are otherwise not special.
691 In the matcher, `$` _name_ `:` _designator_ matches the nonterminal in the Rust
692 syntax named by _designator_. Valid designators are `item`, `block`, `stmt`,
693 `pat`, `expr`, `ty` (type), `ident`, `path`, `tt` (either side of the `=>`
694 in macro rules). In the transcriber, the designator is already known, and so
695 only the name of a matched nonterminal comes after the dollar sign.
697 In both the matcher and transcriber, the Kleene star-like operator indicates
698 repetition. The Kleene star operator consists of `$` and parens, optionally
699 followed by a separator token, followed by `*` or `+`. `*` means zero or more
700 repetitions, `+` means at least one repetition. The parens are not matched or
701 transcribed. On the matcher side, a name is bound to _all_ of the names it
702 matches, in a structure that mimics the structure of the repetition encountered
703 on a successful match. The job of the transcriber is to sort that structure
706 The rules for transcription of these repetitions are called "Macro By Example".
707 Essentially, one "layer" of repetition is discharged at a time, and all of them
708 must be discharged by the time a name is transcribed. Therefore, `( $( $i:ident
709 ),* ) => ( $i )` is an invalid macro, but `( $( $i:ident ),* ) => ( $( $i:ident
710 ),* )` is acceptable (if trivial).
712 When Macro By Example encounters a repetition, it examines all of the `$`
713 _name_ s that occur in its body. At the "current layer", they all must repeat
714 the same number of times, so ` ( $( $i:ident ),* ; $( $j:ident ),* ) => ( $(
715 ($i,$j) ),* )` is valid if given the argument `(a,b,c ; d,e,f)`, but not
716 `(a,b,c ; d,e)`. The repetition walks through the choices at that layer in
717 lockstep, so the former input transcribes to `( (a,d), (b,e), (c,f) )`.
719 Nested repetitions are allowed.
721 ### Parsing limitations
723 The parser used by the macro system is reasonably powerful, but the parsing of
724 Rust syntax is restricted in two ways:
726 1. The parser will always parse as much as possible. If it attempts to match
727 `$i:expr [ , ]` against `8 [ , ]`, it will attempt to parse `i` as an array
728 index operation and fail. Adding a separator can solve this problem.
729 2. The parser must have eliminated all ambiguity by the time it reaches a `$`
730 _name_ `:` _designator_. This requirement most often affects name-designator
731 pairs when they occur at the beginning of, or immediately after, a `$(...)*`;
732 requiring a distinctive token in front can solve the problem.
734 ## Syntax extensions useful for the macro author
736 * `log_syntax!` : print out the arguments at compile time
737 * `trace_macros!` : supply `true` or `false` to enable or disable macro expansion logging
738 * `stringify!` : turn the identifier argument into a string literal
739 * `concat!` : concatenates a comma-separated list of literals
740 * `concat_idents!` : create a new identifier by concatenating the arguments
742 The following attributes are used for quasiquoting in procedural macros:
751 # Crates and source files
753 Rust is a *compiled* language. Its semantics obey a *phase distinction*
754 between compile-time and run-time. Those semantic rules that have a *static
755 interpretation* govern the success or failure of compilation. We refer to
756 these rules as "static semantics". Semantic rules called "dynamic semantics"
757 govern the behavior of programs at run-time. A program that fails to compile
758 due to violation of a compile-time rule has no defined dynamic semantics; the
759 compiler should halt with an error report, and produce no executable artifact.
761 The compilation model centers on artifacts called _crates_. Each compilation
762 processes a single crate in source form, and if successful, produces a single
763 crate in binary form: either an executable or a library.[^cratesourcefile]
765 [^cratesourcefile]: A crate is somewhat analogous to an *assembly* in the
766 ECMA-335 CLI model, a *library* in the SML/NJ Compilation Manager, a *unit*
767 in the Owens and Flatt module system, or a *configuration* in Mesa.
769 A _crate_ is a unit of compilation and linking, as well as versioning,
770 distribution and runtime loading. A crate contains a _tree_ of nested
771 [module](#modules) scopes. The top level of this tree is a module that is
772 anonymous (from the point of view of paths within the module) and any item
773 within a crate has a canonical [module path](#paths) denoting its location
774 within the crate's module tree.
776 The Rust compiler is always invoked with a single source file as input, and
777 always produces a single output crate. The processing of that source file may
778 result in other source files being loaded as modules. Source files have the
781 A Rust source file describes a module, the name and location of which —
782 in the module tree of the current crate — are defined from outside the
783 source file: either by an explicit `mod_item` in a referencing source file, or
784 by the name of the crate itself.
786 Each source file contains a sequence of zero or more `item` definitions, and
787 may optionally begin with any number of `attributes` that apply to the
788 containing module. Attributes on the anonymous crate module define important
789 metadata that influences the behavior of the compiler.
792 # #![allow(unused_attribute)]
794 #![crate_name = "projx"]
796 // Specify the output type
797 #![crate_type = "lib"]
800 #![warn(non_camel_case_types)]
803 A crate that contains a `main` function can be compiled to an executable. If a
804 `main` function is present, its return type must be [`unit`](#primitive-types)
805 and it must take no arguments.
807 # Items and attributes
809 Crates contain [items](#items), each of which may have some number of
810 [attributes](#attributes) attached to it.
815 item : extern_crate_decl | use_decl | mod_item | fn_item | type_item
816 | struct_item | enum_item | static_item | trait_item | impl_item
820 An _item_ is a component of a crate; some module items can be defined in crate
821 files, but most are defined in source files. Items are organized within a crate
822 by a nested set of [modules](#modules). Every crate has a single "outermost"
823 anonymous module; all further items within the crate have [paths](#paths)
824 within the module tree of the crate.
826 Items are entirely determined at compile-time, generally remain fixed during
827 execution, and may reside in read-only memory.
829 There are several kinds of item:
831 * [`extern crate` declarations](#extern-crate-declarations)
832 * [`use` declarations](#use-declarations)
833 * [modules](#modules)
834 * [functions](#functions)
835 * [type definitions](#type-definitions)
836 * [structures](#structures)
837 * [enumerations](#enumerations)
838 * [static items](#static-items)
840 * [implementations](#implementations)
842 Some items form an implicit scope for the declaration of sub-items. In other
843 words, within a function or module, declarations of items can (in many cases)
844 be mixed with the statements, control blocks, and similar artifacts that
845 otherwise compose the item body. The meaning of these scoped items is the same
846 as if the item was declared outside the scope — it is still a static item
847 — except that the item's *path name* within the module namespace is
848 qualified by the name of the enclosing item, or is private to the enclosing
849 item (in the case of functions). The grammar specifies the exact locations in
850 which sub-item declarations may appear.
854 All items except modules may be *parameterized* by type. Type parameters are
855 given as a comma-separated list of identifiers enclosed in angle brackets
856 (`<...>`), after the name of the item and before its definition. The type
857 parameters of an item are considered "part of the name", not part of the type
858 of the item. A referencing [path](#paths) must (in principle) provide type
859 arguments as a list of comma-separated types enclosed within angle brackets, in
860 order to refer to the type-parameterized item. In practice, the type-inference
861 system can usually infer such argument types from context. There are no
862 general type-parametric types, only type-parametric items. That is, Rust has
863 no notion of type abstraction: there are no first-class "forall" types.
868 mod_item : "mod" ident ( ';' | '{' mod '}' );
872 A module is a container for zero or more [items](#items).
874 A _module item_ is a module, surrounded in braces, named, and prefixed with the
875 keyword `mod`. A module item introduces a new, named module into the tree of
876 modules making up a crate. Modules can nest arbitrarily.
878 An example of a module:
882 type Complex = (f64, f64);
883 fn sin(f: f64) -> f64 {
887 fn cos(f: f64) -> f64 {
891 fn tan(f: f64) -> f64 {
898 Modules and types share the same namespace. Declaring a named type with
899 the same name as a module in scope is forbidden: that is, a type definition,
900 trait, struct, enumeration, or type parameter can't shadow the name of a module
901 in scope, or vice versa.
903 A module without a body is loaded from an external file, by default with the
904 same name as the module, plus the `.rs` extension. When a nested submodule is
905 loaded from an external file, it is loaded from a subdirectory path that
906 mirrors the module hierarchy.
909 // Load the `vec` module from `vec.rs`
913 // Load the `local_data` module from `thread/local_data.rs`
918 The directories and files used for loading external file modules can be
919 influenced with the `path` attribute.
922 #[path = "thread_files"]
924 // Load the `local_data` module from `thread_files/tls.rs`
930 ##### Extern crate declarations
933 extern_crate_decl : "extern" "crate" crate_name
934 crate_name: ident | ( string_lit "as" ident )
937 An _`extern crate` declaration_ specifies a dependency on an external crate.
938 The external crate is then bound into the declaring scope as the `ident`
939 provided in the `extern_crate_decl`.
941 The external crate is resolved to a specific `soname` at compile time, and a
942 runtime linkage requirement to that `soname` is passed to the linker for
943 loading at runtime. The `soname` is resolved at compile time by scanning the
944 compiler's library path and matching the optional `crateid` provided as a
945 string literal against the `crateid` attributes that were declared on the
946 external crate when it was compiled. If no `crateid` is provided, a default
947 `name` attribute is assumed, equal to the `ident` given in the
950 Three examples of `extern crate` declarations:
955 extern crate std; // equivalent to: extern crate std as std;
957 extern crate "std" as ruststd; // linking to 'std' under another name
960 ##### Use declarations
963 use_decl : "pub" ? "use" [ path "as" ident
966 path_glob : ident [ "::" [ path_glob
968 | '{' path_item [ ',' path_item ] * '}' ;
970 path_item : ident | "self" ;
973 A _use declaration_ creates one or more local name bindings synonymous with
974 some other [path](#paths). Usually a `use` declaration is used to shorten the
975 path required to refer to a module item. These declarations may appear at the
976 top of [modules](#modules) and [blocks](#blocks).
978 > **Note**: Unlike in many languages,
979 > `use` declarations in Rust do *not* declare linkage dependency with external crates.
980 > Rather, [`extern crate` declarations](#extern-crate-declarations) declare linkage dependencies.
982 Use declarations support a number of convenient shortcuts:
984 * Rebinding the target name as a new local name, using the syntax `use p::q::r as x;`
985 * Simultaneously binding a list of paths differing only in their final element,
986 using the glob-like brace syntax `use a::b::{c,d,e,f};`
987 * Binding all paths matching a given prefix, using the asterisk wildcard syntax
989 * Simultaneously binding a list of paths differing only in their final element
990 and their immediate parent module, using the `self` keyword, such as
991 `use a::b::{self, c, d};`
993 An example of `use` declarations:
996 use std::iter::range_step;
997 use std::option::Option::{Some, None};
998 use std::collections::hash_map::{self, HashMap};
1001 fn bar(map1: HashMap<String, usize>, map2: hash_map::HashMap<String, usize>){}
1004 // Equivalent to 'std::iter::range_step(0us, 10, 2);'
1005 range_step(0us, 10, 2);
1007 // Equivalent to 'foo(vec![std::option::Option::Some(1.0f64),
1008 // std::option::Option::None]);'
1009 foo(vec![Some(1.0f64), None]);
1011 // Both `hash_map` and `HashMap` are in scope.
1012 let map1 = HashMap::new();
1013 let map2 = hash_map::HashMap::new();
1018 Like items, `use` declarations are private to the containing module, by
1019 default. Also like items, a `use` declaration can be public, if qualified by
1020 the `pub` keyword. Such a `use` declaration serves to _re-export_ a name. A
1021 public `use` declaration can therefore _redirect_ some public name to a
1022 different target definition: even a definition with a private canonical path,
1023 inside a different module. If a sequence of such redirections form a cycle or
1024 cannot be resolved unambiguously, they represent a compile-time error.
1026 An example of re-exporting:
1031 pub use quux::foo::{bar, baz};
1040 In this example, the module `quux` re-exports two public names defined in
1043 Also note that the paths contained in `use` items are relative to the crate
1044 root. So, in the previous example, the `use` refers to `quux::foo::{bar,
1045 baz}`, and not simply to `foo::{bar, baz}`. This also means that top-level
1046 module declarations should be at the crate root if direct usage of the declared
1047 modules within `use` items is desired. It is also possible to use `self` and
1048 `super` at the beginning of a `use` item to refer to the current and direct
1049 parent modules respectively. All rules regarding accessing declared modules in
1050 `use` declarations applies to both module declarations and `extern crate`
1053 An example of what will and will not work for `use` items:
1056 # #![allow(unused_imports)]
1057 use foo::core::iter; // good: foo is at the root of the crate
1058 use foo::baz::foobaz; // good: foo is at the root of the crate
1063 use foo::core::iter; // good: foo is at crate root
1064 // use core::iter; // bad: native is not at the crate root
1065 use self::baz::foobaz; // good: self refers to module 'foo'
1066 use foo::bar::foobar; // good: foo is at crate root
1073 use super::bar::foobar; // good: super refers to module 'foo'
1083 A _function item_ defines a sequence of [statements](#statements) and an
1084 optional final [expression](#expressions), along with a name and a set of
1085 parameters. Functions are declared with the keyword `fn`. Functions declare a
1086 set of *input* [*slots*](#memory-slots) as parameters, through which the caller
1087 passes arguments into the function, and an *output* [*slot*](#memory-slots)
1088 through which the function passes results back to the caller.
1090 A function may also be copied into a first-class *value*, in which case the
1091 value has the corresponding [*function type*](#function-types), and can be used
1092 otherwise exactly as a function item (with a minor additional cost of calling
1093 the function indirectly).
1095 Every control path in a function logically ends with a `return` expression or a
1096 diverging expression. If the outermost block of a function has a
1097 value-producing expression in its final-expression position, that expression is
1098 interpreted as an implicit `return` expression applied to the final-expression.
1100 An example of a function:
1103 fn add(x: i32, y: i32) -> i32 {
1108 As with `let` bindings, function arguments are irrefutable patterns, so any
1109 pattern that is valid in a let binding is also valid as an argument.
1112 fn first((value, _): (i32, i32)) -> i32 { value }
1116 #### Generic functions
1118 A _generic function_ allows one or more _parameterized types_ to appear in its
1119 signature. Each type parameter must be explicitly declared, in an
1120 angle-bracket-enclosed, comma-separated list following the function name.
1123 fn iter<T>(seq: &[T], f: |T|) {
1124 for elt in seq.iter() { f(elt); }
1126 fn map<T, U>(seq: &[T], f: |T| -> U) -> Vec<U> {
1127 let mut acc = vec![];
1128 for elt in seq.iter() { acc.push(f(elt)); }
1133 Inside the function signature and body, the name of the type parameter can be
1134 used as a type name.
1136 When a generic function is referenced, its type is instantiated based on the
1137 context of the reference. For example, calling the `iter` function defined
1138 above on `[1, 2]` will instantiate type parameter `T` with `isize`, and require
1139 the closure parameter to have type `fn(isize)`.
1141 The type parameters can also be explicitly supplied in a trailing
1142 [path](#paths) component after the function name. This might be necessary if
1143 there is not sufficient context to determine the type parameters. For example,
1144 `mem::size_of::<u32>() == 4`.
1146 Since a parameter type is opaque to the generic function, the set of operations
1147 that can be performed on it is limited. Values of parameter type can only be
1151 fn id<T>(x: T) -> T { x }
1154 Similarly, [trait](#traits) bounds can be specified for type parameters to
1155 allow methods with that trait to be called on values of that type.
1159 Unsafe operations are those that potentially violate the memory-safety
1160 guarantees of Rust's static semantics.
1162 The following language level features cannot be used in the safe subset of
1165 - Dereferencing a [raw pointer](#pointer-types).
1166 - Reading or writing a [mutable static variable](#mutable-statics).
1167 - Calling an unsafe function (including an intrinsic or foreign function).
1169 ##### Unsafe functions
1171 Unsafe functions are functions that are not safe in all contexts and/or for all
1172 possible inputs. Such a function must be prefixed with the keyword `unsafe` and
1173 can only be called from an `unsafe` block or another `unsafe` function.
1177 A block of code can be prefixed with the `unsafe` keyword, to permit calling
1178 `unsafe` functions or dereferencing raw pointers within a safe function.
1180 When a programmer has sufficient conviction that a sequence of potentially
1181 unsafe operations is actually safe, they can encapsulate that sequence (taken
1182 as a whole) within an `unsafe` block. The compiler will consider uses of such
1183 code safe, in the surrounding context.
1185 Unsafe blocks are used to wrap foreign libraries, make direct use of hardware
1186 or implement features not directly present in the language. For example, Rust
1187 provides the language features necessary to implement memory-safe concurrency
1188 in the language but the implementation of threads and message passing is in the
1191 Rust's type system is a conservative approximation of the dynamic safety
1192 requirements, so in some cases there is a performance cost to using safe code.
1193 For example, a doubly-linked list is not a tree structure and can only be
1194 represented with reference-counted pointers in safe code. By using `unsafe`
1195 blocks to represent the reverse links as raw pointers, it can be implemented
1198 ##### Behavior considered undefined
1200 The following is a list of behavior which is forbidden in all Rust code,
1201 including within `unsafe` blocks and `unsafe` functions. Type checking provides
1202 the guarantee that these issues are never caused by safe code.
1205 * Dereferencing a null/dangling raw pointer
1206 * Mutating an immutable value/reference without `UnsafeCell`
1207 * Reads of [undef](http://llvm.org/docs/LangRef.html#undefined-values)
1208 (uninitialized) memory
1209 * Breaking the [pointer aliasing
1210 rules](http://llvm.org/docs/LangRef.html#pointer-aliasing-rules)
1211 with raw pointers (a subset of the rules used by C)
1212 * Invoking undefined behavior via compiler intrinsics:
1213 * Indexing outside of the bounds of an object with `std::ptr::offset`
1214 (`offset` intrinsic), with
1215 the exception of one byte past the end which is permitted.
1216 * Using `std::ptr::copy_nonoverlapping_memory` (`memcpy32`/`memcpy64`
1217 intrinsics) on overlapping buffers
1218 * Invalid values in primitive types, even in private fields/locals:
1219 * Dangling/null references or boxes
1220 * A value other than `false` (0) or `true` (1) in a `bool`
1221 * A discriminant in an `enum` not included in the type definition
1222 * A value in a `char` which is a surrogate or above `char::MAX`
1223 * Non-UTF-8 byte sequences in a `str`
1224 * Unwinding into Rust from foreign code or unwinding from Rust into foreign
1225 code. Rust's failure system is not compatible with exception handling in
1226 other languages. Unwinding must be caught and handled at FFI boundaries.
1228 ##### Behaviour not considered unsafe
1230 This is a list of behaviour not considered *unsafe* in Rust terms, but that may
1234 * Reading data from private fields (`std::repr`)
1235 * Leaks due to reference count cycles, even in the global heap
1236 * Exiting without calling destructors
1238 * Accessing/modifying the file system
1239 * Unsigned integer overflow (well-defined as wrapping)
1240 * Signed integer overflow (well-defined as two's complement representation
1243 #### Diverging functions
1245 A special kind of function can be declared with a `!` character where the
1246 output slot type would normally be. For example:
1249 fn my_err(s: &str) -> ! {
1255 We call such functions "diverging" because they never return a value to the
1256 caller. Every control path in a diverging function must end with a `panic!()` or
1257 a call to another diverging function on every control path. The `!` annotation
1258 does *not* denote a type. Rather, the result type of a diverging function is a
1259 special type called ⊥ ("bottom") that unifies with any type. Rust has no
1262 It might be necessary to declare a diverging function because as mentioned
1263 previously, the typechecker checks that every control path in a function ends
1264 with a [`return`](#return-expressions) or diverging expression. So, if `my_err`
1265 were declared without the `!` annotation, the following code would not
1269 # fn my_err(s: &str) -> ! { panic!() }
1271 fn f(i: i32) -> i32 {
1276 my_err("Bad number!");
1281 This will not compile without the `!` annotation on `my_err`, since the `else`
1282 branch of the conditional in `f` does not return an `i32`, as required by the
1283 signature of `f`. Adding the `!` annotation to `my_err` informs the
1284 typechecker that, should control ever enter `my_err`, no further type judgments
1285 about `f` need to hold, since control will never resume in any context that
1286 relies on those judgments. Thus the return type on `f` only needs to reflect
1287 the `if` branch of the conditional.
1289 #### Extern functions
1291 Extern functions are part of Rust's foreign function interface, providing the
1292 opposite functionality to [external blocks](#external-blocks). Whereas
1293 external blocks allow Rust code to call foreign code, extern functions with
1294 bodies defined in Rust code _can be called by foreign code_. They are defined
1295 in the same way as any other Rust function, except that they have the `extern`
1299 // Declares an extern fn, the ABI defaults to "C"
1300 extern fn new_i32() -> i32 { 0 }
1302 // Declares an extern fn with "stdcall" ABI
1303 extern "stdcall" fn new_i32_stdcall() -> i32 { 0 }
1306 Unlike normal functions, extern fns have an `extern "ABI" fn()`. This is the
1307 same type as the functions declared in an extern block.
1310 # extern fn new_i32() -> i32 { 0 }
1311 let fptr: extern "C" fn() -> i32 = new_i32;
1314 Extern functions may be called directly from Rust code as Rust uses large,
1315 contiguous stack segments like C.
1319 A _type alias_ defines a new name for an existing [type](#types). Type
1320 aliases are declared with the keyword `type`. Every value has a single,
1321 specific type; the type-specified aspects of a value include:
1323 * Whether the value is composed of sub-values or is indivisible.
1324 * Whether the value represents textual or numerical information.
1325 * Whether the value represents integral or floating-point information.
1326 * The sequence of memory operations required to access the value.
1327 * The [kind](#type-kinds) of the type.
1329 For example, the type `(u8, u8)` defines the set of immutable values that are
1330 composite pairs, each containing two unsigned 8-bit integers accessed by
1331 pattern-matching and laid out in memory with the `x` component preceding the
1335 type Point = (u8, u8);
1336 let p: Point = (41, 68);
1341 A _structure_ is a nominal [structure type](#structure-types) defined with the
1344 An example of a `struct` item and its use:
1347 struct Point {x: i32, y: i32}
1348 let p = Point {x: 10, y: 11};
1352 A _tuple structure_ is a nominal [tuple type](#tuple-types), also defined with
1353 the keyword `struct`. For example:
1356 struct Point(i32, i32);
1357 let p = Point(10, 11);
1358 let px: i32 = match p { Point(x, _) => x };
1361 A _unit-like struct_ is a structure without any fields, defined by leaving off
1362 the list of fields entirely. Such types will have a single value, just like
1363 the [unit value `()`](#unit-and-boolean-literals) of the unit type. For
1368 let c = [Cookie, Cookie, Cookie, Cookie];
1371 The precise memory layout of a structure is not specified. One can specify a
1372 particular layout using the [`repr` attribute](#ffi-attributes).
1376 An _enumeration_ is a simultaneous definition of a nominal [enumerated
1377 type](#enumerated-types) as well as a set of *constructors*, that can be used
1378 to create or pattern-match values of the corresponding enumerated type.
1380 Enumerations are declared with the keyword `enum`.
1382 An example of an `enum` item and its use:
1390 let mut a: Animal = Animal::Dog;
1394 Enumeration constructors can have either named or unnamed fields:
1397 # #![feature(struct_variant)]
1401 Cat { name: String, weight: f64 }
1404 let mut a: Animal = Animal::Dog("Cocoa".to_string(), 37.2);
1405 a = Animal::Cat { name: "Spotty".to_string(), weight: 2.7 };
1409 In this example, `Cat` is a _struct-like enum variant_,
1410 whereas `Dog` is simply called an enum variant.
1412 Enums have a discriminant. You can assign them explicitly:
1420 If a discriminant isn't assigned, they start at zero, and add one for each
1423 You can cast an enum to get this value:
1426 # enum Foo { Bar = 123 }
1427 let x = Foo::Bar as u32; // x is now 123u32
1430 This only works as long as none of the variants have data attached. If
1431 it were `Bar(i32)`, this is disallowed.
1436 const_item : "const" ident ':' type '=' expr ';' ;
1439 A *constant item* is a named _constant value_ which is not associated with a
1440 specific memory location in the program. Constants are essentially inlined
1441 wherever they are used, meaning that they are copied directly into the relevant
1442 context when used. References to the same constant are not necessarily
1443 guaranteed to refer to the same memory address.
1445 Constant values must not have destructors, and otherwise permit most forms of
1446 data. Constants may refer to the address of other constants, in which case the
1447 address will have the `static` lifetime. The compiler is, however, still at
1448 liberty to translate the constant many times, so the address referred to may not
1451 Constants must be explicitly typed. The type may be `bool`, `char`, a number, or
1452 a type derived from those primitive types. The derived types are references with
1453 the `static` lifetime, fixed-size arrays, tuples, enum variants, and structs.
1456 const BIT1: u32 = 1 << 0;
1457 const BIT2: u32 = 1 << 1;
1459 const BITS: [u32; 2] = [BIT1, BIT2];
1460 const STRING: &'static str = "bitstring";
1462 struct BitsNStrings<'a> {
1467 const BITS_N_STRINGS: BitsNStrings<'static> = BitsNStrings {
1476 static_item : "static" ident ':' type '=' expr ';' ;
1479 A *static item* is similar to a *constant*, except that it represents a precise
1480 memory location in the program. A static is never "inlined" at the usage site,
1481 and all references to it refer to the same memory location. Static items have
1482 the `static` lifetime, which outlives all other lifetimes in a Rust program.
1483 Static items may be placed in read-only memory if they do not contain any
1484 interior mutability.
1486 Statics may contain interior mutability through the `UnsafeCell` language item.
1487 All access to a static is safe, but there are a number of restrictions on
1490 * Statics may not contain any destructors.
1491 * The types of static values must ascribe to `Sync` to allow threadsafe access.
1492 * Statics may not refer to other statics by value, only by reference.
1493 * Constants cannot refer to statics.
1495 Constants should in general be preferred over statics, unless large amounts of
1496 data are being stored, or single-address and mutability properties are required.
1499 use std::sync::atomic::{AtomicUsize, Ordering, ATOMIC_USIZE_INIT};
1501 // Note that ATOMIC_USIZE_INIT is a *const*, but it may be used to initialize a
1502 // static. This static can be modified, so it is not placed in read-only memory.
1503 static COUNTER: AtomicUsize = ATOMIC_USIZE_INIT;
1505 // This table is a candidate to be placed in read-only memory.
1506 static TABLE: &'static [usize] = &[1, 2, 3, /* ... */];
1508 for slot in TABLE.iter() {
1509 println!("{}", slot);
1511 COUNTER.fetch_add(1, Ordering::SeqCst);
1514 #### Mutable statics
1516 If a static item is declared with the `mut` keyword, then it is allowed to
1517 be modified by the program. One of Rust's goals is to make concurrency bugs
1518 hard to run into, and this is obviously a very large source of race conditions
1519 or other bugs. For this reason, an `unsafe` block is required when either
1520 reading or writing a mutable static variable. Care should be taken to ensure
1521 that modifications to a mutable static are safe with respect to other threads
1522 running in the same process.
1524 Mutable statics are still very useful, however. They can be used with C
1525 libraries and can also be bound from C libraries (in an `extern` block).
1528 # fn atomic_add(_: &mut u32, _: u32) -> u32 { 2 }
1530 static mut LEVELS: u32 = 0;
1532 // This violates the idea of no shared state, and this doesn't internally
1533 // protect against races, so this function is `unsafe`
1534 unsafe fn bump_levels_unsafe1() -> u32 {
1540 // Assuming that we have an atomic_add function which returns the old value,
1541 // this function is "safe" but the meaning of the return value may not be what
1542 // callers expect, so it's still marked as `unsafe`
1543 unsafe fn bump_levels_unsafe2() -> u32 {
1544 return atomic_add(&mut LEVELS, 1);
1548 Mutable statics have the same restrictions as normal statics, except that the
1549 type of the value is not required to ascribe to `Sync`.
1553 A _trait_ describes a set of method types.
1555 Traits can include default implementations of methods, written in terms of some
1556 unknown [`self` type](#self-types); the `self` type may either be completely
1557 unspecified, or constrained by some other trait.
1559 Traits are implemented for specific types through separate
1560 [implementations](#implementations).
1563 # type Surface = i32;
1564 # type BoundingBox = i32;
1566 fn draw(&self, Surface);
1567 fn bounding_box(&self) -> BoundingBox;
1571 This defines a trait with two methods. All values that have
1572 [implementations](#implementations) of this trait in scope can have their
1573 `draw` and `bounding_box` methods called, using `value.bounding_box()`
1574 [syntax](#method-call-expressions).
1576 Type parameters can be specified for a trait to make it generic. These appear
1577 after the trait name, using the same syntax used in [generic
1578 functions](#generic-functions).
1582 fn len(&self) -> u32;
1583 fn elt_at(&self, n: u32) -> T;
1584 fn iter<F>(&self, F) where F: Fn(T);
1588 Generic functions may use traits as _bounds_ on their type parameters. This
1589 will have two effects: only types that have the trait may instantiate the
1590 parameter, and within the generic function, the methods of the trait can be
1591 called on values that have the parameter's type. For example:
1594 # type Surface = i32;
1595 # trait Shape { fn draw(&self, Surface); }
1596 fn draw_twice<T: Shape>(surface: Surface, sh: T) {
1602 Traits also define an [object type](#object-types) with the same name as the
1603 trait. Values of this type are created by [casting](#type-cast-expressions)
1604 pointer values (pointing to a type for which an implementation of the given
1605 trait is in scope) to pointers to the trait name, used as a type.
1609 # impl Shape for i32 { }
1610 # let mycircle = 0i32;
1611 let myshape: Box<Shape> = Box::new(mycircle) as Box<Shape>;
1614 The resulting value is a box containing the value that was cast, along with
1615 information that identifies the methods of the implementation that was used.
1616 Values with a trait type can have [methods called](#method-call-expressions) on
1617 them, for any method in the trait, and can be used to instantiate type
1618 parameters that are bounded by the trait.
1620 Trait methods may be static, which means that they lack a `self` argument.
1621 This means that they can only be called with function call syntax (`f(x)`) and
1622 not method call syntax (`obj.f()`). The way to refer to the name of a static
1623 method is to qualify it with the trait name, treating the trait name like a
1624 module. For example:
1628 fn from_i32(n: i32) -> Self;
1631 fn from_i32(n: i32) -> f64 { n as f64 }
1633 let x: f64 = Num::from_i32(42);
1636 Traits may inherit from other traits. For example, in
1639 trait Shape { fn area() -> f64; }
1640 trait Circle : Shape { fn radius() -> f64; }
1643 the syntax `Circle : Shape` means that types that implement `Circle` must also
1644 have an implementation for `Shape`. Multiple supertraits are separated by `+`,
1645 `trait Circle : Shape + PartialEq { }`. In an implementation of `Circle` for a
1646 given type `T`, methods can refer to `Shape` methods, since the typechecker
1647 checks that any type with an implementation of `Circle` also has an
1648 implementation of `Shape`.
1650 In type-parameterized functions, methods of the supertrait may be called on
1651 values of subtrait-bound type parameters. Referring to the previous example of
1652 `trait Circle : Shape`:
1655 # trait Shape { fn area(&self) -> f64; }
1656 # trait Circle : Shape { fn radius(&self) -> f64; }
1657 fn radius_times_area<T: Circle>(c: T) -> f64 {
1658 // `c` is both a Circle and a Shape
1659 c.radius() * c.area()
1663 Likewise, supertrait methods may also be called on trait objects.
1666 # trait Shape { fn area(&self) -> f64; }
1667 # trait Circle : Shape { fn radius(&self) -> f64; }
1668 # impl Shape for i32 { fn area(&self) -> f64 { 0.0 } }
1669 # impl Circle for i32 { fn radius(&self) -> f64 { 0.0 } }
1670 # let mycircle = 0i32;
1671 let mycircle = Box::new(mycircle) as Box<Circle>;
1672 let nonsense = mycircle.radius() * mycircle.area();
1677 An _implementation_ is an item that implements a [trait](#traits) for a
1680 Implementations are defined with the keyword `impl`.
1683 # struct Point {x: f64, y: f64};
1684 # impl Copy for Point {}
1685 # type Surface = i32;
1686 # struct BoundingBox {x: f64, y: f64, width: f64, height: f64};
1687 # trait Shape { fn draw(&self, Surface); fn bounding_box(&self) -> BoundingBox; }
1688 # fn do_draw_circle(s: Surface, c: Circle) { }
1694 impl Copy for Circle {}
1696 impl Shape for Circle {
1697 fn draw(&self, s: Surface) { do_draw_circle(s, *self); }
1698 fn bounding_box(&self) -> BoundingBox {
1699 let r = self.radius;
1700 BoundingBox{x: self.center.x - r, y: self.center.y - r,
1701 width: 2.0 * r, height: 2.0 * r}
1706 It is possible to define an implementation without referring to a trait. The
1707 methods in such an implementation can only be used as direct calls on the
1708 values of the type that the implementation targets. In such an implementation,
1709 the trait type and `for` after `impl` are omitted. Such implementations are
1710 limited to nominal types (enums, structs), and the implementation must appear
1711 in the same module or a sub-module as the `self` type:
1714 struct Point {x: i32, y: i32}
1718 println!("Point is at ({}, {})", self.x, self.y);
1722 let my_point = Point {x: 10, y:11};
1726 When a trait _is_ specified in an `impl`, all methods declared as part of the
1727 trait must be implemented, with matching types and type parameter counts.
1729 An implementation can take type parameters, which can be different from the
1730 type parameters taken by the trait it implements. Implementation parameters
1731 are written after the `impl` keyword.
1735 impl<T> Seq<T> for Vec<T> {
1738 impl Seq<bool> for u32 {
1739 /* Treat the integer as a sequence of bits */
1746 extern_block_item : "extern" '{' extern_block '}' ;
1747 extern_block : [ foreign_fn ] * ;
1750 External blocks form the basis for Rust's foreign function interface.
1751 Declarations in an external block describe symbols in external, non-Rust
1754 Functions within external blocks are declared in the same way as other Rust
1755 functions, with the exception that they may not have a body and are instead
1756 terminated by a semicolon.
1760 use libc::{c_char, FILE};
1763 fn fopen(filename: *const c_char, mode: *const c_char) -> *mut FILE;
1768 Functions within external blocks may be called by Rust code, just like
1769 functions defined in Rust. The Rust compiler automatically translates between
1770 the Rust ABI and the foreign ABI.
1772 A number of [attributes](#attributes) control the behavior of external blocks.
1774 By default external blocks assume that the library they are calling uses the
1775 standard C "cdecl" ABI. Other ABIs may be specified using an `abi` string, as
1779 // Interface to the Windows API
1780 extern "stdcall" { }
1783 The `link` attribute allows the name of the library to be specified. When
1784 specified the compiler will attempt to link against the native library of the
1788 #[link(name = "crypto")]
1792 The type of a function declared in an extern block is `extern "abi" fn(A1, ...,
1793 An) -> R`, where `A1...An` are the declared types of its arguments and `R` is
1794 the declared return type.
1796 ## Visibility and Privacy
1798 These two terms are often used interchangeably, and what they are attempting to
1799 convey is the answer to the question "Can this item be used at this location?"
1801 Rust's name resolution operates on a global hierarchy of namespaces. Each level
1802 in the hierarchy can be thought of as some item. The items are one of those
1803 mentioned above, but also include external crates. Declaring or defining a new
1804 module can be thought of as inserting a new tree into the hierarchy at the
1805 location of the definition.
1807 To control whether interfaces can be used across modules, Rust checks each use
1808 of an item to see whether it should be allowed or not. This is where privacy
1809 warnings are generated, or otherwise "you used a private item of another module
1810 and weren't allowed to."
1812 By default, everything in Rust is *private*, with one exception. Enum variants
1813 in a `pub` enum are also public by default. You are allowed to alter this
1814 default visibility with the `priv` keyword. When an item is declared as `pub`,
1815 it can be thought of as being accessible to the outside world. For example:
1818 # #![allow(missing_copy_implementations)]
1820 // Declare a private struct
1823 // Declare a public struct with a private field
1828 // Declare a public enum with two public variants
1830 PubliclyAccessibleState,
1831 PubliclyAccessibleState2,
1835 With the notion of an item being either public or private, Rust allows item
1836 accesses in two cases:
1838 1. If an item is public, then it can be used externally through any of its
1840 2. If an item is private, it may be accessed by the current module and its
1843 These two cases are surprisingly powerful for creating module hierarchies
1844 exposing public APIs while hiding internal implementation details. To help
1845 explain, here's a few use cases and what they would entail:
1847 * A library developer needs to expose functionality to crates which link
1848 against their library. As a consequence of the first case, this means that
1849 anything which is usable externally must be `pub` from the root down to the
1850 destination item. Any private item in the chain will disallow external
1853 * A crate needs a global available "helper module" to itself, but it doesn't
1854 want to expose the helper module as a public API. To accomplish this, the
1855 root of the crate's hierarchy would have a private module which then
1856 internally has a "public api". Because the entire crate is a descendant of
1857 the root, then the entire local crate can access this private module through
1860 * When writing unit tests for a module, it's often a common idiom to have an
1861 immediate child of the module to-be-tested named `mod test`. This module
1862 could access any items of the parent module through the second case, meaning
1863 that internal implementation details could also be seamlessly tested from the
1866 In the second case, it mentions that a private item "can be accessed" by the
1867 current module and its descendants, but the exact meaning of accessing an item
1868 depends on what the item is. Accessing a module, for example, would mean
1869 looking inside of it (to import more items). On the other hand, accessing a
1870 function would mean that it is invoked. Additionally, path expressions and
1871 import statements are considered to access an item in the sense that the
1872 import/expression is only valid if the destination is in the current visibility
1875 Here's an example of a program which exemplifies the three cases outlined
1879 // This module is private, meaning that no external crate can access this
1880 // module. Because it is private at the root of this current crate, however, any
1881 // module in the crate may access any publicly visible item in this module.
1882 mod crate_helper_module {
1884 // This function can be used by anything in the current crate
1885 pub fn crate_helper() {}
1887 // This function *cannot* be used by anything else in the crate. It is not
1888 // publicly visible outside of the `crate_helper_module`, so only this
1889 // current module and its descendants may access it.
1890 fn implementation_detail() {}
1893 // This function is "public to the root" meaning that it's available to external
1894 // crates linking against this one.
1895 pub fn public_api() {}
1897 // Similarly to 'public_api', this module is public so external crates may look
1900 use crate_helper_module;
1902 pub fn my_method() {
1903 // Any item in the local crate may invoke the helper module's public
1904 // interface through a combination of the two rules above.
1905 crate_helper_module::crate_helper();
1908 // This function is hidden to any module which is not a descendant of
1910 fn my_implementation() {}
1916 fn test_my_implementation() {
1917 // Because this module is a descendant of `submodule`, it's allowed
1918 // to access private items inside of `submodule` without a privacy
1920 super::my_implementation();
1928 For a rust program to pass the privacy checking pass, all paths must be valid
1929 accesses given the two rules above. This includes all use statements,
1930 expressions, types, etc.
1932 ### Re-exporting and Visibility
1934 Rust allows publicly re-exporting items through a `pub use` directive. Because
1935 this is a public directive, this allows the item to be used in the current
1936 module through the rules above. It essentially allows public access into the
1937 re-exported item. For example, this program is valid:
1940 pub use self::implementation as api;
1942 mod implementation {
1949 This means that any external crate referencing `implementation::f` would
1950 receive a privacy violation, while the path `api::f` would be allowed.
1952 When re-exporting a private item, it can be thought of as allowing the "privacy
1953 chain" being short-circuited through the reexport instead of passing through
1954 the namespace hierarchy as it normally would.
1959 attribute : "#!" ? '[' meta_item ']' ;
1960 meta_item : ident [ '=' literal
1961 | '(' meta_seq ')' ] ? ;
1962 meta_seq : meta_item [ ',' meta_seq ] ? ;
1965 Any item declaration may have an _attribute_ applied to it. Attributes in Rust
1966 are modeled on Attributes in ECMA-335, with the syntax coming from ECMA-334
1967 (C#). An attribute is a general, free-form metadatum that is interpreted
1968 according to name, convention, and language and compiler version. Attributes
1969 may appear as any of:
1971 * A single identifier, the attribute name
1972 * An identifier followed by the equals sign '=' and a literal, providing a
1974 * An identifier followed by a parenthesized list of sub-attribute arguments
1976 Attributes with a bang ("!") after the hash ("#") apply to the item that the
1977 attribute is declared within. Attributes that do not have a bang after the hash
1978 apply to the item that follows the attribute.
1980 An example of attributes:
1983 // General metadata applied to the enclosing module or crate.
1984 #![crate_type = "lib"]
1986 // A function marked as a unit test
1992 // A conditionally-compiled module
1993 #[cfg(target_os="linux")]
1998 // A lint attribute used to suppress a warning/error
1999 #[allow(non_camel_case_types)]
2003 > **Note:** At some point in the future, the compiler will distinguish between
2004 > language-reserved and user-available attributes. Until then, there is
2005 > effectively no difference between an attribute handled by a loadable syntax
2006 > extension and the compiler.
2008 ### Crate-only attributes
2010 - `crate_name` - specify the this crate's crate name.
2011 - `crate_type` - see [linkage](#linkage).
2012 - `feature` - see [compiler features](#compiler-features).
2013 - `no_builtins` - disable optimizing certain code patterns to invocations of
2014 library functions that are assumed to exist
2015 - `no_main` - disable emitting the `main` symbol. Useful when some other
2016 object being linked to defines `main`.
2017 - `no_start` - disable linking to the `native` crate, which specifies the
2018 "start" language item.
2019 - `no_std` - disable linking to the `std` crate.
2021 ### Module-only attributes
2023 - `no_implicit_prelude` - disable injecting `use std::prelude::*` in this
2025 - `path` - specifies the file to load the module from. `#[path="foo.rs"] mod
2026 bar;` is equivalent to `mod bar { /* contents of foo.rs */ }`. The path is
2027 taken relative to the directory that the current module is in.
2029 ### Function-only attributes
2031 - `main` - indicates that this function should be passed to the entry point,
2032 rather than the function in the crate root named `main`.
2033 - `plugin_registrar` - mark this function as the registration point for
2034 [compiler plugins][plugin], such as loadable syntax extensions.
2035 - `start` - indicates that this function should be used as the entry point,
2036 overriding the "start" language item. See the "start" [language
2037 item](#language-items) for more details.
2038 - `test` - indicates that this function is a test function, to only be compiled
2039 in case of `--test`.
2040 - `should_fail` - indicates that this test function should panic, inverting the success condition.
2041 - `cold` - The function is unlikely to be executed, so optimize it (and calls
2044 ### Static-only attributes
2046 - `thread_local` - on a `static mut`, this signals that the value of this
2047 static may change depending on the current thread. The exact consequences of
2048 this are implementation-defined.
2052 On an `extern` block, the following attributes are interpreted:
2054 - `link_args` - specify arguments to the linker, rather than just the library
2055 name and type. This is feature gated and the exact behavior is
2056 implementation-defined (due to variety of linker invocation syntax).
2057 - `link` - indicate that a native library should be linked to for the
2058 declarations in this block to be linked correctly. `link` supports an optional `kind`
2059 key with three possible values: `dylib`, `static`, and `framework`. See [external blocks](#external-blocks) for more about external blocks. Two
2060 examples: `#[link(name = "readline")]` and
2061 `#[link(name = "CoreFoundation", kind = "framework")]`.
2063 On declarations inside an `extern` block, the following attributes are
2066 - `link_name` - the name of the symbol that this function or static should be
2068 - `linkage` - on a static, this specifies the [linkage
2069 type](http://llvm.org/docs/LangRef.html#linkage-types).
2073 - `repr` - on C-like enums, this sets the underlying type used for
2074 representation. Takes one argument, which is the primitive
2075 type this enum should be represented for, or `C`, which specifies that it
2076 should be the default `enum` size of the C ABI for that platform. Note that
2077 enum representation in C is undefined, and this may be incorrect when the C
2078 code is compiled with certain flags.
2082 - `repr` - specifies the representation to use for this struct. Takes a list
2083 of options. The currently accepted ones are `C` and `packed`, which may be
2084 combined. `C` will use a C ABI compatible struct layout, and `packed` will
2085 remove any padding between fields (note that this is very fragile and may
2086 break platforms which require aligned access).
2088 ### Macro- and plugin-related attributes
2090 - `macro_use` on a `mod` — macros defined in this module will be visible in the
2091 module's parent, after this module has been included.
2093 - `macro_use` on an `extern crate` — load macros from this crate. An optional
2094 list of names `#[macro_use(foo, bar)]` restricts the import to just those
2095 macros named. The `extern crate` must appear at the crate root, not inside
2096 `mod`, which ensures proper function of the [`$crate` macro
2097 variable](book/macros.html#the-variable-$crate).
2099 - `macro_reexport` on an `extern crate` — re-export the named macros.
2101 - `macro_export` - export a macro for cross-crate usage.
2103 - `plugin` on an `extern crate` — load this crate as a [compiler
2104 plugin][plugin]. The `plugin` feature gate is required. Any arguments to
2105 the attribute, e.g. `#[plugin=...]` or `#[plugin(...)]`, are provided to the
2108 - `no_link` on an `extern crate` — even if we load this crate for macros or
2109 compiler plugins, don't link it into the output.
2111 See the [macros section of the
2112 book](book/macros.html#scoping-and-macro-import/export) for more information on
2116 ### Miscellaneous attributes
2118 - `export_name` - on statics and functions, this determines the name of the
2120 - `link_section` - on statics and functions, this specifies the section of the
2121 object file that this item's contents will be placed into.
2122 - `no_mangle` - on any item, do not apply the standard name mangling. Set the
2123 symbol for this item to its identifier.
2124 - `packed` - on structs or enums, eliminate any padding that would be used to
2126 - `simd` - on certain tuple structs, derive the arithmetic operators, which
2127 lower to the target's SIMD instructions, if any; the `simd` feature gate
2128 is necessary to use this attribute.
2129 - `static_assert` - on statics whose type is `bool`, terminates compilation
2130 with an error if it is not initialized to `true`.
2131 - `unsafe_destructor` - allow implementations of the "drop" language item
2132 where the type it is implemented for does not implement the "send" language
2133 item; the `unsafe_destructor` feature gate is needed to use this attribute
2134 - `unsafe_no_drop_flag` - on structs, remove the flag that prevents
2135 destructors from being run twice. Destructors might be run multiple times on
2136 the same object with this attribute.
2137 - `doc` - Doc comments such as `/// foo` are equivalent to `#[doc = "foo"]`.
2138 - `rustc_on_unimplemented` - Write a custom note to be shown along with the error
2139 when the trait is found to be unimplemented on a type.
2140 You may use format arguments like `{T}`, `{A}` to correspond to the
2141 types at the point of use corresponding to the type parameters of the
2142 trait of the same name. `{Self}` will be replaced with the type that is supposed
2143 to implement the trait but doesn't. To use this, the `on_unimplemented` feature gate
2146 ### Conditional compilation
2148 Sometimes one wants to have different compiler outputs from the same code,
2149 depending on build target, such as targeted operating system, or to enable
2152 There are two kinds of configuration options, one that is either defined or not
2153 (`#[cfg(foo)]`), and the other that contains a string that can be checked
2154 against (`#[cfg(bar = "baz")]` (currently only compiler-defined configuration
2155 options can have the latter form).
2158 // The function is only included in the build when compiling for OSX
2159 #[cfg(target_os = "macos")]
2164 // This function is only included when either foo or bar is defined
2165 #[cfg(any(foo, bar))]
2166 fn needs_foo_or_bar() {
2170 // This function is only included when compiling for a unixish OS with a 32-bit
2172 #[cfg(all(unix, target_word_size = "32"))]
2173 fn on_32bit_unix() {
2177 // This function is only included when foo is not defined
2179 fn needs_not_foo() {
2184 This illustrates some conditional compilation can be achieved using the
2185 `#[cfg(...)]` attribute. `any`, `all` and `not` can be used to assemble
2186 arbitrarily complex configurations through nesting.
2188 The following configurations must be defined by the implementation:
2190 * `target_arch = "..."`. Target CPU architecture, such as `"x86"`, `"x86_64"`
2191 `"mips"`, `"powerpc"`, `"arm"`, or `"aarch64"`.
2192 * `target_endian = "..."`. Endianness of the target CPU, either `"little"` or
2194 * `target_family = "..."`. Operating system family of the target, e. g.
2195 `"unix"` or `"windows"`. The value of this configuration option is defined
2196 as a configuration itself, like `unix` or `windows`.
2197 * `target_os = "..."`. Operating system of the target, examples include
2198 `"win32"`, `"macos"`, `"linux"`, `"android"`, `"freebsd"` or `"dragonfly"`.
2199 * `target_word_size = "..."`. Target word size in bits. This is set to `"32"`
2200 for targets with 32-bit pointers, and likewise set to `"64"` for 64-bit
2202 * `unix`. See `target_family`.
2203 * `windows`. See `target_family`.
2205 ### Lint check attributes
2207 A lint check names a potentially undesirable coding pattern, such as
2208 unreachable code or omitted documentation, for the static entity to which the
2211 For any lint check `C`:
2213 * `allow(C)` overrides the check for `C` so that violations will go
2215 * `deny(C)` signals an error after encountering a violation of `C`,
2216 * `forbid(C)` is the same as `deny(C)`, but also forbids changing the lint
2218 * `warn(C)` warns about violations of `C` but continues compilation.
2220 The lint checks supported by the compiler can be found via `rustc -W help`,
2221 along with their default settings. [Compiler
2222 plugins](book/plugin.html#lint-plugins) can provide additional lint checks.
2226 // Missing documentation is ignored here
2227 #[allow(missing_docs)]
2228 pub fn undocumented_one() -> i32 { 1 }
2230 // Missing documentation signals a warning here
2231 #[warn(missing_docs)]
2232 pub fn undocumented_too() -> i32 { 2 }
2234 // Missing documentation signals an error here
2235 #[deny(missing_docs)]
2236 pub fn undocumented_end() -> i32 { 3 }
2240 This example shows how one can use `allow` and `warn` to toggle a particular
2244 #[warn(missing_docs)]
2246 #[allow(missing_docs)]
2248 // Missing documentation is ignored here
2249 pub fn undocumented_one() -> i32 { 1 }
2251 // Missing documentation signals a warning here,
2252 // despite the allow above.
2253 #[warn(missing_docs)]
2254 pub fn undocumented_two() -> i32 { 2 }
2257 // Missing documentation signals a warning here
2258 pub fn undocumented_too() -> i32 { 3 }
2262 This example shows how one can use `forbid` to disallow uses of `allow` for
2266 #[forbid(missing_docs)]
2268 // Attempting to toggle warning signals an error here
2269 #[allow(missing_docs)]
2271 pub fn undocumented_too() -> i32 { 2 }
2277 Some primitive Rust operations are defined in Rust code, rather than being
2278 implemented directly in C or assembly language. The definitions of these
2279 operations have to be easy for the compiler to find. The `lang` attribute
2280 makes it possible to declare these operations. For example, the `str` module
2281 in the Rust standard library defines the string equality function:
2285 pub fn eq_slice(a: &str, b: &str) -> bool {
2290 The name `str_eq` has a special meaning to the Rust compiler, and the presence
2291 of this definition means that it will use this definition when generating calls
2292 to the string equality function.
2294 A complete list of the built-in language items follows:
2296 #### Built-in Traits
2299 : Types that do not move ownership when used by-value.
2303 : Able to be sent across thread boundaries.
2305 : Has a size known at compile time.
2307 : Able to be safely shared between threads when aliased.
2311 These language items are traits:
2314 : Elements can be added (for example, integers and floats).
2316 : Elements can be subtracted.
2318 : Elements can be multiplied.
2320 : Elements have a division operation.
2322 : Elements have a remainder operation.
2324 : Elements can be negated arithmetically.
2326 : Elements can be negated logically.
2328 : Elements have an exclusive-or operation.
2330 : Elements have a bitwise `and` operation.
2332 : Elements have a bitwise `or` operation.
2334 : Elements have a left shift operation.
2336 : Elements have a right shift operation.
2338 : Elements can be indexed.
2340 : ___Needs filling in___
2342 : Elements can be compared for equality.
2344 : Elements have a partial ordering.
2346 : `*` can be applied, yielding a reference to another type.
2348 : `*` can be applied, yielding a mutable reference to another type.
2350 These are functions:
2353 : ___Needs filling in___
2355 : ___Needs filling in___
2357 : ___Needs filling in___
2359 : Compare two strings (`&str`) for equality.
2361 : Return a new unique string
2362 containing a copy of the contents of a unique string.
2367 : The type returned by the `type_id` intrinsic.
2369 : A type whose contents can be mutated through an immutable reference.
2373 These types help drive the compiler's analysis
2376 : ___Needs filling in___
2378 : This type does not implement "copy", even if eligible.
2380 : ___Needs filling in___
2382 : Free memory that was allocated on the exchange heap.
2384 : Allocate memory on the exchange heap.
2385 * `closure_exchange_malloc`
2386 : ___Needs filling in___
2388 : Abort the program with an error.
2389 * `fail_bounds_check`
2390 : Abort the program with a bounds check error.
2392 : Free memory that was allocated on the managed heap.
2394 : ___Needs filling in___
2396 : ___Needs filling in___
2398 : ___Needs filling in___
2399 * `contravariant_lifetime`
2400 : The lifetime parameter should be considered contravariant.
2401 * `covariant_lifetime`
2402 : The lifetime parameter should be considered covariant.
2403 * `invariant_lifetime`
2404 : The lifetime parameter should be considered invariant.
2406 : Allocate memory on the managed heap.
2408 : ___Needs filling in___
2410 : ___Needs filling in___
2412 : ___Needs filling in___
2413 * `contravariant_type`
2414 : The type parameter should be considered contravariant.
2416 : The type parameter should be considered covariant.
2418 : The type parameter should be considered invariant.
2420 : ___Needs filling in___
2422 > **Note:** This list is likely to become out of date. We should auto-generate
2423 > it from `librustc/middle/lang_items.rs`.
2425 ### Inline attributes
2427 The inline attribute is used to suggest to the compiler to perform an inline
2428 expansion and place a copy of the function or static in the caller rather than
2429 generating code to call the function or access the static where it is defined.
2431 The compiler automatically inlines functions based on internal heuristics.
2432 Incorrectly inlining functions can actually making the program slower, so it
2433 should be used with care.
2435 Immutable statics are always considered inlineable unless marked with
2436 `#[inline(never)]`. It is undefined whether two different inlineable statics
2437 have the same memory address. In other words, the compiler is free to collapse
2438 duplicate inlineable statics together.
2440 `#[inline]` and `#[inline(always)]` always causes the function to be serialized
2441 into crate metadata to allow cross-crate inlining.
2443 There are three different types of inline attributes:
2445 * `#[inline]` hints the compiler to perform an inline expansion.
2446 * `#[inline(always)]` asks the compiler to always perform an inline expansion.
2447 * `#[inline(never)]` asks the compiler to never perform an inline expansion.
2451 The `derive` attribute allows certain traits to be automatically implemented
2452 for data structures. For example, the following will create an `impl` for the
2453 `PartialEq` and `Clone` traits for `Foo`, the type parameter `T` will be given
2454 the `PartialEq` or `Clone` constraints for the appropriate `impl`:
2457 #[derive(PartialEq, Clone)]
2464 The generated `impl` for `PartialEq` is equivalent to
2467 # struct Foo<T> { a: i32, b: T }
2468 impl<T: PartialEq> PartialEq for Foo<T> {
2469 fn eq(&self, other: &Foo<T>) -> bool {
2470 self.a == other.a && self.b == other.b
2473 fn ne(&self, other: &Foo<T>) -> bool {
2474 self.a != other.a || self.b != other.b
2479 Supported traits for `derive` are:
2481 * Comparison traits: `PartialEq`, `Eq`, `PartialOrd`, `Ord`.
2482 * Serialization: `Encodable`, `Decodable`. These require `serialize`.
2483 * `Clone`, to create `T` from `&T` via a copy.
2484 * `Default`, to create an empty instance of a data type.
2485 * `FromPrimitive`, to create an instance from a numeric primitive.
2486 * `Hash`, to iterate over the bytes in a data type.
2487 * `Rand`, to create a random instance of a data type.
2488 * `Show`, to format a value using the `{}` formatter.
2489 * `Zero`, to create a zero instance of a numeric data type.
2493 One can indicate the stability of an API using the following attributes:
2495 * `deprecated`: This item should no longer be used, e.g. it has been
2496 replaced. No guarantee of backwards-compatibility.
2497 * `experimental`: This item was only recently introduced or is
2498 otherwise in a state of flux. It may change significantly, or even
2499 be removed. No guarantee of backwards-compatibility.
2500 * `unstable`: This item is still under development, but requires more
2501 testing to be considered stable. No guarantee of backwards-compatibility.
2502 * `stable`: This item is considered stable, and will not change
2503 significantly. Guarantee of backwards-compatibility.
2504 * `frozen`: This item is very stable, and is unlikely to
2505 change. Guarantee of backwards-compatibility.
2506 * `locked`: This item will never change unless a serious bug is
2507 found. Guarantee of backwards-compatibility.
2509 These levels are directly inspired by
2510 [Node.js' "stability index"](http://nodejs.org/api/documentation.html).
2512 Stability levels are inherited, so an item's stability attribute is the default
2513 stability for everything nested underneath it.
2515 There are lints for disallowing items marked with certain levels: `deprecated`,
2516 `experimental` and `unstable`. For now, only `deprecated` warns by default, but
2517 this will change once the standard library has been stabilized. Stability
2518 levels are meant to be promises at the crate level, so these lints only apply
2519 when referencing items from an _external_ crate, not to items defined within
2520 the current crate. Items with no stability level are considered to be unstable
2521 for the purposes of the lint. One can give an optional string that will be
2522 displayed when the lint flags the use of an item.
2524 For example, if we define one crate called `stability_levels`:
2527 #[deprecated="replaced by `best`"]
2529 // delete everything
2533 // delete fewer things
2542 then the lints will work as follows for a client crate:
2546 extern crate stability_levels;
2547 use stability_levels::{bad, better, best};
2550 bad(); // "warning: use of deprecated item: replaced by `best`"
2552 better(); // "warning: use of unmarked item"
2554 best(); // no warning
2558 > **Note:** Currently these are only checked when applied to individual
2559 > functions, structs, methods and enum variants, *not* to entire modules,
2560 > traits, impls or enums themselves.
2562 ### Compiler Features
2564 Certain aspects of Rust may be implemented in the compiler, but they're not
2565 necessarily ready for every-day use. These features are often of "prototype
2566 quality" or "almost production ready", but may not be stable enough to be
2567 considered a full-fledged language feature.
2569 For this reason, Rust recognizes a special crate-level attribute of the form:
2572 #![feature(feature1, feature2, feature3)]
2575 This directive informs the compiler that the feature list: `feature1`,
2576 `feature2`, and `feature3` should all be enabled. This is only recognized at a
2577 crate-level, not at a module-level. Without this directive, all features are
2578 considered off, and using the features will result in a compiler error.
2580 The currently implemented features of the reference compiler are:
2582 * `asm` - The `asm!` macro provides a means for inline assembly. This is often
2583 useful, but the exact syntax for this feature along with its
2584 semantics are likely to change, so this macro usage must be opted
2587 * `concat_idents` - Allows use of the `concat_idents` macro, which is in many
2588 ways insufficient for concatenating identifiers, and may be
2589 removed entirely for something more wholesome.
2591 * `default_type_params` - Allows use of default type parameters. The future of
2592 this feature is uncertain.
2594 * `intrinsics` - Allows use of the "rust-intrinsics" ABI. Compiler intrinsics
2595 are inherently unstable and no promise about them is made.
2597 * `lang_items` - Allows use of the `#[lang]` attribute. Like `intrinsics`,
2598 lang items are inherently unstable and no promise about them
2601 * `link_args` - This attribute is used to specify custom flags to the linker,
2602 but usage is strongly discouraged. The compiler's usage of the
2603 system linker is not guaranteed to continue in the future, and
2604 if the system linker is not used then specifying custom flags
2605 doesn't have much meaning.
2607 * `link_llvm_intrinsics` – Allows linking to LLVM intrinsics via
2608 `#[link_name="llvm.*"]`.
2610 * `linkage` - Allows use of the `linkage` attribute, which is not portable.
2612 * `log_syntax` - Allows use of the `log_syntax` macro attribute, which is a
2613 nasty hack that will certainly be removed.
2615 * `non_ascii_idents` - The compiler supports the use of non-ascii identifiers,
2616 but the implementation is a little rough around the
2617 edges, so this can be seen as an experimental feature
2618 for now until the specification of identifiers is fully
2621 * `once_fns` - Onceness guarantees a closure is only executed once. Defining a
2622 closure as `once` is unlikely to be supported going forward. So
2623 they are hidden behind this feature until they are to be removed.
2625 * `plugin` - Usage of [compiler plugins][plugin] for custom lints or syntax extensions.
2626 These depend on compiler internals and are subject to change.
2628 * `plugin_registrar` - Indicates that a crate provides [compiler plugins][plugin].
2630 * `quote` - Allows use of the `quote_*!` family of macros, which are
2631 implemented very poorly and will likely change significantly
2632 with a proper implementation.
2634 * `rustc_diagnostic_macros`- A mysterious feature, used in the implementation
2635 of rustc, not meant for mortals.
2637 * `simd` - Allows use of the `#[simd]` attribute, which is overly simple and
2638 not the SIMD interface we want to expose in the long term.
2640 * `struct_inherit` - Allows using struct inheritance, which is barely
2641 implemented and will probably be removed. Don't use this.
2643 * `struct_variant` - Structural enum variants (those with named fields). It is
2644 currently unknown whether this style of enum variant is as
2645 fully supported as the tuple-forms, and it's not certain
2646 that this style of variant should remain in the language.
2647 For now this style of variant is hidden behind a feature
2650 * `thread_local` - The usage of the `#[thread_local]` attribute is experimental
2651 and should be seen as unstable. This attribute is used to
2652 declare a `static` as being unique per-thread leveraging
2653 LLVM's implementation which works in concert with the kernel
2654 loader and dynamic linker. This is not necessarily available
2655 on all platforms, and usage of it is discouraged (rust
2656 focuses more on thread-local data instead of thread-local
2659 * `trace_macros` - Allows use of the `trace_macros` macro, which is a nasty
2660 hack that will certainly be removed.
2662 * `unboxed_closures` - Rust's new closure design, which is currently a work in
2663 progress feature with many known bugs.
2665 * `unsafe_destructor` - Allows use of the `#[unsafe_destructor]` attribute,
2666 which is considered wildly unsafe and will be
2667 obsoleted by language improvements.
2669 * `associated_types` - Allows type aliases in traits. Experimental.
2671 If a feature is promoted to a language feature, then all existing programs will
2672 start to receive compilation warnings about #[feature] directives which enabled
2673 the new feature (because the directive is no longer necessary). However, if a
2674 feature is decided to be removed from the language, errors will be issued (if
2675 there isn't a parser error first). The directive in this case is no longer
2676 necessary, and it's likely that existing code will break if the feature isn't
2679 If an unknown feature is found in a directive, it results in a compiler error.
2680 An unknown feature is one which has never been recognized by the compiler.
2682 # Statements and expressions
2684 Rust is _primarily_ an expression language. This means that most forms of
2685 value-producing or effect-causing evaluation are directed by the uniform syntax
2686 category of _expressions_. Each kind of expression can typically _nest_ within
2687 each other kind of expression, and rules for evaluation of expressions involve
2688 specifying both the value produced by the expression and the order in which its
2689 sub-expressions are themselves evaluated.
2691 In contrast, statements in Rust serve _mostly_ to contain and explicitly
2692 sequence expression evaluation.
2696 A _statement_ is a component of a block, which is in turn a component of an
2697 outer [expression](#expressions) or [function](#functions).
2699 Rust has two kinds of statement: [declaration
2700 statements](#declaration-statements) and [expression
2701 statements](#expression-statements).
2703 ### Declaration statements
2705 A _declaration statement_ is one that introduces one or more *names* into the
2706 enclosing statement block. The declared names may denote new slots or new
2709 #### Item declarations
2711 An _item declaration statement_ has a syntactic form identical to an
2712 [item](#items) declaration within a module. Declaring an item — a
2713 function, enumeration, structure, type, static, trait, implementation or module
2714 — locally within a statement block is simply a way of restricting its
2715 scope to a narrow region containing all of its uses; it is otherwise identical
2716 in meaning to declaring the item outside the statement block.
2718 > **Note**: there is no implicit capture of the function's dynamic environment when
2719 > declaring a function-local item.
2721 #### Slot declarations
2724 let_decl : "let" pat [':' type ] ? [ init ] ? ';' ;
2725 init : [ '=' ] expr ;
2728 A _slot declaration_ introduces a new set of slots, given by a pattern. The
2729 pattern may be followed by a type annotation, and/or an initializer expression.
2730 When no type annotation is given, the compiler will infer the type, or signal
2731 an error if insufficient type information is available for definite inference.
2732 Any slots introduced by a slot declaration are visible from the point of
2733 declaration until the end of the enclosing block scope.
2735 ### Expression statements
2737 An _expression statement_ is one that evaluates an [expression](#expressions)
2738 and ignores its result. The type of an expression statement `e;` is always
2739 `()`, regardless of the type of `e`. As a rule, an expression statement's
2740 purpose is to trigger the effects of evaluating its expression.
2744 An expression may have two roles: it always produces a *value*, and it may have
2745 *effects* (otherwise known as "side effects"). An expression *evaluates to* a
2746 value, and has effects during *evaluation*. Many expressions contain
2747 sub-expressions (operands). The meaning of each kind of expression dictates
2750 * Whether or not to evaluate the sub-expressions when evaluating the expression
2751 * The order in which to evaluate the sub-expressions
2752 * How to combine the sub-expressions' values to obtain the value of the expression
2754 In this way, the structure of expressions dictates the structure of execution.
2755 Blocks are just another kind of expression, so blocks, statements, expressions,
2756 and blocks again can recursively nest inside each other to an arbitrary depth.
2758 #### Lvalues, rvalues and temporaries
2760 Expressions are divided into two main categories: _lvalues_ and _rvalues_.
2761 Likewise within each expression, sub-expressions may occur in _lvalue context_
2762 or _rvalue context_. The evaluation of an expression depends both on its own
2763 category and the context it occurs within.
2765 An lvalue is an expression that represents a memory location. These expressions
2766 are [paths](#path-expressions) (which refer to local variables, function and
2767 method arguments, or static variables), dereferences (`*expr`), [indexing
2768 expressions](#index-expressions) (`expr[expr]`), and [field
2769 references](#field-expressions) (`expr.f`). All other expressions are rvalues.
2771 The left operand of an [assignment](#assignment-expressions) or
2772 [compound-assignment](#compound-assignment-expressions) expression is an lvalue
2773 context, as is the single operand of a unary
2774 [borrow](#unary-operator-expressions). All other expression contexts are
2777 When an lvalue is evaluated in an _lvalue context_, it denotes a memory
2778 location; when evaluated in an _rvalue context_, it denotes the value held _in_
2779 that memory location.
2781 When an rvalue is used in an lvalue context, a temporary un-named lvalue is
2782 created and used instead. A temporary's lifetime equals the largest lifetime
2783 of any reference that points to it.
2785 #### Moved and copied types
2787 When a [local variable](#memory-slots) is used as an
2788 [rvalue](#lvalues,-rvalues-and-temporaries) the variable will either be moved
2789 or copied, depending on its type. For types that contain [owning
2790 pointers](#pointer-types) or values that implement the special trait `Drop`,
2791 the variable is moved. All other types are copied.
2793 ### Literal expressions
2795 A _literal expression_ consists of one of the [literal](#literals) forms
2796 described earlier. It directly describes a number, character, string, boolean
2797 value, or the unit value.
2801 "hello"; // string type
2802 '5'; // character type
2806 ### Path expressions
2808 A [path](#paths) used as an expression context denotes either a local variable
2809 or an item. Path expressions are [lvalues](#lvalues,-rvalues-and-temporaries).
2811 ### Tuple expressions
2813 Tuples are written by enclosing zero or more comma-separated expressions in
2814 parentheses. They are used to create [tuple-typed](#tuple-types) values.
2822 ### Unit expressions
2824 The expression `()` denotes the _unit value_, the only value of the type with
2827 ### Structure expressions
2830 struct_expr : expr_path '{' ident ':' expr
2831 [ ',' ident ':' expr ] *
2834 [ ',' expr ] * ')' |
2838 There are several forms of structure expressions. A _structure expression_
2839 consists of the [path](#paths) of a [structure item](#structures), followed by
2840 a brace-enclosed list of one or more comma-separated name-value pairs,
2841 providing the field values of a new instance of the structure. A field name
2842 can be any identifier, and is separated from its value expression by a colon.
2843 The location denoted by a structure field is mutable if and only if the
2844 enclosing structure is mutable.
2846 A _tuple structure expression_ consists of the [path](#paths) of a [structure
2847 item](#structures), followed by a parenthesized list of one or more
2848 comma-separated expressions (in other words, the path of a structure item
2849 followed by a tuple expression). The structure item must be a tuple structure
2852 A _unit-like structure expression_ consists only of the [path](#paths) of a
2853 [structure item](#structures).
2855 The following are examples of structure expressions:
2858 # struct Point { x: f64, y: f64 }
2859 # struct TuplePoint(f64, f64);
2860 # mod game { pub struct User<'a> { pub name: &'a str, pub age: u32, pub score: uint } }
2861 # struct Cookie; fn some_fn<T>(t: T) {}
2862 Point {x: 10.0, y: 20.0};
2863 TuplePoint(10.0, 20.0);
2864 let u = game::User {name: "Joe", age: 35, score: 100_000};
2865 some_fn::<Cookie>(Cookie);
2868 A structure expression forms a new value of the named structure type. Note
2869 that for a given *unit-like* structure type, this will always be the same
2872 A structure expression can terminate with the syntax `..` followed by an
2873 expression to denote a functional update. The expression following `..` (the
2874 base) must have the same structure type as the new structure type being formed.
2875 The entire expression denotes the result of constructing a new structure (with
2876 the same type as the base expression) with the given values for the fields that
2877 were explicitly specified and the values in the base expression for all other
2881 # struct Point3d { x: i32, y: i32, z: i32 }
2882 let base = Point3d {x: 1, y: 2, z: 3};
2883 Point3d {y: 0, z: 10, .. base};
2886 ### Block expressions
2889 block_expr : '{' [ stmt ';' | item ] *
2893 A _block expression_ is similar to a module in terms of the declarations that
2894 are possible. Each block conceptually introduces a new namespace scope. Use
2895 items can bring new names into scopes and declared items are in scope for only
2898 A block will execute each statement sequentially, and then execute the
2899 expression (if given). If the final expression is omitted, the type and return
2900 value of the block are `()`, but if it is provided, the type and return value
2901 of the block are that of the expression itself.
2903 ### Method-call expressions
2906 method_call_expr : expr '.' ident paren_expr_list ;
2909 A _method call_ consists of an expression followed by a single dot, an
2910 identifier, and a parenthesized expression-list. Method calls are resolved to
2911 methods on specific traits, either statically dispatching to a method if the
2912 exact `self`-type of the left-hand-side is known, or dynamically dispatching if
2913 the left-hand-side expression is an indirect [object type](#object-types).
2915 ### Field expressions
2918 field_expr : expr '.' ident ;
2921 A _field expression_ consists of an expression followed by a single dot and an
2922 identifier, when not immediately followed by a parenthesized expression-list
2923 (the latter is a [method call expression](#method-call-expressions)). A field
2924 expression denotes a field of a [structure](#structure-types).
2929 (Struct {a: 10, b: 20}).a;
2932 A field access is an [lvalue](#lvalues,-rvalues-and-temporaries) referring to
2933 the value of that field. When the type providing the field inherits mutability,
2934 it can be [assigned](#assignment-expressions) to.
2936 Also, if the type of the expression to the left of the dot is a pointer, it is
2937 automatically dereferenced to make the field access possible.
2939 ### Array expressions
2942 array_expr : '[' "mut" ? vec_elems? ']' ;
2944 array_elems : [expr [',' expr]*] | [expr ';' expr] ;
2947 An [array](#array,-and-slice-types) _expression_ is written by enclosing zero
2948 or more comma-separated expressions of uniform type in square brackets.
2950 In the `[expr ';' expr]` form, the expression after the `';'` must be a
2951 constant expression that can be evaluated at compile time, such as a
2952 [literal](#literals) or a [static item](#static-items).
2956 ["a", "b", "c", "d"];
2957 [0is; 128]; // array with 128 zeros
2958 [0u8, 0u8, 0u8, 0u8];
2961 ### Index expressions
2964 idx_expr : expr '[' expr ']' ;
2967 [Array](#array,-and-slice-types)-typed expressions can be indexed by
2968 writing a square-bracket-enclosed expression (the index) after them. When the
2969 array is mutable, the resulting [lvalue](#lvalues,-rvalues-and-temporaries) can
2972 Indices are zero-based, and may be of any integral type. Vector access is
2973 bounds-checked at run-time. When the check fails, it will put the thread in a
2978 (["a", "b"])[10]; // panics
2981 ### Unary operator expressions
2983 Rust defines three unary operators. They are all written as prefix operators,
2984 before the expression they apply to.
2987 : Negation. May only be applied to numeric types.
2989 : Dereference. When applied to a [pointer](#pointer-types) it denotes the
2990 pointed-to location. For pointers to mutable locations, the resulting
2991 [lvalue](#lvalues,-rvalues-and-temporaries) can be assigned to.
2992 On non-pointer types, it calls the `deref` method of the `std::ops::Deref`
2993 trait, or the `deref_mut` method of the `std::ops::DerefMut` trait (if
2994 implemented by the type and required for an outer expression that will or
2995 could mutate the dereference), and produces the result of dereferencing the
2996 `&` or `&mut` borrowed pointer returned from the overload method.
2999 : Logical negation. On the boolean type, this flips between `true` and
3000 `false`. On integer types, this inverts the individual bits in the
3001 two's complement representation of the value.
3003 ### Binary operator expressions
3006 binop_expr : expr binop expr ;
3009 Binary operators expressions are given in terms of [operator
3010 precedence](#operator-precedence).
3012 #### Arithmetic operators
3014 Binary arithmetic expressions are syntactic sugar for calls to built-in traits,
3015 defined in the `std::ops` module of the `std` library. This means that
3016 arithmetic operators can be overridden for user-defined types. The default
3017 meaning of the operators on standard types is given here.
3020 : Addition and array/string concatenation.
3021 Calls the `add` method on the `std::ops::Add` trait.
3024 Calls the `sub` method on the `std::ops::Sub` trait.
3027 Calls the `mul` method on the `std::ops::Mul` trait.
3030 Calls the `div` method on the `std::ops::Div` trait.
3033 Calls the `rem` method on the `std::ops::Rem` trait.
3035 #### Bitwise operators
3037 Like the [arithmetic operators](#arithmetic-operators), bitwise operators are
3038 syntactic sugar for calls to methods of built-in traits. This means that
3039 bitwise operators can be overridden for user-defined types. The default
3040 meaning of the operators on standard types is given here.
3044 Calls the `bitand` method of the `std::ops::BitAnd` trait.
3047 Calls the `bitor` method of the `std::ops::BitOr` trait.
3050 Calls the `bitxor` method of the `std::ops::BitXor` trait.
3052 : Logical left shift.
3053 Calls the `shl` method of the `std::ops::Shl` trait.
3055 : Logical right shift.
3056 Calls the `shr` method of the `std::ops::Shr` trait.
3058 #### Lazy boolean operators
3060 The operators `||` and `&&` may be applied to operands of boolean type. The
3061 `||` operator denotes logical 'or', and the `&&` operator denotes logical
3062 'and'. They differ from `|` and `&` in that the right-hand operand is only
3063 evaluated when the left-hand operand does not already determine the result of
3064 the expression. That is, `||` only evaluates its right-hand operand when the
3065 left-hand operand evaluates to `false`, and `&&` only when it evaluates to
3068 #### Comparison operators
3070 Comparison operators are, like the [arithmetic
3071 operators](#arithmetic-operators), and [bitwise operators](#bitwise-operators),
3072 syntactic sugar for calls to built-in traits. This means that comparison
3073 operators can be overridden for user-defined types. The default meaning of the
3074 operators on standard types is given here.
3078 Calls the `eq` method on the `std::cmp::PartialEq` trait.
3081 Calls the `ne` method on the `std::cmp::PartialEq` trait.
3084 Calls the `lt` method on the `std::cmp::PartialOrd` trait.
3087 Calls the `gt` method on the `std::cmp::PartialOrd` trait.
3089 : Less than or equal.
3090 Calls the `le` method on the `std::cmp::PartialOrd` trait.
3092 : Greater than or equal.
3093 Calls the `ge` method on the `std::cmp::PartialOrd` trait.
3095 #### Type cast expressions
3097 A type cast expression is denoted with the binary operator `as`.
3099 Executing an `as` expression casts the value on the left-hand side to the type
3100 on the right-hand side.
3102 A numeric value can be cast to any numeric type. A raw pointer value can be
3103 cast to or from any integral type or raw pointer type. Any other cast is
3104 unsupported and will fail to compile.
3106 An example of an `as` expression:
3109 # fn sum(v: &[f64]) -> f64 { 0.0 }
3110 # fn len(v: &[f64]) -> i32 { 0 }
3112 fn avg(v: &[f64]) -> f64 {
3113 let sum: f64 = sum(v);
3114 let sz: f64 = len(v) as f64;
3119 #### Assignment expressions
3121 An _assignment expression_ consists of an
3122 [lvalue](#lvalues,-rvalues-and-temporaries) expression followed by an equals
3123 sign (`=`) and an [rvalue](#lvalues,-rvalues-and-temporaries) expression.
3125 Evaluating an assignment expression [either copies or
3126 moves](#moved-and-copied-types) its right-hand operand to its left-hand
3136 #### Compound assignment expressions
3138 The `+`, `-`, `*`, `/`, `%`, `&`, `|`, `^`, `<<`, and `>>` operators may be
3139 composed with the `=` operator. The expression `lval OP= val` is equivalent to
3140 `lval = lval OP val`. For example, `x = x + 1` may be written as `x += 1`.
3142 Any such expression always has the [`unit`](#primitive-types) type.
3144 #### Operator precedence
3146 The precedence of Rust binary operators is ordered as follows, going from
3149 ```{.text .precedence}
3163 Operators at the same precedence level are evaluated left-to-right. [Unary
3164 operators](#unary-operator-expressions) have the same precedence level and are
3165 stronger than any of the binary operators.
3167 ### Grouped expressions
3169 An expression enclosed in parentheses evaluates to the result of the enclosed
3170 expression. Parentheses can be used to explicitly specify evaluation order
3171 within an expression.
3174 paren_expr : '(' expr ')' ;
3177 An example of a parenthesized expression:
3180 let x: i32 = (2 + 3) * 4;
3184 ### Call expressions
3187 expr_list : [ expr [ ',' expr ]* ] ? ;
3188 paren_expr_list : '(' expr_list ')' ;
3189 call_expr : expr paren_expr_list ;
3192 A _call expression_ invokes a function, providing zero or more input slots and
3193 an optional reference slot to serve as the function's output, bound to the
3194 `lval` on the right hand side of the call. If the function eventually returns,
3195 then the expression completes.
3197 Some examples of call expressions:
3200 # fn add(x: i32, y: i32) -> i32 { 0 }
3202 let x: i32 = add(1i32, 2i32);
3203 let pi: Option<f32> = "3.14".parse();
3206 ### Lambda expressions
3209 ident_list : [ ident [ ',' ident ]* ] ? ;
3210 lambda_expr : '|' ident_list '|' expr ;
3213 A _lambda expression_ (sometimes called an "anonymous function expression")
3214 defines a function and denotes it as a value, in a single expression. A lambda
3215 expression is a pipe-symbol-delimited (`|`) list of identifiers followed by an
3218 A lambda expression denotes a function that maps a list of parameters
3219 (`ident_list`) onto the expression that follows the `ident_list`. The
3220 identifiers in the `ident_list` are the parameters to the function. These
3221 parameters' types need not be specified, as the compiler infers them from
3224 Lambda expressions are most useful when passing functions as arguments to other
3225 functions, as an abbreviation for defining and capturing a separate function.
3227 Significantly, lambda expressions _capture their environment_, which regular
3228 [function definitions](#functions) do not. The exact type of capture depends
3229 on the [function type](#function-types) inferred for the lambda expression. In
3230 the simplest and least-expensive form (analogous to a ```|| { }``` expression),
3231 the lambda expression captures its environment by reference, effectively
3232 borrowing pointers to all outer variables mentioned inside the function.
3233 Alternately, the compiler may infer that a lambda expression should copy or
3234 move values (depending on their type) from the environment into the lambda
3235 expression's captured environment.
3237 In this example, we define a function `ten_times` that takes a higher-order
3238 function argument, and call it with a lambda expression as an argument:
3241 fn ten_times<F>(f: F) where F: Fn(i32) {
3249 ten_times(|j| println!("hello, {}", j));
3255 while_expr : "while" no_struct_literal_expr '{' block '}' ;
3258 A `while` loop begins by evaluating the boolean loop conditional expression.
3259 If the loop conditional expression evaluates to `true`, the loop body block
3260 executes and control returns to the loop conditional expression. If the loop
3261 conditional expression evaluates to `false`, the `while` expression completes.
3276 A `loop` expression denotes an infinite loop.
3279 loop_expr : [ lifetime ':' ] "loop" '{' block '}';
3282 A `loop` expression may optionally have a _label_. If a label is present, then
3283 labeled `break` and `continue` expressions nested within this loop may exit out
3284 of this loop or return control to its head. See [Break
3285 expressions](#break-expressions) and [Continue
3286 expressions](#continue-expressions).
3288 ### Break expressions
3291 break_expr : "break" [ lifetime ];
3294 A `break` expression has an optional _label_. If the label is absent, then
3295 executing a `break` expression immediately terminates the innermost loop
3296 enclosing it. It is only permitted in the body of a loop. If the label is
3297 present, then `break foo` terminates the loop with label `foo`, which need not
3298 be the innermost label enclosing the `break` expression, but must enclose it.
3300 ### Continue expressions
3303 continue_expr : "continue" [ lifetime ];
3306 A `continue` expression has an optional _label_. If the label is absent, then
3307 executing a `continue` expression immediately terminates the current iteration
3308 of the innermost loop enclosing it, returning control to the loop *head*. In
3309 the case of a `while` loop, the head is the conditional expression controlling
3310 the loop. In the case of a `for` loop, the head is the call-expression
3311 controlling the loop. If the label is present, then `continue foo` returns
3312 control to the head of the loop with label `foo`, which need not be the
3313 innermost label enclosing the `break` expression, but must enclose it.
3315 A `continue` expression is only permitted in the body of a loop.
3320 for_expr : "for" pat "in" no_struct_literal_expr '{' block '}' ;
3323 A `for` expression is a syntactic construct for looping over elements provided
3324 by an implementation of `std::iter::Iterator`.
3326 An example of a for loop over the contents of an array:
3330 # fn bar(f: Foo) { }
3335 let v: &[Foo] = &[a, b, c];
3342 An example of a for loop over a series of integers:
3345 # fn bar(b:usize) { }
3346 for i in range(0us, 256) {
3354 if_expr : "if" no_struct_literal_expr '{' block '}'
3357 else_tail : "else" [ if_expr | if_let_expr
3361 An `if` expression is a conditional branch in program control. The form of an
3362 `if` expression is a condition expression, followed by a consequent block, any
3363 number of `else if` conditions and blocks, and an optional trailing `else`
3364 block. The condition expressions must have type `bool`. If a condition
3365 expression evaluates to `true`, the consequent block is executed and any
3366 subsequent `else if` or `else` block is skipped. If a condition expression
3367 evaluates to `false`, the consequent block is skipped and any subsequent `else
3368 if` condition is evaluated. If all `if` and `else if` conditions evaluate to
3369 `false` then any `else` block is executed.
3371 ### Match expressions
3374 match_expr : "match" no_struct_literal_expr '{' match_arm * '}' ;
3376 match_arm : attribute * match_pat "=>" [ expr "," | '{' block '}' ] ;
3378 match_pat : pat [ '|' pat ] * [ "if" expr ] ? ;
3381 A `match` expression branches on a *pattern*. The exact form of matching that
3382 occurs depends on the pattern. Patterns consist of some combination of
3383 literals, destructured arrays or enum constructors, structures and tuples,
3384 variable binding specifications, wildcards (`..`), and placeholders (`_`). A
3385 `match` expression has a *head expression*, which is the value to compare to
3386 the patterns. The type of the patterns must equal the type of the head
3389 In a pattern whose head expression has an `enum` type, a placeholder (`_`)
3390 stands for a *single* data field, whereas a wildcard `..` stands for *all* the
3391 fields of a particular variant. For example:
3394 #![feature(box_syntax)]
3395 enum List<X> { Nil, Cons(X, Box<List<X>>) }
3398 let x: List<i32> = List::Cons(10, box List::Cons(11, box List::Nil));
3401 List::Cons(_, box List::Nil) => panic!("singleton list"),
3402 List::Cons(..) => return,
3403 List::Nil => panic!("empty list")
3408 The first pattern matches lists constructed by applying `Cons` to any head
3409 value, and a tail value of `box Nil`. The second pattern matches _any_ list
3410 constructed with `Cons`, ignoring the values of its arguments. The difference
3411 between `_` and `..` is that the pattern `C(_)` is only type-correct if `C` has
3412 exactly one argument, while the pattern `C(..)` is type-correct for any enum
3413 variant `C`, regardless of how many arguments `C` has.
3415 Used inside an array pattern, `..` stands for any number of elements, when the
3416 `advanced_slice_patterns` feature gate is turned on. This wildcard can be used
3417 at most once for a given array, which implies that it cannot be used to
3418 specifically match elements that are at an unknown distance from both ends of a
3419 array, like `[.., 42, ..]`. If preceded by a variable name, it will bind the
3420 corresponding slice to the variable. Example:
3423 # #![feature(advanced_slice_patterns)]
3424 fn is_symmetric(list: &[u32]) -> bool {
3427 [x, inside.., y] if x == y => is_symmetric(inside),
3433 let sym = &[0, 1, 4, 2, 4, 1, 0];
3434 let not_sym = &[0, 1, 7, 2, 4, 1, 0];
3435 assert!(is_symmetric(sym));
3436 assert!(!is_symmetric(not_sym));
3440 A `match` behaves differently depending on whether or not the head expression
3441 is an [lvalue or an rvalue](#lvalues,-rvalues-and-temporaries). If the head
3442 expression is an rvalue, it is first evaluated into a temporary location, and
3443 the resulting value is sequentially compared to the patterns in the arms until
3444 a match is found. The first arm with a matching pattern is chosen as the branch
3445 target of the `match`, any variables bound by the pattern are assigned to local
3446 variables in the arm's block, and control enters the block.
3448 When the head expression is an lvalue, the match does not allocate a temporary
3449 location (however, a by-value binding may copy or move from the lvalue). When
3450 possible, it is preferable to match on lvalues, as the lifetime of these
3451 matches inherits the lifetime of the lvalue, rather than being restricted to
3452 the inside of the match.
3454 An example of a `match` expression:
3457 #![feature(box_syntax)]
3458 # fn process_pair(a: i32, b: i32) { }
3459 # fn process_ten() { }
3461 enum List<X> { Nil, Cons(X, Box<List<X>>) }
3464 let x: List<i32> = List::Cons(10, box List::Cons(11, box List::Nil));
3467 List::Cons(a, box List::Cons(b, _)) => {
3470 List::Cons(10, _) => {
3483 Patterns that bind variables default to binding to a copy or move of the
3484 matched value (depending on the matched value's type). This can be changed to
3485 bind to a reference by using the `ref` keyword, or to a mutable reference using
3488 Subpatterns can also be bound to variables by the use of the syntax `variable @
3489 subpattern`. For example:
3492 #![feature(box_syntax)]
3494 enum List { Nil, Cons(uint, Box<List>) }
3496 fn is_sorted(list: &List) -> bool {
3498 List::Nil | List::Cons(_, box List::Nil) => true,
3499 List::Cons(x, ref r @ box List::Cons(_, _)) => {
3501 box List::Cons(y, _) => (x <= y) && is_sorted(&**r),
3509 let a = List::Cons(6, box List::Cons(7, box List::Cons(42, box List::Nil)));
3510 assert!(is_sorted(&a));
3515 Patterns can also dereference pointers by using the `&`, `&mut` and `box`
3516 symbols, as appropriate. For example, these two matches on `x: &isize` are
3521 let y = match *x { 0 => "zero", _ => "some" };
3522 let z = match x { &0 => "zero", _ => "some" };
3527 A pattern that's just an identifier, like `Nil` in the previous example, could
3528 either refer to an enum variant that's in scope, or bind a new variable. The
3529 compiler resolves this ambiguity by forbidding variable bindings that occur in
3530 `match` patterns from shadowing names of variants that are in scope. For
3531 example, wherever `List` is in scope, a `match` pattern would not be able to
3532 bind `Nil` as a new name. The compiler interprets a variable pattern `x` as a
3533 binding _only_ if there is no variant named `x` in scope. A convention you can
3534 use to avoid conflicts is simply to name variants with upper-case letters, and
3535 local variables with lower-case letters.
3537 Multiple match patterns may be joined with the `|` operator. A range of values
3538 may be specified with `...`. For example:
3543 let message = match x {
3544 0 | 1 => "not many",
3550 Range patterns only work on scalar types (like integers and characters; not
3551 like arrays and structs, which have sub-components). A range pattern may not
3552 be a sub-range of another range pattern inside the same `match`.
3554 Finally, match patterns can accept *pattern guards* to further refine the
3555 criteria for matching a case. Pattern guards appear after the pattern and
3556 consist of a bool-typed expression following the `if` keyword. A pattern guard
3557 may refer to the variables bound within the pattern they follow.
3560 # let maybe_digit = Some(0);
3561 # fn process_digit(i: i32) { }
3562 # fn process_other(i: i32) { }
3564 let message = match maybe_digit {
3565 Some(x) if x < 10 => process_digit(x),
3566 Some(x) => process_other(x),
3571 ### If let expressions
3574 if_let_expr : "if" "let" pat '=' expr '{' block '}'
3576 else_tail : "else" [ if_expr | if_let_expr | '{' block '}' ] ;
3579 An `if let` expression is semantically identical to an `if` expression but in place
3580 of a condition expression it expects a refutable let statement. If the value of the
3581 expression on the right hand side of the let statement matches the pattern, the corresponding
3582 block will execute, otherwise flow proceeds to the first `else` block that follows.
3587 while_let_expr : "while" "let" pat '=' expr '{' block '}' ;
3590 A `while let` loop is semantically identical to a `while` loop but in place of a
3591 condition expression it expects a refutable let statement. If the value of the
3592 expression on the right hand side of the let statement matches the pattern, the
3593 loop body block executes and control returns to the pattern matching statement.
3594 Otherwise, the while expression completes.
3596 ### Return expressions
3599 return_expr : "return" expr ? ;
3602 Return expressions are denoted with the keyword `return`. Evaluating a `return`
3603 expression moves its argument into the output slot of the current function,
3604 destroys the current function activation frame, and transfers control to the
3607 An example of a `return` expression:
3610 fn max(a: i32, b: i32) -> i32 {
3622 Every slot, item and value in a Rust program has a type. The _type_ of a
3623 *value* defines the interpretation of the memory holding it.
3625 Built-in types and type-constructors are tightly integrated into the language,
3626 in nontrivial ways that are not possible to emulate in user-defined types.
3627 User-defined types have limited capabilities.
3631 The primitive types are the following:
3633 * The "unit" type `()`, having the single "unit" value `()` (occasionally called
3635 * The boolean type `bool` with values `true` and `false`.
3636 * The machine types.
3637 * The machine-dependent integer and floating-point types.
3639 [^unittype]: The "unit" value `()` is *not* a sentinel "null pointer" value for
3640 reference slots; the "unit" type is the implicit return type from functions
3641 otherwise lacking a return type, and can be used in other contexts (such as
3642 message-sending or type-parametric code) as a zero-size type.]
3646 The machine types are the following:
3648 * The unsigned word types `u8`, `u16`, `u32` and `u64`, with values drawn from
3649 the integer intervals [0, 2^8 - 1], [0, 2^16 - 1], [0, 2^32 - 1] and
3650 [0, 2^64 - 1] respectively.
3652 * The signed two's complement word types `i8`, `i16`, `i32` and `i64`, with
3653 values drawn from the integer intervals [-(2^(7)), 2^7 - 1],
3654 [-(2^(15)), 2^15 - 1], [-(2^(31)), 2^31 - 1], [-(2^(63)), 2^63 - 1]
3657 * The IEEE 754-2008 `binary32` and `binary64` floating-point types: `f32` and
3658 `f64`, respectively.
3660 #### Machine-dependent integer types
3662 The `usize` type is an unsigned integer type with the same number of bits as the
3663 platform's pointer type. It can represent every memory address in the process.
3665 The `isize` type is a signed integer type with the same number of bits as the
3666 platform's pointer type. The theoretical upper bound on object and array size
3667 is the maximum `isize` value. This ensures that `isize` can be used to calculate
3668 differences between pointers into an object or array and can address every byte
3669 within an object along with one byte past the end.
3673 The types `char` and `str` hold textual data.
3675 A value of type `char` is a [Unicode scalar value](
3676 http://www.unicode.org/glossary/#unicode_scalar_value) (i.e. a code point that
3677 is not a surrogate), represented as a 32-bit unsigned word in the 0x0000 to
3678 0xD7FF or 0xE000 to 0x10FFFF range. A `[char]` array is effectively an UCS-4 /
3681 A value of type `str` is a Unicode string, represented as an array of 8-bit
3682 unsigned bytes holding a sequence of UTF-8 codepoints. Since `str` is of
3683 unknown size, it is not a _first-class_ type, but can only be instantiated
3684 through a pointer type, such as `&str` or `String`.
3688 A tuple *type* is a heterogeneous product of other types, called the *elements*
3689 of the tuple. It has no nominal name and is instead structurally typed.
3691 Tuple types and values are denoted by listing the types or values of their
3692 elements, respectively, in a parenthesized, comma-separated list.
3694 Because tuple elements don't have a name, they can only be accessed by
3697 The members of a tuple are laid out in memory contiguously, in order specified
3700 An example of a tuple type and its use:
3703 type Pair<'a> = (i32, &'a str);
3704 let p: Pair<'static> = (10, "hello");
3706 assert!(b != "world");
3709 ### Array, and Slice types
3711 Rust has two different types for a list of items:
3713 * `[T; N]`, an 'array'.
3714 * `&[T]`, a 'slice'.
3716 An array has a fixed size, and can be allocated on either the stack or the
3719 A slice is a 'view' into an array. It doesn't own the data it points
3722 An example of each kind:
3725 let vec: Vec<i32> = vec![1, 2, 3];
3726 let arr: [i32; 3] = [1, 2, 3];
3727 let s: &[i32] = vec.as_slice();
3730 As you can see, the `vec!` macro allows you to create a `Vec<T>` easily. The
3731 `vec!` macro is also part of the standard library, rather than the language.
3733 All in-bounds elements of arrays, and slices are always initialized, and access
3734 to an array or slice is always bounds-checked.
3738 A `struct` *type* is a heterogeneous product of other types, called the
3739 *fields* of the type.[^structtype]
3741 [^structtype]: `struct` types are analogous `struct` types in C,
3742 the *record* types of the ML family,
3743 or the *structure* types of the Lisp family.
3745 New instances of a `struct` can be constructed with a [struct
3746 expression](#structure-expressions).
3748 The memory layout of a `struct` is undefined by default to allow for compiler
3749 optimizations like field reordering, but it can be fixed with the
3750 `#[repr(...)]` attribute. In either case, fields may be given in any order in
3751 a corresponding struct *expression*; the resulting `struct` value will always
3752 have the same memory layout.
3754 The fields of a `struct` may be qualified by [visibility
3755 modifiers](#re-exporting-and-visibility), to allow access to data in a
3756 structure outside a module.
3758 A _tuple struct_ type is just like a structure type, except that the fields are
3761 A _unit-like struct_ type is like a structure type, except that it has no
3762 fields. The one value constructed by the associated [structure
3763 expression](#structure-expressions) is the only value that inhabits such a
3766 ### Enumerated types
3768 An *enumerated type* is a nominal, heterogeneous disjoint union type, denoted
3769 by the name of an [`enum` item](#enumerations). [^enumtype]
3771 [^enumtype]: The `enum` type is analogous to a `data` constructor declaration in
3772 ML, or a *pick ADT* in Limbo.
3774 An [`enum` item](#enumerations) declares both the type and a number of *variant
3775 constructors*, each of which is independently named and takes an optional tuple
3778 New instances of an `enum` can be constructed by calling one of the variant
3779 constructors, in a [call expression](#call-expressions).
3781 Any `enum` value consumes as much memory as the largest variant constructor for
3782 its corresponding `enum` type.
3784 Enum types cannot be denoted *structurally* as types, but must be denoted by
3785 named reference to an [`enum` item](#enumerations).
3789 Nominal types — [enumerations](#enumerated-types) and
3790 [structures](#structure-types) — may be recursive. That is, each `enum`
3791 constructor or `struct` field may refer, directly or indirectly, to the
3792 enclosing `enum` or `struct` type itself. Such recursion has restrictions:
3794 * Recursive types must include a nominal type in the recursion
3795 (not mere [type definitions](#type-definitions),
3796 or other structural types such as [arrays](#array,-and-slice-types) or [tuples](#tuple-types)).
3797 * A recursive `enum` item must have at least one non-recursive constructor
3798 (in order to give the recursion a basis case).
3799 * The size of a recursive type must be finite;
3800 in other words the recursive fields of the type must be [pointer types](#pointer-types).
3801 * Recursive type definitions can cross module boundaries, but not module *visibility* boundaries,
3802 or crate boundaries (in order to simplify the module system and type checker).
3804 An example of a *recursive* type and its use:
3809 Cons(T, Box<List<T>>)
3812 let a: List<i32> = List::Cons(7, Box::new(List::Cons(13, Box::new(List::Nil))));
3817 All pointers in Rust are explicit first-class values. They can be copied,
3818 stored into data structures, and returned from functions. There are two
3819 varieties of pointer in Rust:
3822 : These point to memory _owned by some other value_.
3823 A reference type is written `&type` for some lifetime-variable `f`,
3824 or just `&'a type` when you need an explicit lifetime.
3825 Copying a reference is a "shallow" operation:
3826 it involves only copying the pointer itself.
3827 Releasing a reference typically has no effect on the value it points to,
3828 with the exception of temporary values, which are released when the last
3829 reference to them is released.
3831 * Raw pointers (`*`)
3832 : Raw pointers are pointers without safety or liveness guarantees.
3833 Raw pointers are written as `*const T` or `*mut T`,
3834 for example `*const int` means a raw pointer to an integer.
3835 Copying or dropping a raw pointer has no effect on the lifecycle of any
3836 other value. Dereferencing a raw pointer or converting it to any other
3837 pointer type is an [`unsafe` operation](#unsafe-functions).
3838 Raw pointers are generally discouraged in Rust code;
3839 they exist to support interoperability with foreign code,
3840 and writing performance-critical or low-level functions.
3842 The standard library contains additional 'smart pointer' types beyond references
3847 The function type constructor `fn` forms new function types. A function type
3848 consists of a possibly-empty set of function-type modifiers (such as `unsafe`
3849 or `extern`), a sequence of input types and an output type.
3851 An example of a `fn` type:
3854 fn add(x: i32, y: i32) -> i32 {
3858 let mut x = add(5,7);
3860 type Binop = fn(i32, i32) -> i32;
3861 let bo: Binop = add;
3867 ```{.ebnf .notation}
3868 closure_type := [ 'unsafe' ] [ '<' lifetime-list '>' ] '|' arg-list '|'
3869 [ ':' bound-list ] [ '->' type ]
3870 lifetime-list := lifetime | lifetime ',' lifetime-list
3871 arg-list := ident ':' type | ident ':' type ',' arg-list
3872 bound-list := bound | bound '+' bound-list
3873 bound := path | lifetime
3876 The type of a closure mapping an input of type `A` to an output of type `B` is
3877 `|A| -> B`. A closure with no arguments or return values has type `||`.
3879 An example of creating and calling a closure:
3882 let captured_var = 10is;
3884 let closure_no_args = |&:| println!("captured_var={}", captured_var);
3886 let closure_args = |&: arg: isize| -> isize {
3887 println!("captured_var={}, arg={}", captured_var, arg);
3888 arg // Note lack of semicolon after 'arg'
3891 fn call_closure<F: Fn(), G: Fn(isize) -> isize>(c1: F, c2: G) {
3896 call_closure(closure_no_args, closure_args);
3902 Every trait item (see [traits](#traits)) defines a type with the same name as
3903 the trait. This type is called the _object type_ of the trait. Object types
3904 permit "late binding" of methods, dispatched using _virtual method tables_
3905 ("vtables"). Whereas most calls to trait methods are "early bound" (statically
3906 resolved) to specific implementations at compile time, a call to a method on an
3907 object type is only resolved to a vtable entry at compile time. The actual
3908 implementation for each vtable entry can vary on an object-by-object basis.
3910 Given a pointer-typed expression `E` of type `&T` or `Box<T>`, where `T`
3911 implements trait `R`, casting `E` to the corresponding pointer type `&R` or
3912 `Box<R>` results in a value of the _object type_ `R`. This result is
3913 represented as a pair of pointers: the vtable pointer for the `T`
3914 implementation of `R`, and the pointer value of `E`.
3916 An example of an object type:
3920 fn stringify(&self) -> String;
3923 impl Printable for isize {
3924 fn stringify(&self) -> String { self.to_string() }
3927 fn print(a: Box<Printable>) {
3928 println!("{}", a.stringify());
3932 print(Box::new(10is) as Box<Printable>);
3936 In this example, the trait `Printable` occurs as an object type in both the
3937 type signature of `print`, and the cast expression in `main`.
3941 Within the body of an item that has type parameter declarations, the names of
3942 its type parameters are types:
3945 fn map<A: Clone, B: Clone>(f: |A| -> B, xs: &[A]) -> Vec<B> {
3949 let first: B = f(xs[0].clone());
3950 let mut rest: Vec<B> = map(f, xs.slice(1, xs.len()));
3951 rest.insert(0, first);
3956 Here, `first` has type `B`, referring to `map`'s `B` type parameter; and `rest`
3957 has type `Vec<B>`, a vector type with element type `B`.
3961 The special type `self` has a meaning within methods inside an impl item. It
3962 refers to the type of the implicit `self` argument. For example, in:
3966 fn make_string(&self) -> String;
3969 impl Printable for String {
3970 fn make_string(&self) -> String {
3976 `self` refers to the value of type `String` that is the receiver for a call to
3977 the method `make_string`.
3981 Types in Rust are categorized into kinds, based on various properties of the
3982 components of the type. The kinds are:
3985 : Types of this kind can be safely sent between threads.
3986 This kind includes scalars, boxes, procs, and
3987 structural types containing only other owned types.
3988 All `Send` types are `'static`.
3990 : Types of this kind consist of "Plain Old Data"
3991 which can be copied by simply moving bits.
3992 All values of this kind can be implicitly copied.
3993 This kind includes scalars and immutable references,
3994 as well as structural types containing other `Copy` types.
3996 : Types of this kind do not contain any references (except for
3997 references with the `static` lifetime, which are allowed).
3998 This can be a useful guarantee for code
3999 that breaks borrowing assumptions
4000 using [`unsafe` operations](#unsafe-functions).
4002 : This is not strictly a kind,
4003 but its presence interacts with kinds:
4004 the `Drop` trait provides a single method `drop`
4005 that takes no parameters,
4006 and is run when values of the type are dropped.
4007 Such a method is called a "destructor",
4008 and are always executed in "top-down" order:
4009 a value is completely destroyed
4010 before any of the values it owns run their destructors.
4011 Only `Send` types can implement `Drop`.
4014 : Types with destructors, closure environments,
4015 and various other _non-first-class_ types,
4016 are not copyable at all.
4017 Such types can usually only be accessed through pointers,
4018 or in some cases, moved between mutable locations.
4020 Kinds can be supplied as _bounds_ on type parameters, like traits, in which
4021 case the parameter is constrained to types satisfying that kind.
4023 By default, type parameters do not carry any assumed kind-bounds at all. When
4024 instantiating a type parameter, the kind bounds on the parameter are checked to
4025 be the same or narrower than the kind of the type that it is instantiated with.
4027 Sending operations are not part of the Rust language, but are implemented in
4028 the library. Generic functions that send values bound the kind of these values
4031 # Memory and concurrency models
4033 Rust has a memory model centered around concurrently-executing _threads_. Thus
4034 its memory model and its concurrency model are best discussed simultaneously,
4035 as parts of each only make sense when considered from the perspective of the
4038 When reading about the memory model, keep in mind that it is partitioned in
4039 order to support threads; and when reading about threads, keep in mind that their
4040 isolation and communication mechanisms are only possible due to the ownership
4041 and lifetime semantics of the memory model.
4045 A Rust program's memory consists of a static set of *items*, a set of
4046 [threads](#threads) each with its own *stack*, and a *heap*. Immutable portions of
4047 the heap may be shared between threads, mutable portions may not.
4049 Allocations in the stack consist of *slots*, and allocations in the heap
4052 ### Memory allocation and lifetime
4054 The _items_ of a program are those functions, modules and types that have their
4055 value calculated at compile-time and stored uniquely in the memory image of the
4056 rust process. Items are neither dynamically allocated nor freed.
4058 A thread's _stack_ consists of activation frames automatically allocated on entry
4059 to each function as the thread executes. A stack allocation is reclaimed when
4060 control leaves the frame containing it.
4062 The _heap_ is a general term that describes boxes. The lifetime of an
4063 allocation in the heap depends on the lifetime of the box values pointing to
4064 it. Since box values may themselves be passed in and out of frames, or stored
4065 in the heap, heap allocations may outlive the frame they are allocated within.
4067 ### Memory ownership
4069 A thread owns all memory it can *safely* reach through local variables, as well
4070 as boxes and references.
4072 When a thread sends a value that has the `Send` trait to another thread, it loses
4073 ownership of the value sent and can no longer refer to it. This is statically
4074 guaranteed by the combined use of "move semantics", and the compiler-checked
4075 _meaning_ of the `Send` trait: it is only instantiated for (transitively)
4076 sendable kinds of data constructor and pointers, never including references.
4078 When a stack frame is exited, its local allocations are all released, and its
4079 references to boxes are dropped.
4081 When a thread finishes, its stack is necessarily empty and it therefore has no
4082 references to any boxes; the remainder of its heap is immediately freed.
4086 A thread's stack contains slots.
4088 A _slot_ is a component of a stack frame, either a function parameter, a
4089 [temporary](#lvalues,-rvalues-and-temporaries), or a local variable.
4091 A _local variable_ (or *stack-local* allocation) holds a value directly,
4092 allocated within the stack's memory. The value is a part of the stack frame.
4094 Local variables are immutable unless declared otherwise like: `let mut x = ...`.
4096 Function parameters are immutable unless declared with `mut`. The `mut` keyword
4097 applies only to the following parameter (so `|mut x, y|` and `fn f(mut x:
4098 Box<i32>, y: Box<i32>)` declare one mutable variable `x` and one immutable
4101 Methods that take either `self` or `Box<Self>` can optionally place them in a
4102 mutable slot by prefixing them with `mut` (similar to regular arguments):
4106 fn change(mut self) -> Self;
4107 fn modify(mut self: Box<Self>) -> Box<Self>;
4111 Local variables are not initialized when allocated; the entire frame worth of
4112 local variables are allocated at once, on frame-entry, in an uninitialized
4113 state. Subsequent statements within a function may or may not initialize the
4114 local variables. Local variables can be used only after they have been
4115 initialized; this is enforced by the compiler.
4119 A _box_ is a reference to a heap allocation holding another value, which is
4120 constructed by the prefix operator `box`. When the standard library is in use,
4121 the type of a box is `std::owned::Box<T>`.
4123 An example of a box type and value:
4126 let x: Box<i32> = Box::new(10);
4129 Box values exist in 1:1 correspondence with their heap allocation, copying a
4130 box value makes a shallow copy of the pointer. Rust will consider a shallow
4131 copy of a box to move ownership of the value. After a value has been moved,
4132 the source location cannot be used unless it is reinitialized.
4135 let x: Box<i32> = Box::new(10);
4137 // attempting to use `x` will result in an error here
4142 Rust's primary concurrency mechanism is called a **thread**.
4144 ### Communication between threads
4146 Rust threads are isolated and generally unable to interfere with one another's
4147 memory directly, except through [`unsafe` code](#unsafe-functions). All
4148 contact between threads is mediated by safe forms of ownership transfer, and data
4149 races on memory are prohibited by the type system.
4151 When you wish to send data between threads, the values are restricted to the
4152 [`Send` type-kind](#type-kinds). Restricting communication interfaces to this
4153 kind ensures that no references move between threads. Thus access to an entire
4154 data structure can be mediated through its owning "root" value; no further
4155 locking or copying is required to avoid data races within the substructure of
4160 The _lifecycle_ of a threads consists of a finite set of states and events that
4161 cause transitions between the states. The lifecycle states of a thread are:
4168 A thread begins its lifecycle — once it has been spawned — in the
4169 *running* state. In this state it executes the statements of its entry
4170 function, and any functions called by the entry function.
4172 A thread may transition from the *running* state to the *blocked* state any time
4173 it makes a blocking communication call. When the call can be completed —
4174 when a message arrives at a sender, or a buffer opens to receive a message
4175 — then the blocked thread will unblock and transition back to *running*.
4177 A thread may transition to the *panicked* state at any time, due being killed by
4178 some external event or internally, from the evaluation of a `panic!()` macro.
4179 Once *panicking*, a thread unwinds its stack and transitions to the *dead* state.
4180 Unwinding the stack of a thread is done by the thread itself, on its own control
4181 stack. If a value with a destructor is freed during unwinding, the code for the
4182 destructor is run, also on the thread's control stack. Running the destructor
4183 code causes a temporary transition to a *running* state, and allows the
4184 destructor code to cause any subsequent state transitions. The original thread
4185 of unwinding and panicking thereby may suspend temporarily, and may involve
4186 (recursive) unwinding of the stack of a failed destructor. Nonetheless, the
4187 outermost unwinding activity will continue until the stack is unwound and the
4188 thread transitions to the *dead* state. There is no way to "recover" from thread
4189 panics. Once a thread has temporarily suspended its unwinding in the *panicking*
4190 state, a panic occurring from within this destructor results in *hard* panic.
4191 A hard panic currently results in the process aborting.
4193 A thread in the *dead* state cannot transition to other states; it exists only to
4194 have its termination status inspected by other threads, and/or to await
4195 reclamation when the last reference to it drops.
4197 # Runtime services, linkage and debugging
4199 The Rust _runtime_ is a relatively compact collection of Rust code that
4200 provides fundamental services and datatypes to all Rust threads at run-time. It
4201 is smaller and simpler than many modern language runtimes. It is tightly
4202 integrated into the language's execution model of memory, threads, communication
4205 ### Memory allocation
4207 The runtime memory-management system is based on a _service-provider
4208 interface_, through which the runtime requests blocks of memory from its
4209 environment and releases them back to its environment when they are no longer
4210 needed. The default implementation of the service-provider interface consists
4211 of the C runtime functions `malloc` and `free`.
4213 The runtime memory-management system, in turn, supplies Rust threads with
4214 facilities for allocating releasing stacks, as well as allocating and freeing
4219 The runtime provides C and Rust code to assist with various built-in types,
4220 such as arrays, strings, and the low level communication system (ports,
4223 Support for other built-in types such as simple types, tuples and enums is
4224 open-coded by the Rust compiler.
4226 ### Thread scheduling and communication
4228 The runtime provides code to manage inter-thread communication. This includes
4229 the system of thread-lifecycle state transitions depending on the contents of
4230 queues, as well as code to copy values between queues and their recipients and
4231 to serialize values for transmission over operating-system inter-process
4232 communication facilities.
4236 The Rust compiler supports various methods to link crates together both
4237 statically and dynamically. This section will explore the various methods to
4238 link Rust crates together, and more information about native libraries can be
4239 found in the [ffi section of the book][ffi].
4241 In one session of compilation, the compiler can generate multiple artifacts
4242 through the usage of either command line flags or the `crate_type` attribute.
4243 If one or more command line flag is specified, all `crate_type` attributes will
4244 be ignored in favor of only building the artifacts specified by command line.
4246 * `--crate-type=bin`, `#[crate_type = "bin"]` - A runnable executable will be
4247 produced. This requires that there is a `main` function in the crate which
4248 will be run when the program begins executing. This will link in all Rust and
4249 native dependencies, producing a distributable binary.
4251 * `--crate-type=lib`, `#[crate_type = "lib"]` - A Rust library will be produced.
4252 This is an ambiguous concept as to what exactly is produced because a library
4253 can manifest itself in several forms. The purpose of this generic `lib` option
4254 is to generate the "compiler recommended" style of library. The output library
4255 will always be usable by rustc, but the actual type of library may change from
4256 time-to-time. The remaining output types are all different flavors of
4257 libraries, and the `lib` type can be seen as an alias for one of them (but the
4258 actual one is compiler-defined).
4260 * `--crate-type=dylib`, `#[crate_type = "dylib"]` - A dynamic Rust library will
4261 be produced. This is different from the `lib` output type in that this forces
4262 dynamic library generation. The resulting dynamic library can be used as a
4263 dependency for other libraries and/or executables. This output type will
4264 create `*.so` files on linux, `*.dylib` files on osx, and `*.dll` files on
4267 * `--crate-type=staticlib`, `#[crate_type = "staticlib"]` - A static system
4268 library will be produced. This is different from other library outputs in that
4269 the Rust compiler will never attempt to link to `staticlib` outputs. The
4270 purpose of this output type is to create a static library containing all of
4271 the local crate's code along with all upstream dependencies. The static
4272 library is actually a `*.a` archive on linux and osx and a `*.lib` file on
4273 windows. This format is recommended for use in situations such as linking
4274 Rust code into an existing non-Rust application because it will not have
4275 dynamic dependencies on other Rust code.
4277 * `--crate-type=rlib`, `#[crate_type = "rlib"]` - A "Rust library" file will be
4278 produced. This is used as an intermediate artifact and can be thought of as a
4279 "static Rust library". These `rlib` files, unlike `staticlib` files, are
4280 interpreted by the Rust compiler in future linkage. This essentially means
4281 that `rustc` will look for metadata in `rlib` files like it looks for metadata
4282 in dynamic libraries. This form of output is used to produce statically linked
4283 executables as well as `staticlib` outputs.
4285 Note that these outputs are stackable in the sense that if multiple are
4286 specified, then the compiler will produce each form of output at once without
4287 having to recompile. However, this only applies for outputs specified by the
4288 same method. If only `crate_type` attributes are specified, then they will all
4289 be built, but if one or more `--crate-type` command line flag is specified,
4290 then only those outputs will be built.
4292 With all these different kinds of outputs, if crate A depends on crate B, then
4293 the compiler could find B in various different forms throughout the system. The
4294 only forms looked for by the compiler, however, are the `rlib` format and the
4295 dynamic library format. With these two options for a dependent library, the
4296 compiler must at some point make a choice between these two formats. With this
4297 in mind, the compiler follows these rules when determining what format of
4298 dependencies will be used:
4300 1. If a static library is being produced, all upstream dependencies are
4301 required to be available in `rlib` formats. This requirement stems from the
4302 reason that a dynamic library cannot be converted into a static format.
4304 Note that it is impossible to link in native dynamic dependencies to a static
4305 library, and in this case warnings will be printed about all unlinked native
4306 dynamic dependencies.
4308 2. If an `rlib` file is being produced, then there are no restrictions on what
4309 format the upstream dependencies are available in. It is simply required that
4310 all upstream dependencies be available for reading metadata from.
4312 The reason for this is that `rlib` files do not contain any of their upstream
4313 dependencies. It wouldn't be very efficient for all `rlib` files to contain a
4314 copy of `libstd.rlib`!
4316 3. If an executable is being produced and the `-C prefer-dynamic` flag is not
4317 specified, then dependencies are first attempted to be found in the `rlib`
4318 format. If some dependencies are not available in an rlib format, then
4319 dynamic linking is attempted (see below).
4321 4. If a dynamic library or an executable that is being dynamically linked is
4322 being produced, then the compiler will attempt to reconcile the available
4323 dependencies in either the rlib or dylib format to create a final product.
4325 A major goal of the compiler is to ensure that a library never appears more
4326 than once in any artifact. For example, if dynamic libraries B and C were
4327 each statically linked to library A, then a crate could not link to B and C
4328 together because there would be two copies of A. The compiler allows mixing
4329 the rlib and dylib formats, but this restriction must be satisfied.
4331 The compiler currently implements no method of hinting what format a library
4332 should be linked with. When dynamically linking, the compiler will attempt to
4333 maximize dynamic dependencies while still allowing some dependencies to be
4334 linked in via an rlib.
4336 For most situations, having all libraries available as a dylib is recommended
4337 if dynamically linking. For other situations, the compiler will emit a
4338 warning if it is unable to determine which formats to link each library with.
4340 In general, `--crate-type=bin` or `--crate-type=lib` should be sufficient for
4341 all compilation needs, and the other options are just available if more
4342 fine-grained control is desired over the output format of a Rust crate.
4344 # Appendix: Rationales and design tradeoffs
4348 # Appendix: Influences
4350 Rust is not a particularly original language, with design elements coming from
4351 a wide range of sources. Some of these are listed below (including elements
4352 that have since been removed):
4354 * SML, OCaml: algebraic datatypes, pattern matching, type inference,
4355 semicolon statement separation
4356 * C++: references, RAII, smart pointers, move semantics, monomorphisation,
4358 * ML Kit, Cyclone: region based memory management
4359 * Haskell (GHC): typeclasses, type families
4360 * Newsqueak, Alef, Limbo: channels, concurrency
4361 * Erlang: message passing, task failure, ~~linked task failure~~,
4362 ~~lightweight concurrency~~
4363 * Swift: optional bindings
4364 * Scheme: hygienic macros
4366 * Ruby: ~~block syntax~~
4367 * NIL, Hermes: ~~typestate~~
4368 * [Unicode Annex #31](http://www.unicode.org/reports/tr31/): identifier and
4371 [ffi]: book/ffi.html
4372 [plugin]: book/plugin.html