5 This document is the primary reference for the Rust programming language. It
6 provides three kinds of material:
8 - Chapters that informally describe each language construct and their use.
9 - Chapters that informally describe the memory model, concurrency model,
10 runtime services, linkage model and debugging facilities.
11 - Appendix chapters providing rationale and references to languages that
12 influenced the design.
14 This document does not serve as an introduction to the language. Background
15 familiarity with the language is assumed. A separate [book] is available to
16 help acquire such background familiarity.
18 This document also does not serve as a reference to the [standard] library
19 included in the language distribution. Those libraries are documented
20 separately by extracting documentation attributes from their source code. Many
21 of the features that one might expect to be language features are library
22 features in Rust, so what you're looking for may be there, not here.
24 You may also be interested in the [grammar].
26 [book]: book/index.html
27 [standard]: std/index.html
28 [grammar]: grammar.html
32 Rust's grammar is defined over Unicode codepoints, each conventionally denoted
33 `U+XXXX`, for 4 or more hexadecimal digits `X`. _Most_ of Rust's grammar is
34 confined to the ASCII range of Unicode, and is described in this document by a
35 dialect of Extended Backus-Naur Form (EBNF), specifically a dialect of EBNF
36 supported by common automated LL(k) parsing tools such as `llgen`, rather than
37 the dialect given in ISO 14977. The dialect can be defined self-referentially
42 rule : nonterminal ':' productionrule ';' ;
43 productionrule : production [ '|' production ] * ;
45 term : element repeats ;
46 element : LITERAL | IDENTIFIER | '[' productionrule ']' ;
47 repeats : [ '*' | '+' ] NUMBER ? | NUMBER ? | '?' ;
52 - Whitespace in the grammar is ignored.
53 - Square brackets are used to group rules.
54 - `LITERAL` is a single printable ASCII character, or an escaped hexadecimal
55 ASCII code of the form `\xQQ`, in single quotes, denoting the corresponding
56 Unicode codepoint `U+00QQ`.
57 - `IDENTIFIER` is a nonempty string of ASCII letters and underscores.
58 - The `repeat` forms apply to the adjacent `element`, and are as follows:
59 - `?` means zero or one repetition
60 - `*` means zero or more repetitions
61 - `+` means one or more repetitions
62 - NUMBER trailing a repeat symbol gives a maximum repetition count
63 - NUMBER on its own gives an exact repetition count
65 This EBNF dialect should hopefully be familiar to many readers.
67 ## Unicode productions
69 A few productions in Rust's grammar permit Unicode codepoints outside the ASCII
70 range. We define these productions in terms of character properties specified
71 in the Unicode standard, rather than in terms of ASCII-range codepoints. The
72 section [Special Unicode Productions](#special-unicode-productions) lists these
75 ## String table productions
77 Some rules in the grammar — notably [unary
78 operators](#unary-operator-expressions), [binary
79 operators](#binary-operator-expressions), and [keywords](#keywords) — are
80 given in a simplified form: as a listing of a table of unquoted, printable
81 whitespace-separated strings. These cases form a subset of the rules regarding
82 the [token](#tokens) rule, and are assumed to be the result of a
83 lexical-analysis phase feeding the parser, driven by a DFA, operating over the
84 disjunction of all such string table entries.
86 When such a string enclosed in double-quotes (`"`) occurs inside the grammar,
87 it is an implicit reference to a single member of such a string table
88 production. See [tokens](#tokens) for more information.
94 Rust input is interpreted as a sequence of Unicode codepoints encoded in UTF-8.
95 Most Rust grammar rules are defined in terms of printable ASCII-range
96 codepoints, but a small number are defined in terms of Unicode properties or
97 explicit codepoint lists. [^inputformat]
99 [^inputformat]: Substitute definitions for the special Unicode productions are
100 provided to the grammar verifier, restricted to ASCII range, when verifying the
101 grammar in this document.
103 ## Special Unicode Productions
105 The following productions in the Rust grammar are defined in terms of Unicode
106 properties: `ident`, `non_null`, `non_star`, `non_eol`, `non_slash_or_star`,
107 `non_single_quote` and `non_double_quote`.
111 The `ident` production is any nonempty Unicode string of the following form:
113 - The first character has property `XID_start`
114 - The remaining characters have property `XID_continue`
116 that does _not_ occur in the set of [keywords](#keywords).
118 > **Note**: `XID_start` and `XID_continue` as character properties cover the
119 > character ranges used to form the more familiar C and Java language-family
122 ### Delimiter-restricted productions
124 Some productions are defined by exclusion of particular Unicode characters:
126 - `non_null` is any single Unicode character aside from `U+0000` (null)
127 - `non_eol` is `non_null` restricted to exclude `U+000A` (`'\n'`)
128 - `non_star` is `non_null` restricted to exclude `U+002A` (`*`)
129 - `non_slash_or_star` is `non_null` restricted to exclude `U+002F` (`/`) and `U+002A` (`*`)
130 - `non_single_quote` is `non_null` restricted to exclude `U+0027` (`'`)
131 - `non_double_quote` is `non_null` restricted to exclude `U+0022` (`"`)
136 comment : block_comment | line_comment ;
137 block_comment : "/*" block_comment_body * "*/" ;
138 block_comment_body : [block_comment | character] * ;
139 line_comment : "//" non_eol * ;
142 Comments in Rust code follow the general C++ style of line and block-comment
143 forms. Nested block comments are supported.
145 Line comments beginning with exactly _three_ slashes (`///`), and block
146 comments beginning with exactly one repeated asterisk in the block-open
147 sequence (`/**`), are interpreted as a special syntax for `doc`
148 [attributes](#attributes). That is, they are equivalent to writing
149 `#[doc="..."]` around the body of the comment (this includes the comment
150 characters themselves, ie `/// Foo` turns into `#[doc="/// Foo"]`).
152 `//!` comments apply to the parent of the comment, rather than the item that
153 follows. `//!` comments are usually used to display information on the crate
156 Non-doc comments are interpreted as a form of whitespace.
161 whitespace_char : '\x20' | '\x09' | '\x0a' | '\x0d' ;
162 whitespace : [ whitespace_char | comment ] + ;
165 The `whitespace_char` production is any nonempty Unicode string consisting of
166 any of the following Unicode characters: `U+0020` (space, `' '`), `U+0009`
167 (tab, `'\t'`), `U+000A` (LF, `'\n'`), `U+000D` (CR, `'\r'`).
169 Rust is a "free-form" language, meaning that all forms of whitespace serve only
170 to separate _tokens_ in the grammar, and have no semantic significance.
172 A Rust program has identical meaning if each whitespace element is replaced
173 with any other legal whitespace element, such as a single space character.
178 simple_token : keyword | unop | binop ;
179 token : simple_token | ident | literal | symbol | whitespace token ;
182 Tokens are primitive productions in the grammar defined by regular
183 (non-recursive) languages. "Simple" tokens are given in [string table
184 production](#string-table-productions) form, and occur in the rest of the
185 grammar as double-quoted strings. Other tokens have exact rules given.
189 <p id="keyword-table-marker"></p>
192 |----------|----------|----------|----------|---------|
193 | abstract | alignof | as | become | box |
194 | break | const | continue | crate | do |
195 | else | enum | extern | false | final |
196 | fn | for | if | impl | in |
197 | let | loop | macro | match | mod |
198 | move | mut | offsetof | override | priv |
199 | pub | pure | ref | return | sizeof |
200 | static | self | struct | super | true |
201 | trait | type | typeof | unsafe | unsized |
202 | use | virtual | where | while | yield |
205 Each of these keywords has special meaning in its grammar, and all of them are
206 excluded from the `ident` rule.
208 Note that some of these keywords are reserved, and do not currently do
213 A literal is an expression consisting of a single token, rather than a sequence
214 of tokens, that immediately and directly denotes the value it evaluates to,
215 rather than referring to it by name or some other evaluation rule. A literal is
216 a form of constant expression, so is evaluated (primarily) at compile time.
220 literal : [ string_lit | char_lit | byte_string_lit | byte_lit | num_lit ] lit_suffix ?;
223 The optional suffix is only used for certain numeric literals, but is
224 reserved for future extension, that is, the above gives the lexical
225 grammar, but a Rust parser will reject everything but the 12 special
226 cases mentioned in [Number literals](#number-literals) below.
230 ##### Characters and strings
232 | | Example | `#` sets | Characters | Escapes |
233 |----------------------------------------------|-----------------|------------|-------------|---------------------|
234 | [Character](#character-literals) | `'H'` | `N/A` | All Unicode | `\'` & [Byte](#byte-escapes) & [Unicode](#unicode-escapes) |
235 | [String](#string-literals) | `"hello"` | `N/A` | All Unicode | `\"` & [Byte](#byte-escapes) & [Unicode](#unicode-escapes) |
236 | [Raw](#raw-string-literals) | `r#"hello"#` | `0...` | All Unicode | `N/A` |
237 | [Byte](#byte-literals) | `b'H'` | `N/A` | All ASCII | `\'` & [Byte](#byte-escapes) |
238 | [Byte string](#byte-string-literals) | `b"hello"` | `N/A` | All ASCII | `\"` & [Byte](#byte-escapes) |
239 | [Raw byte string](#raw-byte-string-literals) | `br#"hello"#` | `0...` | All ASCII | `N/A` |
245 | `\x7F` | 8-bit character code (exactly 2 digits) |
247 | `\r` | Carriage return |
251 ##### Unicode escapes
254 | `\u{7FFF}` | 24-bit Unicode character code (up to 6 digits) |
258 | [Number literals](#number-literals)`*` | Example | Exponentiation | Suffixes |
259 |----------------------------------------|---------|----------------|----------|
260 | Decimal integer | `98_222` | `N/A` | Integer suffixes |
261 | Hex integer | `0xff` | `N/A` | Integer suffixes |
262 | Octal integer | `0o77` | `N/A` | Integer suffixes |
263 | Binary integer | `0b1111_0000` | `N/A` | Integer suffixes |
264 | Floating-point | `123.0E+77` | `Optional` | Floating-point suffixes |
266 `*` All number literals allow `_` as a visual separator: `1_234.0E+18f64`
269 | Integer | Floating-point |
270 |---------|----------------|
271 | `u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`, `i64`, `is` (`isize`), `us` (`usize`) | `f32`, `f64` |
273 #### Character and string literals
276 char_lit : '\x27' char_body '\x27' ;
277 string_lit : '"' string_body * '"' | 'r' raw_string ;
279 char_body : non_single_quote
280 | '\x5c' [ '\x27' | common_escape | unicode_escape ] ;
282 string_body : non_double_quote
283 | '\x5c' [ '\x22' | common_escape | unicode_escape ] ;
284 raw_string : '"' raw_string_body '"' | '#' raw_string '#' ;
286 common_escape : '\x5c'
287 | 'n' | 'r' | 't' | '0'
290 unicode_escape : 'u' '{' hex_digit+ 6 '}';
292 hex_digit : 'a' | 'b' | 'c' | 'd' | 'e' | 'f'
293 | 'A' | 'B' | 'C' | 'D' | 'E' | 'F'
295 oct_digit : '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' ;
296 dec_digit : '0' | nonzero_dec ;
297 nonzero_dec: '1' | '2' | '3' | '4'
298 | '5' | '6' | '7' | '8' | '9' ;
301 ##### Character literals
303 A _character literal_ is a single Unicode character enclosed within two
304 `U+0027` (single-quote) characters, with the exception of `U+0027` itself,
305 which must be _escaped_ by a preceding `U+005C` character (`\`).
307 ##### String literals
309 A _string literal_ is a sequence of any Unicode characters enclosed within two
310 `U+0022` (double-quote) characters, with the exception of `U+0022` itself,
311 which must be _escaped_ by a preceding `U+005C` character (`\`), or a _raw
314 A multi-line string literal may be defined by terminating each line with a
315 `U+005C` character (`\`) immediately before the newline. This causes the
316 `U+005C` character, the newline, and all whitespace at the beginning of the
317 next line to be ignored.
327 ##### Character escapes
329 Some additional _escapes_ are available in either character or non-raw string
330 literals. An escape starts with a `U+005C` (`\`) and continues with one of the
333 * An _8-bit codepoint escape_ escape starts with `U+0078` (`x`) and is
334 followed by exactly two _hex digits_. It denotes the Unicode codepoint
335 equal to the provided hex value.
336 * A _24-bit codepoint escape_ starts with `U+0075` (`u`) and is followed
337 by up to six _hex digits_ surrounded by braces `U+007B` (`{`) and `U+007D`
338 (`}`). It denotes the Unicode codepoint equal to the provided hex value.
339 * A _whitespace escape_ is one of the characters `U+006E` (`n`), `U+0072`
340 (`r`), or `U+0074` (`t`), denoting the unicode values `U+000A` (LF),
341 `U+000D` (CR) or `U+0009` (HT) respectively.
342 * The _backslash escape_ is the character `U+005C` (`\`) which must be
343 escaped in order to denote *itself*.
345 ##### Raw string literals
347 Raw string literals do not process any escapes. They start with the character
348 `U+0072` (`r`), followed by zero or more of the character `U+0023` (`#`) and a
349 `U+0022` (double-quote) character. The _raw string body_ is not defined in the
350 EBNF grammar above: it can contain any sequence of Unicode characters and is
351 terminated only by another `U+0022` (double-quote) character, followed by the
352 same number of `U+0023` (`#`) characters that preceded the opening `U+0022`
353 (double-quote) character.
355 All Unicode characters contained in the raw string body represent themselves,
356 the characters `U+0022` (double-quote) (except when followed by at least as
357 many `U+0023` (`#`) characters as were used to start the raw string literal) or
358 `U+005C` (`\`) do not have any special meaning.
360 Examples for string literals:
363 "foo"; r"foo"; // foo
364 "\"foo\""; r#""foo""#; // "foo"
367 r##"foo #"# bar"##; // foo #"# bar
369 "\x52"; "R"; r"R"; // R
370 "\\x52"; r"\x52"; // \x52
373 #### Byte and byte string literals
376 byte_lit : "b\x27" byte_body '\x27' ;
377 byte_string_lit : "b\x22" string_body * '\x22' | "br" raw_byte_string ;
379 byte_body : ascii_non_single_quote
380 | '\x5c' [ '\x27' | common_escape ] ;
382 byte_string_body : ascii_non_double_quote
383 | '\x5c' [ '\x22' | common_escape ] ;
384 raw_byte_string : '"' raw_byte_string_body '"' | '#' raw_byte_string '#' ;
390 A _byte literal_ is a single ASCII character (in the `U+0000` to `U+007F`
391 range) enclosed within two `U+0027` (single-quote) characters, with the
392 exception of `U+0027` itself, which must be _escaped_ by a preceding U+005C
393 character (`\`), or a single _escape_. It is equivalent to a `u8` unsigned
394 8-bit integer _number literal_.
396 ##### Byte string literals
398 A non-raw _byte string literal_ is a sequence of ASCII characters and _escapes_,
399 preceded by the characters `U+0062` (`b`) and `U+0022` (double-quote), and
400 followed by the character `U+0022`. If the character `U+0022` is present within
401 the literal, it must be _escaped_ by a preceding `U+005C` (`\`) character.
402 Alternatively, a byte string literal can be a _raw byte string literal_, defined
403 below. A byte string literal is equivalent to a `&'static [u8]` borrowed array
404 of unsigned 8-bit integers.
406 Some additional _escapes_ are available in either byte or non-raw byte string
407 literals. An escape starts with a `U+005C` (`\`) and continues with one of the
410 * An _byte escape_ escape starts with `U+0078` (`x`) and is
411 followed by exactly two _hex digits_. It denotes the byte
412 equal to the provided hex value.
413 * A _whitespace escape_ is one of the characters `U+006E` (`n`), `U+0072`
414 (`r`), or `U+0074` (`t`), denoting the bytes values `0x0A` (ASCII LF),
415 `0x0D` (ASCII CR) or `0x09` (ASCII HT) respectively.
416 * The _backslash escape_ is the character `U+005C` (`\`) which must be
417 escaped in order to denote its ASCII encoding `0x5C`.
419 ##### Raw byte string literals
421 Raw byte string literals do not process any escapes. They start with the
422 character `U+0062` (`b`), followed by `U+0072` (`r`), followed by zero or more
423 of the character `U+0023` (`#`), and a `U+0022` (double-quote) character. The
424 _raw string body_ is not defined in the EBNF grammar above: it can contain any
425 sequence of ASCII characters and is terminated only by another `U+0022`
426 (double-quote) character, followed by the same number of `U+0023` (`#`)
427 characters that preceded the opening `U+0022` (double-quote) character. A raw
428 byte string literal can not contain any non-ASCII byte.
430 All characters contained in the raw string body represent their ASCII encoding,
431 the characters `U+0022` (double-quote) (except when followed by at least as
432 many `U+0023` (`#`) characters as were used to start the raw string literal) or
433 `U+005C` (`\`) do not have any special meaning.
435 Examples for byte string literals:
438 b"foo"; br"foo"; // foo
439 b"\"foo\""; br#""foo""#; // "foo"
442 br##"foo #"# bar"##; // foo #"# bar
444 b"\x52"; b"R"; br"R"; // R
445 b"\\x52"; br"\x52"; // \x52
451 num_lit : nonzero_dec [ dec_digit | '_' ] * float_suffix ?
452 | '0' [ [ dec_digit | '_' ] * float_suffix ?
453 | 'b' [ '1' | '0' | '_' ] +
454 | 'o' [ oct_digit | '_' ] +
455 | 'x' [ hex_digit | '_' ] + ] ;
457 float_suffix : [ exponent | '.' dec_lit exponent ? ] ? ;
459 exponent : ['E' | 'e'] ['-' | '+' ] ? dec_lit ;
460 dec_lit : [ dec_digit | '_' ] + ;
463 A _number literal_ is either an _integer literal_ or a _floating-point
464 literal_. The grammar for recognizing the two kinds of literals is mixed.
466 ##### Integer literals
468 An _integer literal_ has one of four forms:
470 * A _decimal literal_ starts with a *decimal digit* and continues with any
471 mixture of *decimal digits* and _underscores_.
472 * A _hex literal_ starts with the character sequence `U+0030` `U+0078`
473 (`0x`) and continues as any mixture of hex digits and underscores.
474 * An _octal literal_ starts with the character sequence `U+0030` `U+006F`
475 (`0o`) and continues as any mixture of octal digits and underscores.
476 * A _binary literal_ starts with the character sequence `U+0030` `U+0062`
477 (`0b`) and continues as any mixture of binary digits and underscores.
479 Like any literal, an integer literal may be followed (immediately,
480 without any spaces) by an _integer suffix_, which forcibly sets the
481 type of the literal. The integer suffix must be the name of one of the
482 integral types: `u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`, `i64`,
485 The type of an _unsuffixed_ integer literal is determined by type inference.
486 If an integer type can be _uniquely_ determined from the surrounding program
487 context, the unsuffixed integer literal has that type. If the program context
488 underconstrains the type, it defaults to the signed 32-bit integer `i32`; if
489 the program context overconstrains the type, it is considered a static type
492 Examples of integer literals of various forms:
499 0o70_i16; // type i16
500 0b1111_1111_1001_0000_i32; // type i32
501 0usize; // type usize
504 ##### Floating-point literals
506 A _floating-point literal_ has one of two forms:
508 * A _decimal literal_ followed by a period character `U+002E` (`.`). This is
509 optionally followed by another decimal literal, with an optional _exponent_.
510 * A single _decimal literal_ followed by an _exponent_.
512 By default, a floating-point literal has a generic type, and, like integer
513 literals, the type must be uniquely determined from the context. There are two valid
514 _floating-point suffixes_, `f32` and `f64` (the 32-bit and 64-bit floating point
515 types), which explicitly determine the type of the literal.
517 Examples of floating-point literals of various forms:
520 123.0f64; // type f64
523 12E+99_f64; // type f64
524 let x: f64 = 2.; // type f64
527 This last example is different because it is not possible to use the suffix
528 syntax with a floating point literal ending in a period. `2.f64` would attempt
529 to call a method named `f64` on `2`.
531 The representation semantics of floating-point numbers are described in
532 ["Machine Types"](#machine-types).
534 #### Boolean literals
536 The two values of the boolean type are written `true` and `false`.
542 | '#' | '[' | ']' | '(' | ')' | '{' | '}'
546 Symbols are a general class of printable [token](#tokens) that play structural
547 roles in a variety of grammar productions. They are catalogued here for
548 completeness as the set of remaining miscellaneous printable tokens that do not
549 otherwise appear as [unary operators](#unary-operator-expressions), [binary
550 operators](#binary-operator-expressions), or [keywords](#keywords).
556 expr_path : [ "::" ] ident [ "::" expr_path_tail ] + ;
557 expr_path_tail : '<' type_expr [ ',' type_expr ] + '>'
560 type_path : ident [ type_path_tail ] + ;
561 type_path_tail : '<' type_expr [ ',' type_expr ] + '>'
565 A _path_ is a sequence of one or more path components _logically_ separated by
566 a namespace qualifier (`::`). If a path consists of only one component, it may
567 refer to either an [item](#items) or a [variable](#variables) in a local control
568 scope. If a path has multiple components, it refers to an item.
570 Every item has a _canonical path_ within its crate, but the path naming an item
571 is only meaningful within a given crate. There is no global namespace across
572 crates; an item's canonical path merely identifies it within the crate.
574 Two examples of simple paths consisting of only identifier components:
581 Path components are usually [identifiers](#identifiers), but the trailing
582 component of a path may be an angle-bracket-enclosed list of type arguments. In
583 [expression](#expressions) context, the type argument list is given after a
584 final (`::`) namespace qualifier in order to disambiguate it from a relational
585 expression involving the less-than symbol (`<`). In type expression context,
586 the final namespace qualifier is omitted.
588 Two examples of paths with type arguments:
591 # struct HashMap<K, V>(K,V);
593 # fn id<T>(t: T) -> T { t }
594 type T = HashMap<i32,String>; // Type arguments used in a type expression
595 let x = id::<i32>(10); // Type arguments used in a call expression
599 Paths can be denoted with various leading qualifiers to change the meaning of
602 * Paths starting with `::` are considered to be global paths where the
603 components of the path start being resolved from the crate root. Each
604 identifier in the path must resolve to an item.
612 ::a::foo(); // call a's foo function
618 * Paths starting with the keyword `super` begin resolution relative to the
619 parent module. Each further identifier must resolve to an item.
627 super::a::foo(); // call a's foo function
633 * Paths starting with the keyword `self` begin resolution relative to the
634 current module. Each further identifier must resolve to an item.
646 A number of minor features of Rust are not central enough to have their own
647 syntax, and yet are not implementable as functions. Instead, they are given
648 names, and invoked through a consistent syntax: `some_extension!(...)`.
650 Users of `rustc` can define new syntax extensions in two ways:
652 * [Compiler plugins][plugin] can include arbitrary
653 Rust code that manipulates syntax trees at compile time.
655 * [Macros](book/macros.html) define new syntax in a higher-level,
661 expr_macro_rules : "macro_rules" '!' ident '(' macro_rule * ')' ;
662 macro_rule : '(' matcher * ')' "=>" '(' transcriber * ')' ';' ;
663 matcher : '(' matcher * ')' | '[' matcher * ']'
664 | '{' matcher * '}' | '$' ident ':' ident
665 | '$' '(' matcher * ')' sep_token? [ '*' | '+' ]
666 | non_special_token ;
667 transcriber : '(' transcriber * ')' | '[' transcriber * ']'
668 | '{' transcriber * '}' | '$' ident
669 | '$' '(' transcriber * ')' sep_token? [ '*' | '+' ]
670 | non_special_token ;
673 `macro_rules` allows users to define syntax extension in a declarative way. We
674 call such extensions "macros by example" or simply "macros" — to be distinguished
675 from the "procedural macros" defined in [compiler plugins][plugin].
677 Currently, macros can expand to expressions, statements, items, or patterns.
679 (A `sep_token` is any token other than `*` and `+`. A `non_special_token` is
680 any token other than a delimiter or `$`.)
682 The macro expander looks up macro invocations by name, and tries each macro
683 rule in turn. It transcribes the first successful match. Matching and
684 transcription are closely related to each other, and we will describe them
689 The macro expander matches and transcribes every token that does not begin with
690 a `$` literally, including delimiters. For parsing reasons, delimiters must be
691 balanced, but they are otherwise not special.
693 In the matcher, `$` _name_ `:` _designator_ matches the nonterminal in the Rust
694 syntax named by _designator_. Valid designators are `item`, `block`, `stmt`,
695 `pat`, `expr`, `ty` (type), `ident`, `path`, `tt` (either side of the `=>`
696 in macro rules). In the transcriber, the designator is already known, and so
697 only the name of a matched nonterminal comes after the dollar sign.
699 In both the matcher and transcriber, the Kleene star-like operator indicates
700 repetition. The Kleene star operator consists of `$` and parens, optionally
701 followed by a separator token, followed by `*` or `+`. `*` means zero or more
702 repetitions, `+` means at least one repetition. The parens are not matched or
703 transcribed. On the matcher side, a name is bound to _all_ of the names it
704 matches, in a structure that mimics the structure of the repetition encountered
705 on a successful match. The job of the transcriber is to sort that structure
708 The rules for transcription of these repetitions are called "Macro By Example".
709 Essentially, one "layer" of repetition is discharged at a time, and all of them
710 must be discharged by the time a name is transcribed. Therefore, `( $( $i:ident
711 ),* ) => ( $i )` is an invalid macro, but `( $( $i:ident ),* ) => ( $( $i:ident
712 ),* )` is acceptable (if trivial).
714 When Macro By Example encounters a repetition, it examines all of the `$`
715 _name_ s that occur in its body. At the "current layer", they all must repeat
716 the same number of times, so ` ( $( $i:ident ),* ; $( $j:ident ),* ) => ( $(
717 ($i,$j) ),* )` is valid if given the argument `(a,b,c ; d,e,f)`, but not
718 `(a,b,c ; d,e)`. The repetition walks through the choices at that layer in
719 lockstep, so the former input transcribes to `( (a,d), (b,e), (c,f) )`.
721 Nested repetitions are allowed.
723 ### Parsing limitations
725 The parser used by the macro system is reasonably powerful, but the parsing of
726 Rust syntax is restricted in two ways:
728 1. The parser will always parse as much as possible. If it attempts to match
729 `$i:expr [ , ]` against `8 [ , ]`, it will attempt to parse `i` as an array
730 index operation and fail. Adding a separator can solve this problem.
731 2. The parser must have eliminated all ambiguity by the time it reaches a `$`
732 _name_ `:` _designator_. This requirement most often affects name-designator
733 pairs when they occur at the beginning of, or immediately after, a `$(...)*`;
734 requiring a distinctive token in front can solve the problem.
736 # Crates and source files
738 Although Rust, like any other language, can be implemented by an interpreter as
739 well as a compiler, the only existing implementation is a compiler —
740 from now on referred to as *the* Rust compiler — and the language has
741 always been designed to be compiled. For these reasons, this section assumes a
744 Rust's semantics obey a *phase distinction* between compile-time and
745 run-time.[^phase-distinction] Those semantic rules that have a *static
746 interpretation* govern the success or failure of compilation. Those semantics
747 that have a *dynamic interpretation* govern the behavior of the program at
750 [^phase-distinction]: This distinction would also exist in an interpreter.
751 Static checks like syntactic analysis, type checking, and lints should
752 happen before the program is executed regardless of when it is executed.
754 The compilation model centers on artifacts called _crates_. Each compilation
755 processes a single crate in source form, and if successful, produces a single
756 crate in binary form: either an executable or some sort of
757 library.[^cratesourcefile]
759 [^cratesourcefile]: A crate is somewhat analogous to an *assembly* in the
760 ECMA-335 CLI model, a *library* in the SML/NJ Compilation Manager, a *unit*
761 in the Owens and Flatt module system, or a *configuration* in Mesa.
763 A _crate_ is a unit of compilation and linking, as well as versioning,
764 distribution and runtime loading. A crate contains a _tree_ of nested
765 [module](#modules) scopes. The top level of this tree is a module that is
766 anonymous (from the point of view of paths within the module) and any item
767 within a crate has a canonical [module path](#paths) denoting its location
768 within the crate's module tree.
770 The Rust compiler is always invoked with a single source file as input, and
771 always produces a single output crate. The processing of that source file may
772 result in other source files being loaded as modules. Source files have the
775 A Rust source file describes a module, the name and location of which —
776 in the module tree of the current crate — are defined from outside the
777 source file: either by an explicit `mod_item` in a referencing source file, or
778 by the name of the crate itself. Every source file is a module, but not every
779 module needs its own source file: [module definitions](#modules) can be nested
782 Each source file contains a sequence of zero or more `item` definitions, and
783 may optionally begin with any number of [attributes](#Items and attributes)
784 that apply to the containing module, most of which influence the behavior of
785 the compiler. The anonymous crate module can have additional attributes that
786 apply to the crate as a whole.
789 // Specify the crate name.
790 #![crate_name = "projx"]
792 // Specify the type of output artifact.
793 #![crate_type = "lib"]
795 // Turn on a warning.
796 // This can be done in any module, not just the anonymous crate module.
797 #![warn(non_camel_case_types)]
800 A crate that contains a `main` function can be compiled to an executable. If a
801 `main` function is present, its return type must be [`unit`](#primitive-types)
802 and it must take no arguments.
804 # Items and attributes
806 Crates contain [items](#items), each of which may have some number of
807 [attributes](#attributes) attached to it.
812 item : extern_crate_decl | use_decl | mod_item | fn_item | type_item
813 | struct_item | enum_item | static_item | trait_item | impl_item
817 An _item_ is a component of a crate. Items are organized within a crate by a
818 nested set of [modules](#modules). Every crate has a single "outermost"
819 anonymous module; all further items within the crate have [paths](#paths)
820 within the module tree of the crate.
822 Items are entirely determined at compile-time, generally remain fixed during
823 execution, and may reside in read-only memory.
825 There are several kinds of item:
827 * [`extern crate` declarations](#extern-crate-declarations)
828 * [`use` declarations](#use-declarations)
829 * [modules](#modules)
830 * [functions](#functions)
831 * [type definitions](#type-definitions)
832 * [structures](#structures)
833 * [enumerations](#enumerations)
834 * [static items](#static-items)
836 * [implementations](#implementations)
838 Some items form an implicit scope for the declaration of sub-items. In other
839 words, within a function or module, declarations of items can (in many cases)
840 be mixed with the statements, control blocks, and similar artifacts that
841 otherwise compose the item body. The meaning of these scoped items is the same
842 as if the item was declared outside the scope — it is still a static item
843 — except that the item's *path name* within the module namespace is
844 qualified by the name of the enclosing item, or is private to the enclosing
845 item (in the case of functions). The grammar specifies the exact locations in
846 which sub-item declarations may appear.
850 All items except modules may be *parameterized* by type. Type parameters are
851 given as a comma-separated list of identifiers enclosed in angle brackets
852 (`<...>`), after the name of the item and before its definition. The type
853 parameters of an item are considered "part of the name", not part of the type
854 of the item. A referencing [path](#paths) must (in principle) provide type
855 arguments as a list of comma-separated types enclosed within angle brackets, in
856 order to refer to the type-parameterized item. In practice, the type-inference
857 system can usually infer such argument types from context. There are no
858 general type-parametric types, only type-parametric items. That is, Rust has
859 no notion of type abstraction: there are no first-class "forall" types.
864 mod_item : "mod" ident ( ';' | '{' mod '}' );
868 A module is a container for zero or more [items](#items).
870 A _module item_ is a module, surrounded in braces, named, and prefixed with the
871 keyword `mod`. A module item introduces a new, named module into the tree of
872 modules making up a crate. Modules can nest arbitrarily.
874 An example of a module:
878 type Complex = (f64, f64);
879 fn sin(f: f64) -> f64 {
883 fn cos(f: f64) -> f64 {
887 fn tan(f: f64) -> f64 {
894 Modules and types share the same namespace. Declaring a named type with
895 the same name as a module in scope is forbidden: that is, a type definition,
896 trait, struct, enumeration, or type parameter can't shadow the name of a module
897 in scope, or vice versa.
899 A module without a body is loaded from an external file, by default with the
900 same name as the module, plus the `.rs` extension. When a nested submodule is
901 loaded from an external file, it is loaded from a subdirectory path that
902 mirrors the module hierarchy.
905 // Load the `vec` module from `vec.rs`
909 // Load the `local_data` module from `thread/local_data.rs`
914 The directories and files used for loading external file modules can be
915 influenced with the `path` attribute.
918 #[path = "thread_files"]
920 // Load the `local_data` module from `thread_files/tls.rs`
926 ##### Extern crate declarations
929 extern_crate_decl : "extern" "crate" crate_name
930 crate_name: ident | ( string_lit "as" ident )
933 An _`extern crate` declaration_ specifies a dependency on an external crate.
934 The external crate is then bound into the declaring scope as the `ident`
935 provided in the `extern_crate_decl`.
937 The external crate is resolved to a specific `soname` at compile time, and a
938 runtime linkage requirement to that `soname` is passed to the linker for
939 loading at runtime. The `soname` is resolved at compile time by scanning the
940 compiler's library path and matching the optional `crateid` provided as a
941 string literal against the `crateid` attributes that were declared on the
942 external crate when it was compiled. If no `crateid` is provided, a default
943 `name` attribute is assumed, equal to the `ident` given in the
946 Three examples of `extern crate` declarations:
951 extern crate std; // equivalent to: extern crate std as std;
953 extern crate std as ruststd; // linking to 'std' under another name
956 ##### Use declarations
959 use_decl : "pub" ? "use" [ path "as" ident
962 path_glob : ident [ "::" [ path_glob
964 | '{' path_item [ ',' path_item ] * '}' ;
966 path_item : ident | "self" ;
969 A _use declaration_ creates one or more local name bindings synonymous with
970 some other [path](#paths). Usually a `use` declaration is used to shorten the
971 path required to refer to a module item. These declarations may appear at the
972 top of [modules](#modules) and [blocks](#blocks).
974 > **Note**: Unlike in many languages,
975 > `use` declarations in Rust do *not* declare linkage dependency with external crates.
976 > Rather, [`extern crate` declarations](#extern-crate-declarations) declare linkage dependencies.
978 Use declarations support a number of convenient shortcuts:
980 * Rebinding the target name as a new local name, using the syntax `use p::q::r as x;`
981 * Simultaneously binding a list of paths differing only in their final element,
982 using the glob-like brace syntax `use a::b::{c,d,e,f};`
983 * Binding all paths matching a given prefix, using the asterisk wildcard syntax
985 * Simultaneously binding a list of paths differing only in their final element
986 and their immediate parent module, using the `self` keyword, such as
987 `use a::b::{self, c, d};`
989 An example of `use` declarations:
992 use std::option::Option::{Some, None};
993 use std::collections::hash_map::{self, HashMap};
996 fn bar(map1: HashMap<String, usize>, map2: hash_map::HashMap<String, usize>){}
999 // Equivalent to 'foo(vec![std::option::Option::Some(1.0f64),
1000 // std::option::Option::None]);'
1001 foo(vec![Some(1.0f64), None]);
1003 // Both `hash_map` and `HashMap` are in scope.
1004 let map1 = HashMap::new();
1005 let map2 = hash_map::HashMap::new();
1010 Like items, `use` declarations are private to the containing module, by
1011 default. Also like items, a `use` declaration can be public, if qualified by
1012 the `pub` keyword. Such a `use` declaration serves to _re-export_ a name. A
1013 public `use` declaration can therefore _redirect_ some public name to a
1014 different target definition: even a definition with a private canonical path,
1015 inside a different module. If a sequence of such redirections form a cycle or
1016 cannot be resolved unambiguously, they represent a compile-time error.
1018 An example of re-exporting:
1023 pub use quux::foo::{bar, baz};
1032 In this example, the module `quux` re-exports two public names defined in
1035 Also note that the paths contained in `use` items are relative to the crate
1036 root. So, in the previous example, the `use` refers to `quux::foo::{bar,
1037 baz}`, and not simply to `foo::{bar, baz}`. This also means that top-level
1038 module declarations should be at the crate root if direct usage of the declared
1039 modules within `use` items is desired. It is also possible to use `self` and
1040 `super` at the beginning of a `use` item to refer to the current and direct
1041 parent modules respectively. All rules regarding accessing declared modules in
1042 `use` declarations applies to both module declarations and `extern crate`
1045 An example of what will and will not work for `use` items:
1048 # #![allow(unused_imports)]
1049 use foo::baz::foobaz; // good: foo is at the root of the crate
1057 use foo::example::iter; // good: foo is at crate root
1058 // use example::iter; // bad: core is not at the crate root
1059 use self::baz::foobaz; // good: self refers to module 'foo'
1060 use foo::bar::foobar; // good: foo is at crate root
1067 use super::bar::foobar; // good: super refers to module 'foo'
1077 A _function item_ defines a sequence of [statements](#statements) and an
1078 optional final [expression](#expressions), along with a name and a set of
1079 parameters. Functions are declared with the keyword `fn`. Functions declare a
1080 set of *input* [*variables*](#variables) as parameters, through which the caller
1081 passes arguments into the function, and the *output* [*type*](#types)
1082 of the value the function will return to its caller on completion.
1084 A function may also be copied into a first-class *value*, in which case the
1085 value has the corresponding [*function type*](#function-types), and can be used
1086 otherwise exactly as a function item (with a minor additional cost of calling
1087 the function indirectly).
1089 Every control path in a function logically ends with a `return` expression or a
1090 diverging expression. If the outermost block of a function has a
1091 value-producing expression in its final-expression position, that expression is
1092 interpreted as an implicit `return` expression applied to the final-expression.
1094 An example of a function:
1097 fn add(x: i32, y: i32) -> i32 {
1102 As with `let` bindings, function arguments are irrefutable patterns, so any
1103 pattern that is valid in a let binding is also valid as an argument.
1106 fn first((value, _): (i32, i32)) -> i32 { value }
1110 #### Generic functions
1112 A _generic function_ allows one or more _parameterized types_ to appear in its
1113 signature. Each type parameter must be explicitly declared, in an
1114 angle-bracket-enclosed, comma-separated list following the function name.
1117 fn iter<T>(seq: &[T], f: |T|) {
1118 for elt in seq.iter() { f(elt); }
1120 fn map<T, U>(seq: &[T], f: |T| -> U) -> Vec<U> {
1121 let mut acc = vec![];
1122 for elt in seq.iter() { acc.push(f(elt)); }
1127 Inside the function signature and body, the name of the type parameter can be
1128 used as a type name.
1130 When a generic function is referenced, its type is instantiated based on the
1131 context of the reference. For example, calling the `iter` function defined
1132 above on `[1, 2]` will instantiate type parameter `T` with `i32`, and require
1133 the closure parameter to have type `fn(i32)`.
1135 The type parameters can also be explicitly supplied in a trailing
1136 [path](#paths) component after the function name. This might be necessary if
1137 there is not sufficient context to determine the type parameters. For example,
1138 `mem::size_of::<u32>() == 4`.
1140 Since a parameter type is opaque to the generic function, the set of operations
1141 that can be performed on it is limited. Values of parameter type can only be
1145 fn id<T>(x: T) -> T { x }
1148 Similarly, [trait](#traits) bounds can be specified for type parameters to
1149 allow methods with that trait to be called on values of that type.
1153 Unsafe operations are those that potentially violate the memory-safety
1154 guarantees of Rust's static semantics.
1156 The following language level features cannot be used in the safe subset of
1159 - Dereferencing a [raw pointer](#pointer-types).
1160 - Reading or writing a [mutable static variable](#mutable-statics).
1161 - Calling an unsafe function (including an intrinsic or foreign function).
1163 ##### Unsafe functions
1165 Unsafe functions are functions that are not safe in all contexts and/or for all
1166 possible inputs. Such a function must be prefixed with the keyword `unsafe` and
1167 can only be called from an `unsafe` block or another `unsafe` function.
1171 A block of code can be prefixed with the `unsafe` keyword, to permit calling
1172 `unsafe` functions or dereferencing raw pointers within a safe function.
1174 When a programmer has sufficient conviction that a sequence of potentially
1175 unsafe operations is actually safe, they can encapsulate that sequence (taken
1176 as a whole) within an `unsafe` block. The compiler will consider uses of such
1177 code safe, in the surrounding context.
1179 Unsafe blocks are used to wrap foreign libraries, make direct use of hardware
1180 or implement features not directly present in the language. For example, Rust
1181 provides the language features necessary to implement memory-safe concurrency
1182 in the language but the implementation of threads and message passing is in the
1185 Rust's type system is a conservative approximation of the dynamic safety
1186 requirements, so in some cases there is a performance cost to using safe code.
1187 For example, a doubly-linked list is not a tree structure and can only be
1188 represented with reference-counted pointers in safe code. By using `unsafe`
1189 blocks to represent the reverse links as raw pointers, it can be implemented
1192 ##### Behavior considered undefined
1194 The following is a list of behavior which is forbidden in all Rust code,
1195 including within `unsafe` blocks and `unsafe` functions. Type checking provides
1196 the guarantee that these issues are never caused by safe code.
1199 * Dereferencing a null/dangling raw pointer
1200 * Reads of [undef](http://llvm.org/docs/LangRef.html#undefined-values)
1201 (uninitialized) memory
1202 * Breaking the [pointer aliasing
1203 rules](http://llvm.org/docs/LangRef.html#pointer-aliasing-rules)
1204 with raw pointers (a subset of the rules used by C)
1205 * `&mut` and `&` follow LLVM’s scoped [noalias] model, except if the `&T`
1206 contains an `UnsafeCell<U>`. Unsafe code must not violate these aliasing
1208 * Mutating an immutable value/reference without `UnsafeCell<U>`
1209 * Invoking undefined behavior via compiler intrinsics:
1210 * Indexing outside of the bounds of an object with `std::ptr::offset`
1211 (`offset` intrinsic), with
1212 the exception of one byte past the end which is permitted.
1213 * Using `std::ptr::copy_nonoverlapping_memory` (`memcpy32`/`memcpy64`
1214 intrinsics) on overlapping buffers
1215 * Invalid values in primitive types, even in private fields/locals:
1216 * Dangling/null references or boxes
1217 * A value other than `false` (0) or `true` (1) in a `bool`
1218 * A discriminant in an `enum` not included in the type definition
1219 * A value in a `char` which is a surrogate or above `char::MAX`
1220 * Non-UTF-8 byte sequences in a `str`
1221 * Unwinding into Rust from foreign code or unwinding from Rust into foreign
1222 code. Rust's failure system is not compatible with exception handling in
1223 other languages. Unwinding must be caught and handled at FFI boundaries.
1225 [noalias]: http://llvm.org/docs/LangRef.html#noalias
1227 ##### Behaviour not considered unsafe
1229 This is a list of behaviour not considered *unsafe* in Rust terms, but that may
1233 * Reading data from private fields (`std::repr`)
1234 * Leaks due to reference count cycles, even in the global heap
1235 * Exiting without calling destructors
1237 * Accessing/modifying the file system
1238 * Unsigned integer overflow (well-defined as wrapping)
1239 * Signed integer overflow (well-defined as two’s complement representation
1242 #### Diverging functions
1244 A special kind of function can be declared with a `!` character where the
1245 output type would normally be. For example:
1248 fn my_err(s: &str) -> ! {
1254 We call such functions "diverging" because they never return a value to the
1255 caller. Every control path in a diverging function must end with a `panic!()` or
1256 a call to another diverging function on every control path. The `!` annotation
1257 does *not* denote a type.
1259 It might be necessary to declare a diverging function because as mentioned
1260 previously, the typechecker checks that every control path in a function ends
1261 with a [`return`](#return-expressions) or diverging expression. So, if `my_err`
1262 were declared without the `!` annotation, the following code would not
1266 # fn my_err(s: &str) -> ! { panic!() }
1268 fn f(i: i32) -> i32 {
1273 my_err("Bad number!");
1278 This will not compile without the `!` annotation on `my_err`, since the `else`
1279 branch of the conditional in `f` does not return an `i32`, as required by the
1280 signature of `f`. Adding the `!` annotation to `my_err` informs the
1281 typechecker that, should control ever enter `my_err`, no further type judgments
1282 about `f` need to hold, since control will never resume in any context that
1283 relies on those judgments. Thus the return type on `f` only needs to reflect
1284 the `if` branch of the conditional.
1286 #### Extern functions
1288 Extern functions are part of Rust's foreign function interface, providing the
1289 opposite functionality to [external blocks](#external-blocks). Whereas
1290 external blocks allow Rust code to call foreign code, extern functions with
1291 bodies defined in Rust code _can be called by foreign code_. They are defined
1292 in the same way as any other Rust function, except that they have the `extern`
1296 // Declares an extern fn, the ABI defaults to "C"
1297 extern fn new_i32() -> i32 { 0 }
1299 // Declares an extern fn with "stdcall" ABI
1300 extern "stdcall" fn new_i32_stdcall() -> i32 { 0 }
1303 Unlike normal functions, extern fns have an `extern "ABI" fn()`. This is the
1304 same type as the functions declared in an extern block.
1307 # extern fn new_i32() -> i32 { 0 }
1308 let fptr: extern "C" fn() -> i32 = new_i32;
1311 Extern functions may be called directly from Rust code as Rust uses large,
1312 contiguous stack segments like C.
1316 A _type alias_ defines a new name for an existing [type](#types). Type
1317 aliases are declared with the keyword `type`. Every value has a single,
1318 specific type, but may implement several different traits, or be compatible with
1319 several different type constraints.
1321 For example, the following defines the type `Point` as a synonym for the type
1322 `(u8, u8)`, the type of pairs of unsigned 8 bit integers.:
1325 type Point = (u8, u8);
1326 let p: Point = (41, 68);
1331 A _structure_ is a nominal [structure type](#structure-types) defined with the
1334 An example of a `struct` item and its use:
1337 struct Point {x: i32, y: i32}
1338 let p = Point {x: 10, y: 11};
1342 A _tuple structure_ is a nominal [tuple type](#tuple-types), also defined with
1343 the keyword `struct`. For example:
1346 struct Point(i32, i32);
1347 let p = Point(10, 11);
1348 let px: i32 = match p { Point(x, _) => x };
1351 A _unit-like struct_ is a structure without any fields, defined by leaving off
1352 the list of fields entirely. Such types will have a single value, just like
1353 the [unit value `()`](#unit-and-boolean-literals) of the unit type. For
1358 let c = [Cookie, Cookie, Cookie, Cookie];
1361 The precise memory layout of a structure is not specified. One can specify a
1362 particular layout using the [`repr` attribute](#ffi-attributes).
1366 An _enumeration_ is a simultaneous definition of a nominal [enumerated
1367 type](#enumerated-types) as well as a set of *constructors*, that can be used
1368 to create or pattern-match values of the corresponding enumerated type.
1370 Enumerations are declared with the keyword `enum`.
1372 An example of an `enum` item and its use:
1380 let mut a: Animal = Animal::Dog;
1384 Enumeration constructors can have either named or unnamed fields:
1389 Cat { name: String, weight: f64 }
1392 let mut a: Animal = Animal::Dog("Cocoa".to_string(), 37.2);
1393 a = Animal::Cat { name: "Spotty".to_string(), weight: 2.7 };
1396 In this example, `Cat` is a _struct-like enum variant_,
1397 whereas `Dog` is simply called an enum variant.
1399 Enums have a discriminant. You can assign them explicitly:
1407 If a discriminant isn't assigned, they start at zero, and add one for each
1410 You can cast an enum to get this value:
1413 # enum Foo { Bar = 123 }
1414 let x = Foo::Bar as u32; // x is now 123u32
1417 This only works as long as none of the variants have data attached. If
1418 it were `Bar(i32)`, this is disallowed.
1423 const_item : "const" ident ':' type '=' expr ';' ;
1426 A *constant item* is a named _constant value_ which is not associated with a
1427 specific memory location in the program. Constants are essentially inlined
1428 wherever they are used, meaning that they are copied directly into the relevant
1429 context when used. References to the same constant are not necessarily
1430 guaranteed to refer to the same memory address.
1432 Constant values must not have destructors, and otherwise permit most forms of
1433 data. Constants may refer to the address of other constants, in which case the
1434 address will have the `static` lifetime. The compiler is, however, still at
1435 liberty to translate the constant many times, so the address referred to may not
1438 Constants must be explicitly typed. The type may be `bool`, `char`, a number, or
1439 a type derived from those primitive types. The derived types are references with
1440 the `static` lifetime, fixed-size arrays, tuples, enum variants, and structs.
1443 const BIT1: u32 = 1 << 0;
1444 const BIT2: u32 = 1 << 1;
1446 const BITS: [u32; 2] = [BIT1, BIT2];
1447 const STRING: &'static str = "bitstring";
1449 struct BitsNStrings<'a> {
1454 const BITS_N_STRINGS: BitsNStrings<'static> = BitsNStrings {
1463 static_item : "static" ident ':' type '=' expr ';' ;
1466 A *static item* is similar to a *constant*, except that it represents a precise
1467 memory location in the program. A static is never "inlined" at the usage site,
1468 and all references to it refer to the same memory location. Static items have
1469 the `static` lifetime, which outlives all other lifetimes in a Rust program.
1470 Static items may be placed in read-only memory if they do not contain any
1471 interior mutability.
1473 Statics may contain interior mutability through the `UnsafeCell` language item.
1474 All access to a static is safe, but there are a number of restrictions on
1477 * Statics may not contain any destructors.
1478 * The types of static values must ascribe to `Sync` to allow threadsafe access.
1479 * Statics may not refer to other statics by value, only by reference.
1480 * Constants cannot refer to statics.
1482 Constants should in general be preferred over statics, unless large amounts of
1483 data are being stored, or single-address and mutability properties are required.
1485 #### Mutable statics
1487 If a static item is declared with the `mut` keyword, then it is allowed to
1488 be modified by the program. One of Rust's goals is to make concurrency bugs
1489 hard to run into, and this is obviously a very large source of race conditions
1490 or other bugs. For this reason, an `unsafe` block is required when either
1491 reading or writing a mutable static variable. Care should be taken to ensure
1492 that modifications to a mutable static are safe with respect to other threads
1493 running in the same process.
1495 Mutable statics are still very useful, however. They can be used with C
1496 libraries and can also be bound from C libraries (in an `extern` block).
1499 # fn atomic_add(_: &mut u32, _: u32) -> u32 { 2 }
1501 static mut LEVELS: u32 = 0;
1503 // This violates the idea of no shared state, and this doesn't internally
1504 // protect against races, so this function is `unsafe`
1505 unsafe fn bump_levels_unsafe1() -> u32 {
1511 // Assuming that we have an atomic_add function which returns the old value,
1512 // this function is "safe" but the meaning of the return value may not be what
1513 // callers expect, so it's still marked as `unsafe`
1514 unsafe fn bump_levels_unsafe2() -> u32 {
1515 return atomic_add(&mut LEVELS, 1);
1519 Mutable statics have the same restrictions as normal statics, except that the
1520 type of the value is not required to ascribe to `Sync`.
1524 A _trait_ describes a set of method types.
1526 Traits can include default implementations of methods, written in terms of some
1527 unknown [`self` type](#self-types); the `self` type may either be completely
1528 unspecified, or constrained by some other trait.
1530 Traits are implemented for specific types through separate
1531 [implementations](#implementations).
1534 # type Surface = i32;
1535 # type BoundingBox = i32;
1537 fn draw(&self, Surface);
1538 fn bounding_box(&self) -> BoundingBox;
1542 This defines a trait with two methods. All values that have
1543 [implementations](#implementations) of this trait in scope can have their
1544 `draw` and `bounding_box` methods called, using `value.bounding_box()`
1545 [syntax](#method-call-expressions).
1547 Type parameters can be specified for a trait to make it generic. These appear
1548 after the trait name, using the same syntax used in [generic
1549 functions](#generic-functions).
1553 fn len(&self) -> u32;
1554 fn elt_at(&self, n: u32) -> T;
1555 fn iter<F>(&self, F) where F: Fn(T);
1559 Generic functions may use traits as _bounds_ on their type parameters. This
1560 will have two effects: only types that have the trait may instantiate the
1561 parameter, and within the generic function, the methods of the trait can be
1562 called on values that have the parameter's type. For example:
1565 # type Surface = i32;
1566 # trait Shape { fn draw(&self, Surface); }
1567 fn draw_twice<T: Shape>(surface: Surface, sh: T) {
1573 Traits also define an [object type](#object-types) with the same name as the
1574 trait. Values of this type are created by [casting](#type-cast-expressions)
1575 pointer values (pointing to a type for which an implementation of the given
1576 trait is in scope) to pointers to the trait name, used as a type.
1579 # trait Shape { fn dummy(&self) { } }
1580 # impl Shape for i32 { }
1581 # let mycircle = 0i32;
1582 let myshape: Box<Shape> = Box::new(mycircle) as Box<Shape>;
1585 The resulting value is a box containing the value that was cast, along with
1586 information that identifies the methods of the implementation that was used.
1587 Values with a trait type can have [methods called](#method-call-expressions) on
1588 them, for any method in the trait, and can be used to instantiate type
1589 parameters that are bounded by the trait.
1591 Trait methods may be static, which means that they lack a `self` argument.
1592 This means that they can only be called with function call syntax (`f(x)`) and
1593 not method call syntax (`obj.f()`). The way to refer to the name of a static
1594 method is to qualify it with the trait name, treating the trait name like a
1595 module. For example:
1599 fn from_i32(n: i32) -> Self;
1602 fn from_i32(n: i32) -> f64 { n as f64 }
1604 let x: f64 = Num::from_i32(42);
1607 Traits may inherit from other traits. For example, in
1610 trait Shape { fn area(&self) -> f64; }
1611 trait Circle : Shape { fn radius(&self) -> f64; }
1614 the syntax `Circle : Shape` means that types that implement `Circle` must also
1615 have an implementation for `Shape`. Multiple supertraits are separated by `+`,
1616 `trait Circle : Shape + PartialEq { }`. In an implementation of `Circle` for a
1617 given type `T`, methods can refer to `Shape` methods, since the typechecker
1618 checks that any type with an implementation of `Circle` also has an
1619 implementation of `Shape`.
1621 In type-parameterized functions, methods of the supertrait may be called on
1622 values of subtrait-bound type parameters. Referring to the previous example of
1623 `trait Circle : Shape`:
1626 # trait Shape { fn area(&self) -> f64; }
1627 # trait Circle : Shape { fn radius(&self) -> f64; }
1628 fn radius_times_area<T: Circle>(c: T) -> f64 {
1629 // `c` is both a Circle and a Shape
1630 c.radius() * c.area()
1634 Likewise, supertrait methods may also be called on trait objects.
1637 # trait Shape { fn area(&self) -> f64; }
1638 # trait Circle : Shape { fn radius(&self) -> f64; }
1639 # impl Shape for i32 { fn area(&self) -> f64 { 0.0 } }
1640 # impl Circle for i32 { fn radius(&self) -> f64 { 0.0 } }
1641 # let mycircle = 0i32;
1642 let mycircle = Box::new(mycircle) as Box<Circle>;
1643 let nonsense = mycircle.radius() * mycircle.area();
1648 An _implementation_ is an item that implements a [trait](#traits) for a
1651 Implementations are defined with the keyword `impl`.
1654 # #[derive(Copy, Clone)]
1655 # struct Point {x: f64, y: f64};
1656 # type Surface = i32;
1657 # struct BoundingBox {x: f64, y: f64, width: f64, height: f64};
1658 # trait Shape { fn draw(&self, Surface); fn bounding_box(&self) -> BoundingBox; }
1659 # fn do_draw_circle(s: Surface, c: Circle) { }
1665 impl Copy for Circle {}
1667 impl Clone for Circle {
1668 fn clone(&self) -> Circle { *self }
1671 impl Shape for Circle {
1672 fn draw(&self, s: Surface) { do_draw_circle(s, *self); }
1673 fn bounding_box(&self) -> BoundingBox {
1674 let r = self.radius;
1675 BoundingBox{x: self.center.x - r, y: self.center.y - r,
1676 width: 2.0 * r, height: 2.0 * r}
1681 It is possible to define an implementation without referring to a trait. The
1682 methods in such an implementation can only be used as direct calls on the
1683 values of the type that the implementation targets. In such an implementation,
1684 the trait type and `for` after `impl` are omitted. Such implementations are
1685 limited to nominal types (enums, structs), and the implementation must appear
1686 in the same module or a sub-module as the `self` type:
1689 struct Point {x: i32, y: i32}
1693 println!("Point is at ({}, {})", self.x, self.y);
1697 let my_point = Point {x: 10, y:11};
1701 When a trait _is_ specified in an `impl`, all methods declared as part of the
1702 trait must be implemented, with matching types and type parameter counts.
1704 An implementation can take type parameters, which can be different from the
1705 type parameters taken by the trait it implements. Implementation parameters
1706 are written after the `impl` keyword.
1709 # trait Seq<T> { fn dummy(&self, _: T) { } }
1710 impl<T> Seq<T> for Vec<T> {
1713 impl Seq<bool> for u32 {
1714 /* Treat the integer as a sequence of bits */
1721 extern_block_item : "extern" '{' extern_block '}' ;
1722 extern_block : [ foreign_fn ] * ;
1725 External blocks form the basis for Rust's foreign function interface.
1726 Declarations in an external block describe symbols in external, non-Rust
1729 Functions within external blocks are declared in the same way as other Rust
1730 functions, with the exception that they may not have a body and are instead
1731 terminated by a semicolon.
1733 Functions within external blocks may be called by Rust code, just like
1734 functions defined in Rust. The Rust compiler automatically translates between
1735 the Rust ABI and the foreign ABI.
1737 A number of [attributes](#attributes) control the behavior of external blocks.
1739 By default external blocks assume that the library they are calling uses the
1740 standard C "cdecl" ABI. Other ABIs may be specified using an `abi` string, as
1744 // Interface to the Windows API
1745 extern "stdcall" { }
1748 The `link` attribute allows the name of the library to be specified. When
1749 specified the compiler will attempt to link against the native library of the
1753 #[link(name = "crypto")]
1757 The type of a function declared in an extern block is `extern "abi" fn(A1, ...,
1758 An) -> R`, where `A1...An` are the declared types of its arguments and `R` is
1759 the declared return type.
1761 ## Visibility and Privacy
1763 These two terms are often used interchangeably, and what they are attempting to
1764 convey is the answer to the question "Can this item be used at this location?"
1766 Rust's name resolution operates on a global hierarchy of namespaces. Each level
1767 in the hierarchy can be thought of as some item. The items are one of those
1768 mentioned above, but also include external crates. Declaring or defining a new
1769 module can be thought of as inserting a new tree into the hierarchy at the
1770 location of the definition.
1772 To control whether interfaces can be used across modules, Rust checks each use
1773 of an item to see whether it should be allowed or not. This is where privacy
1774 warnings are generated, or otherwise "you used a private item of another module
1775 and weren't allowed to."
1777 By default, everything in Rust is *private*, with one exception. Enum variants
1778 in a `pub` enum are also public by default. You are allowed to alter this
1779 default visibility with the `priv` keyword. When an item is declared as `pub`,
1780 it can be thought of as being accessible to the outside world. For example:
1784 // Declare a private struct
1787 // Declare a public struct with a private field
1792 // Declare a public enum with two public variants
1794 PubliclyAccessibleState,
1795 PubliclyAccessibleState2,
1799 With the notion of an item being either public or private, Rust allows item
1800 accesses in two cases:
1802 1. If an item is public, then it can be used externally through any of its
1804 2. If an item is private, it may be accessed by the current module and its
1807 These two cases are surprisingly powerful for creating module hierarchies
1808 exposing public APIs while hiding internal implementation details. To help
1809 explain, here's a few use cases and what they would entail:
1811 * A library developer needs to expose functionality to crates which link
1812 against their library. As a consequence of the first case, this means that
1813 anything which is usable externally must be `pub` from the root down to the
1814 destination item. Any private item in the chain will disallow external
1817 * A crate needs a global available "helper module" to itself, but it doesn't
1818 want to expose the helper module as a public API. To accomplish this, the
1819 root of the crate's hierarchy would have a private module which then
1820 internally has a "public api". Because the entire crate is a descendant of
1821 the root, then the entire local crate can access this private module through
1824 * When writing unit tests for a module, it's often a common idiom to have an
1825 immediate child of the module to-be-tested named `mod test`. This module
1826 could access any items of the parent module through the second case, meaning
1827 that internal implementation details could also be seamlessly tested from the
1830 In the second case, it mentions that a private item "can be accessed" by the
1831 current module and its descendants, but the exact meaning of accessing an item
1832 depends on what the item is. Accessing a module, for example, would mean
1833 looking inside of it (to import more items). On the other hand, accessing a
1834 function would mean that it is invoked. Additionally, path expressions and
1835 import statements are considered to access an item in the sense that the
1836 import/expression is only valid if the destination is in the current visibility
1839 Here's an example of a program which exemplifies the three cases outlined
1843 // This module is private, meaning that no external crate can access this
1844 // module. Because it is private at the root of this current crate, however, any
1845 // module in the crate may access any publicly visible item in this module.
1846 mod crate_helper_module {
1848 // This function can be used by anything in the current crate
1849 pub fn crate_helper() {}
1851 // This function *cannot* be used by anything else in the crate. It is not
1852 // publicly visible outside of the `crate_helper_module`, so only this
1853 // current module and its descendants may access it.
1854 fn implementation_detail() {}
1857 // This function is "public to the root" meaning that it's available to external
1858 // crates linking against this one.
1859 pub fn public_api() {}
1861 // Similarly to 'public_api', this module is public so external crates may look
1864 use crate_helper_module;
1866 pub fn my_method() {
1867 // Any item in the local crate may invoke the helper module's public
1868 // interface through a combination of the two rules above.
1869 crate_helper_module::crate_helper();
1872 // This function is hidden to any module which is not a descendant of
1874 fn my_implementation() {}
1880 fn test_my_implementation() {
1881 // Because this module is a descendant of `submodule`, it's allowed
1882 // to access private items inside of `submodule` without a privacy
1884 super::my_implementation();
1892 For a rust program to pass the privacy checking pass, all paths must be valid
1893 accesses given the two rules above. This includes all use statements,
1894 expressions, types, etc.
1896 ### Re-exporting and Visibility
1898 Rust allows publicly re-exporting items through a `pub use` directive. Because
1899 this is a public directive, this allows the item to be used in the current
1900 module through the rules above. It essentially allows public access into the
1901 re-exported item. For example, this program is valid:
1904 pub use self::implementation::api;
1906 mod implementation {
1915 This means that any external crate referencing `implementation::api::f` would
1916 receive a privacy violation, while the path `api::f` would be allowed.
1918 When re-exporting a private item, it can be thought of as allowing the "privacy
1919 chain" being short-circuited through the reexport instead of passing through
1920 the namespace hierarchy as it normally would.
1925 attribute : '#' '!' ? '[' meta_item ']' ;
1926 meta_item : ident [ '=' literal
1927 | '(' meta_seq ')' ] ? ;
1928 meta_seq : meta_item [ ',' meta_seq ] ? ;
1931 Any item declaration may have an _attribute_ applied to it. Attributes in Rust
1932 are modeled on Attributes in ECMA-335, with the syntax coming from ECMA-334
1933 (C#). An attribute is a general, free-form metadatum that is interpreted
1934 according to name, convention, and language and compiler version. Attributes
1935 may appear as any of:
1937 * A single identifier, the attribute name
1938 * An identifier followed by the equals sign '=' and a literal, providing a
1940 * An identifier followed by a parenthesized list of sub-attribute arguments
1942 Attributes with a bang ("!") after the hash ("#") apply to the item that the
1943 attribute is declared within. Attributes that do not have a bang after the hash
1944 apply to the item that follows the attribute.
1946 An example of attributes:
1949 // General metadata applied to the enclosing module or crate.
1950 #![crate_type = "lib"]
1952 // A function marked as a unit test
1958 // A conditionally-compiled module
1959 #[cfg(target_os="linux")]
1964 // A lint attribute used to suppress a warning/error
1965 #[allow(non_camel_case_types)]
1969 > **Note:** At some point in the future, the compiler will distinguish between
1970 > language-reserved and user-available attributes. Until then, there is
1971 > effectively no difference between an attribute handled by a loadable syntax
1972 > extension and the compiler.
1974 ### Crate-only attributes
1976 - `crate_name` - specify the this crate's crate name.
1977 - `crate_type` - see [linkage](#linkage).
1978 - `feature` - see [compiler features](#compiler-features).
1979 - `no_builtins` - disable optimizing certain code patterns to invocations of
1980 library functions that are assumed to exist
1981 - `no_main` - disable emitting the `main` symbol. Useful when some other
1982 object being linked to defines `main`.
1983 - `no_start` - disable linking to the `native` crate, which specifies the
1984 "start" language item.
1985 - `no_std` - disable linking to the `std` crate.
1986 - `plugin` — load a list of named crates as compiler plugins, e.g.
1987 `#![plugin(foo, bar)]`. Optional arguments for each plugin,
1988 i.e. `#![plugin(foo(... args ...))]`, are provided to the plugin's
1989 registrar function. The `plugin` feature gate is required to use
1992 ### Module-only attributes
1994 - `no_implicit_prelude` - disable injecting `use std::prelude::*` in this
1996 - `path` - specifies the file to load the module from. `#[path="foo.rs"] mod
1997 bar;` is equivalent to `mod bar { /* contents of foo.rs */ }`. The path is
1998 taken relative to the directory that the current module is in.
2000 ### Function-only attributes
2002 - `main` - indicates that this function should be passed to the entry point,
2003 rather than the function in the crate root named `main`.
2004 - `plugin_registrar` - mark this function as the registration point for
2005 [compiler plugins][plugin], such as loadable syntax extensions.
2006 - `start` - indicates that this function should be used as the entry point,
2007 overriding the "start" language item. See the "start" [language
2008 item](#language-items) for more details.
2009 - `test` - indicates that this function is a test function, to only be compiled
2010 in case of `--test`.
2011 - `should_panic` - indicates that this test function should panic, inverting the success condition.
2012 - `cold` - The function is unlikely to be executed, so optimize it (and calls
2015 ### Static-only attributes
2017 - `thread_local` - on a `static mut`, this signals that the value of this
2018 static may change depending on the current thread. The exact consequences of
2019 this are implementation-defined.
2023 On an `extern` block, the following attributes are interpreted:
2025 - `link_args` - specify arguments to the linker, rather than just the library
2026 name and type. This is feature gated and the exact behavior is
2027 implementation-defined (due to variety of linker invocation syntax).
2028 - `link` - indicate that a native library should be linked to for the
2029 declarations in this block to be linked correctly. `link` supports an optional `kind`
2030 key with three possible values: `dylib`, `static`, and `framework`. See [external blocks](#external-blocks) for more about external blocks. Two
2031 examples: `#[link(name = "readline")]` and
2032 `#[link(name = "CoreFoundation", kind = "framework")]`.
2034 On declarations inside an `extern` block, the following attributes are
2037 - `link_name` - the name of the symbol that this function or static should be
2039 - `linkage` - on a static, this specifies the [linkage
2040 type](http://llvm.org/docs/LangRef.html#linkage-types).
2044 - `repr` - on C-like enums, this sets the underlying type used for
2045 representation. Takes one argument, which is the primitive
2046 type this enum should be represented for, or `C`, which specifies that it
2047 should be the default `enum` size of the C ABI for that platform. Note that
2048 enum representation in C is undefined, and this may be incorrect when the C
2049 code is compiled with certain flags.
2053 - `repr` - specifies the representation to use for this struct. Takes a list
2054 of options. The currently accepted ones are `C` and `packed`, which may be
2055 combined. `C` will use a C ABI compatible struct layout, and `packed` will
2056 remove any padding between fields (note that this is very fragile and may
2057 break platforms which require aligned access).
2059 ### Macro-related attributes
2061 - `macro_use` on a `mod` — macros defined in this module will be visible in the
2062 module's parent, after this module has been included.
2064 - `macro_use` on an `extern crate` — load macros from this crate. An optional
2065 list of names `#[macro_use(foo, bar)]` restricts the import to just those
2066 macros named. The `extern crate` must appear at the crate root, not inside
2067 `mod`, which ensures proper function of the [`$crate` macro
2068 variable](book/macros.html#the-variable-$crate).
2070 - `macro_reexport` on an `extern crate` — re-export the named macros.
2072 - `macro_export` - export a macro for cross-crate usage.
2074 - `no_link` on an `extern crate` — even if we load this crate for macros, don't
2075 link it into the output.
2077 See the [macros section of the
2078 book](book/macros.html#scoping-and-macro-import/export) for more information on
2082 ### Miscellaneous attributes
2084 - `export_name` - on statics and functions, this determines the name of the
2086 - `link_section` - on statics and functions, this specifies the section of the
2087 object file that this item's contents will be placed into.
2088 - `no_mangle` - on any item, do not apply the standard name mangling. Set the
2089 symbol for this item to its identifier.
2090 - `packed` - on structs or enums, eliminate any padding that would be used to
2092 - `simd` - on certain tuple structs, derive the arithmetic operators, which
2093 lower to the target's SIMD instructions, if any; the `simd` feature gate
2094 is necessary to use this attribute.
2095 - `static_assert` - on statics whose type is `bool`, terminates compilation
2096 with an error if it is not initialized to `true`.
2097 - `unsafe_destructor` - allow implementations of the "drop" language item
2098 where the type it is implemented for does not implement the "send" language
2099 item; the `unsafe_destructor` feature gate is needed to use this attribute
2100 - `unsafe_no_drop_flag` - on structs, remove the flag that prevents
2101 destructors from being run twice. Destructors might be run multiple times on
2102 the same object with this attribute.
2103 - `doc` - Doc comments such as `/// foo` are equivalent to `#[doc = "foo"]`.
2104 - `rustc_on_unimplemented` - Write a custom note to be shown along with the error
2105 when the trait is found to be unimplemented on a type.
2106 You may use format arguments like `{T}`, `{A}` to correspond to the
2107 types at the point of use corresponding to the type parameters of the
2108 trait of the same name. `{Self}` will be replaced with the type that is supposed
2109 to implement the trait but doesn't. To use this, the `on_unimplemented` feature gate
2112 ### Conditional compilation
2114 Sometimes one wants to have different compiler outputs from the same code,
2115 depending on build target, such as targeted operating system, or to enable
2118 There are two kinds of configuration options, one that is either defined or not
2119 (`#[cfg(foo)]`), and the other that contains a string that can be checked
2120 against (`#[cfg(bar = "baz")]` (currently only compiler-defined configuration
2121 options can have the latter form).
2124 // The function is only included in the build when compiling for OSX
2125 #[cfg(target_os = "macos")]
2130 // This function is only included when either foo or bar is defined
2131 #[cfg(any(foo, bar))]
2132 fn needs_foo_or_bar() {
2136 // This function is only included when compiling for a unixish OS with a 32-bit
2138 #[cfg(all(unix, target_pointer_width = "32"))]
2139 fn on_32bit_unix() {
2143 // This function is only included when foo is not defined
2145 fn needs_not_foo() {
2150 This illustrates some conditional compilation can be achieved using the
2151 `#[cfg(...)]` attribute. `any`, `all` and `not` can be used to assemble
2152 arbitrarily complex configurations through nesting.
2154 The following configurations must be defined by the implementation:
2156 * `target_arch = "..."`. Target CPU architecture, such as `"x86"`, `"x86_64"`
2157 `"mips"`, `"powerpc"`, `"arm"`, or `"aarch64"`.
2158 * `target_endian = "..."`. Endianness of the target CPU, either `"little"` or
2160 * `target_family = "..."`. Operating system family of the target, e. g.
2161 `"unix"` or `"windows"`. The value of this configuration option is defined
2162 as a configuration itself, like `unix` or `windows`.
2163 * `target_os = "..."`. Operating system of the target, examples include
2164 `"win32"`, `"macos"`, `"linux"`, `"android"`, `"freebsd"`, `"dragonfly"`,
2165 `"bitrig"` or `"openbsd"`.
2166 * `target_pointer_width = "..."`. Target pointer width in bits. This is set
2167 to `"32"` for targets with 32-bit pointers, and likewise set to `"64"` for
2169 * `unix`. See `target_family`.
2170 * `windows`. See `target_family`.
2172 You can also set another attribute based on a `cfg` variable with `cfg_attr`:
2178 Will be the same as `#[b]` if `a` is set by `cfg`, and nothing otherwise.
2180 ### Lint check attributes
2182 A lint check names a potentially undesirable coding pattern, such as
2183 unreachable code or omitted documentation, for the static entity to which the
2186 For any lint check `C`:
2188 * `allow(C)` overrides the check for `C` so that violations will go
2190 * `deny(C)` signals an error after encountering a violation of `C`,
2191 * `forbid(C)` is the same as `deny(C)`, but also forbids changing the lint
2193 * `warn(C)` warns about violations of `C` but continues compilation.
2195 The lint checks supported by the compiler can be found via `rustc -W help`,
2196 along with their default settings. [Compiler
2197 plugins](book/plugins.html#lint-plugins) can provide additional lint checks.
2201 // Missing documentation is ignored here
2202 #[allow(missing_docs)]
2203 pub fn undocumented_one() -> i32 { 1 }
2205 // Missing documentation signals a warning here
2206 #[warn(missing_docs)]
2207 pub fn undocumented_too() -> i32 { 2 }
2209 // Missing documentation signals an error here
2210 #[deny(missing_docs)]
2211 pub fn undocumented_end() -> i32 { 3 }
2215 This example shows how one can use `allow` and `warn` to toggle a particular
2219 #[warn(missing_docs)]
2221 #[allow(missing_docs)]
2223 // Missing documentation is ignored here
2224 pub fn undocumented_one() -> i32 { 1 }
2226 // Missing documentation signals a warning here,
2227 // despite the allow above.
2228 #[warn(missing_docs)]
2229 pub fn undocumented_two() -> i32 { 2 }
2232 // Missing documentation signals a warning here
2233 pub fn undocumented_too() -> i32 { 3 }
2237 This example shows how one can use `forbid` to disallow uses of `allow` for
2241 #[forbid(missing_docs)]
2243 // Attempting to toggle warning signals an error here
2244 #[allow(missing_docs)]
2246 pub fn undocumented_too() -> i32 { 2 }
2252 Some primitive Rust operations are defined in Rust code, rather than being
2253 implemented directly in C or assembly language. The definitions of these
2254 operations have to be easy for the compiler to find. The `lang` attribute
2255 makes it possible to declare these operations. For example, the `str` module
2256 in the Rust standard library defines the string equality function:
2260 pub fn eq_slice(a: &str, b: &str) -> bool {
2265 The name `str_eq` has a special meaning to the Rust compiler, and the presence
2266 of this definition means that it will use this definition when generating calls
2267 to the string equality function.
2269 A complete list of the built-in language items will be added in the future.
2271 ### Inline attributes
2273 The inline attribute is used to suggest to the compiler to perform an inline
2274 expansion and place a copy of the function or static in the caller rather than
2275 generating code to call the function or access the static where it is defined.
2277 The compiler automatically inlines functions based on internal heuristics.
2278 Incorrectly inlining functions can actually making the program slower, so it
2279 should be used with care.
2281 Immutable statics are always considered inlineable unless marked with
2282 `#[inline(never)]`. It is undefined whether two different inlineable statics
2283 have the same memory address. In other words, the compiler is free to collapse
2284 duplicate inlineable statics together.
2286 `#[inline]` and `#[inline(always)]` always causes the function to be serialized
2287 into crate metadata to allow cross-crate inlining.
2289 There are three different types of inline attributes:
2291 * `#[inline]` hints the compiler to perform an inline expansion.
2292 * `#[inline(always)]` asks the compiler to always perform an inline expansion.
2293 * `#[inline(never)]` asks the compiler to never perform an inline expansion.
2297 The `derive` attribute allows certain traits to be automatically implemented
2298 for data structures. For example, the following will create an `impl` for the
2299 `PartialEq` and `Clone` traits for `Foo`, the type parameter `T` will be given
2300 the `PartialEq` or `Clone` constraints for the appropriate `impl`:
2303 #[derive(PartialEq, Clone)]
2310 The generated `impl` for `PartialEq` is equivalent to
2313 # struct Foo<T> { a: i32, b: T }
2314 impl<T: PartialEq> PartialEq for Foo<T> {
2315 fn eq(&self, other: &Foo<T>) -> bool {
2316 self.a == other.a && self.b == other.b
2319 fn ne(&self, other: &Foo<T>) -> bool {
2320 self.a != other.a || self.b != other.b
2325 ### Compiler Features
2327 Certain aspects of Rust may be implemented in the compiler, but they're not
2328 necessarily ready for every-day use. These features are often of "prototype
2329 quality" or "almost production ready", but may not be stable enough to be
2330 considered a full-fledged language feature.
2332 For this reason, Rust recognizes a special crate-level attribute of the form:
2335 #![feature(feature1, feature2, feature3)]
2338 This directive informs the compiler that the feature list: `feature1`,
2339 `feature2`, and `feature3` should all be enabled. This is only recognized at a
2340 crate-level, not at a module-level. Without this directive, all features are
2341 considered off, and using the features will result in a compiler error.
2343 The currently implemented features of the reference compiler are:
2345 * `advanced_slice_patterns` - See the [match expressions](#match-expressions)
2346 section for discussion; the exact semantics of
2347 slice patterns are subject to change, so some types
2350 * `slice_patterns` - OK, actually, slice patterns are just scary and
2351 completely unstable.
2353 * `asm` - The `asm!` macro provides a means for inline assembly. This is often
2354 useful, but the exact syntax for this feature along with its
2355 semantics are likely to change, so this macro usage must be opted
2358 * `associated_types` - Allows type aliases in traits. Experimental.
2360 * `box_patterns` - Allows `box` patterns, the exact semantics of which
2361 is subject to change.
2363 * `box_syntax` - Allows use of `box` expressions, the exact semantics of which
2364 is subject to change.
2366 * `concat_idents` - Allows use of the `concat_idents` macro, which is in many
2367 ways insufficient for concatenating identifiers, and may be
2368 removed entirely for something more wholesome.
2370 * `custom_attribute` - Allows the usage of attributes unknown to the compiler
2371 so that new attributes can be added in a backwards compatible
2374 * `custom_derive` - Allows the use of `#[derive(Foo,Bar)]` as sugar for
2375 `#[derive_Foo] #[derive_Bar]`, which can be user-defined syntax
2378 * `intrinsics` - Allows use of the "rust-intrinsics" ABI. Compiler intrinsics
2379 are inherently unstable and no promise about them is made.
2381 * `lang_items` - Allows use of the `#[lang]` attribute. Like `intrinsics`,
2382 lang items are inherently unstable and no promise about them
2385 * `link_args` - This attribute is used to specify custom flags to the linker,
2386 but usage is strongly discouraged. The compiler's usage of the
2387 system linker is not guaranteed to continue in the future, and
2388 if the system linker is not used then specifying custom flags
2389 doesn't have much meaning.
2391 * `link_llvm_intrinsics` – Allows linking to LLVM intrinsics via
2392 `#[link_name="llvm.*"]`.
2394 * `linkage` - Allows use of the `linkage` attribute, which is not portable.
2396 * `log_syntax` - Allows use of the `log_syntax` macro attribute, which is a
2397 nasty hack that will certainly be removed.
2399 * `main` - Allows use of the `#[main]` attribute, which changes the entry point
2400 into a Rust program. This capability is subject to change.
2402 * `macro_reexport` - Allows macros to be re-exported from one crate after being imported
2403 from another. This feature was originally designed with the sole
2404 use case of the Rust standard library in mind, and is subject to
2407 * `non_ascii_idents` - The compiler supports the use of non-ascii identifiers,
2408 but the implementation is a little rough around the
2409 edges, so this can be seen as an experimental feature
2410 for now until the specification of identifiers is fully
2413 * `no_std` - Allows the `#![no_std]` crate attribute, which disables the implicit
2414 `extern crate std`. This typically requires use of the unstable APIs
2415 behind the libstd "facade", such as libcore and libcollections. It
2416 may also cause problems when using syntax extensions, including
2419 * `on_unimplemented` - Allows the `#[rustc_on_unimplemented]` attribute, which allows
2420 trait definitions to add specialized notes to error messages
2421 when an implementation was expected but not found.
2423 * `optin_builtin_traits` - Allows the definition of default and negative trait
2424 implementations. Experimental.
2426 * `plugin` - Usage of [compiler plugins][plugin] for custom lints or syntax extensions.
2427 These depend on compiler internals and are subject to change.
2429 * `plugin_registrar` - Indicates that a crate provides [compiler plugins][plugin].
2431 * `quote` - Allows use of the `quote_*!` family of macros, which are
2432 implemented very poorly and will likely change significantly
2433 with a proper implementation.
2435 * `rustc_attrs` - Gates internal `#[rustc_*]` attributes which may be
2436 for internal use only or have meaning added to them in the future.
2438 * `rustc_diagnostic_macros`- A mysterious feature, used in the implementation
2439 of rustc, not meant for mortals.
2441 * `simd` - Allows use of the `#[simd]` attribute, which is overly simple and
2442 not the SIMD interface we want to expose in the long term.
2444 * `simd_ffi` - Allows use of SIMD vectors in signatures for foreign functions.
2445 The SIMD interface is subject to change.
2447 * `staged_api` - Allows usage of stability markers and `#![staged_api]` in a
2448 crate. Stability markers are also attributes: `#[stable]`,
2449 `#[unstable]`, and `#[deprecated]` are the three levels.
2451 * `static_assert` - The `#[static_assert]` functionality is experimental and
2452 unstable. The attribute can be attached to a `static` of
2453 type `bool` and the compiler will error if the `bool` is
2454 `false` at compile time. This version of this functionality
2455 is unintuitive and suboptimal.
2457 * `start` - Allows use of the `#[start]` attribute, which changes the entry point
2458 into a Rust program. This capability, especially the signature for the
2459 annotated function, is subject to change.
2461 * `struct_inherit` - Allows using struct inheritance, which is barely
2462 implemented and will probably be removed. Don't use this.
2464 * `struct_variant` - Structural enum variants (those with named fields). It is
2465 currently unknown whether this style of enum variant is as
2466 fully supported as the tuple-forms, and it's not certain
2467 that this style of variant should remain in the language.
2468 For now this style of variant is hidden behind a feature
2471 * `thread_local` - The usage of the `#[thread_local]` attribute is experimental
2472 and should be seen as unstable. This attribute is used to
2473 declare a `static` as being unique per-thread leveraging
2474 LLVM's implementation which works in concert with the kernel
2475 loader and dynamic linker. This is not necessarily available
2476 on all platforms, and usage of it is discouraged.
2478 * `trace_macros` - Allows use of the `trace_macros` macro, which is a nasty
2479 hack that will certainly be removed.
2481 * `unboxed_closures` - Rust's new closure design, which is currently a work in
2482 progress feature with many known bugs.
2484 * `unsafe_destructor` - Allows use of the `#[unsafe_destructor]` attribute,
2485 which is considered wildly unsafe and will be
2486 obsoleted by language improvements.
2488 * `unsafe_no_drop_flag` - Allows use of the `#[unsafe_no_drop_flag]` attribute,
2489 which removes hidden flag added to a type that
2490 implements the `Drop` trait. The design for the
2491 `Drop` flag is subject to change, and this feature
2492 may be removed in the future.
2494 * `unmarked_api` - Allows use of items within a `#![staged_api]` crate
2495 which have not been marked with a stability marker.
2496 Such items should not be allowed by the compiler to exist,
2497 so if you need this there probably is a compiler bug.
2499 * `visible_private_types` - Allows public APIs to expose otherwise private
2500 types, e.g. as the return type of a public function.
2501 This capability may be removed in the future.
2503 * `allow_internal_unstable` - Allows `macro_rules!` macros to be tagged with the
2504 `#[allow_internal_unstable]` attribute, designed
2505 to allow `std` macros to call
2506 `#[unstable]`/feature-gated functionality
2507 internally without imposing on callers
2508 (i.e. making them behave like function calls in
2509 terms of encapsulation).
2511 If a feature is promoted to a language feature, then all existing programs will
2512 start to receive compilation warnings about #[feature] directives which enabled
2513 the new feature (because the directive is no longer necessary). However, if a
2514 feature is decided to be removed from the language, errors will be issued (if
2515 there isn't a parser error first). The directive in this case is no longer
2516 necessary, and it's likely that existing code will break if the feature isn't
2519 If an unknown feature is found in a directive, it results in a compiler error.
2520 An unknown feature is one which has never been recognized by the compiler.
2522 # Statements and expressions
2524 Rust is _primarily_ an expression language. This means that most forms of
2525 value-producing or effect-causing evaluation are directed by the uniform syntax
2526 category of _expressions_. Each kind of expression can typically _nest_ within
2527 each other kind of expression, and rules for evaluation of expressions involve
2528 specifying both the value produced by the expression and the order in which its
2529 sub-expressions are themselves evaluated.
2531 In contrast, statements in Rust serve _mostly_ to contain and explicitly
2532 sequence expression evaluation.
2536 A _statement_ is a component of a block, which is in turn a component of an
2537 outer [expression](#expressions) or [function](#functions).
2539 Rust has two kinds of statement: [declaration
2540 statements](#declaration-statements) and [expression
2541 statements](#expression-statements).
2543 ### Declaration statements
2545 A _declaration statement_ is one that introduces one or more *names* into the
2546 enclosing statement block. The declared names may denote new variables or new
2549 #### Item declarations
2551 An _item declaration statement_ has a syntactic form identical to an
2552 [item](#items) declaration within a module. Declaring an item — a
2553 function, enumeration, structure, type, static, trait, implementation or module
2554 — locally within a statement block is simply a way of restricting its
2555 scope to a narrow region containing all of its uses; it is otherwise identical
2556 in meaning to declaring the item outside the statement block.
2558 > **Note**: there is no implicit capture of the function's dynamic environment when
2559 > declaring a function-local item.
2561 #### Variable declarations
2564 let_decl : "let" pat [':' type ] ? [ init ] ? ';' ;
2565 init : [ '=' ] expr ;
2568 A _variable declaration_ introduces a new set of variable, given by a pattern. The
2569 pattern may be followed by a type annotation, and/or an initializer expression.
2570 When no type annotation is given, the compiler will infer the type, or signal
2571 an error if insufficient type information is available for definite inference.
2572 Any variables introduced by a variable declaration are visible from the point of
2573 declaration until the end of the enclosing block scope.
2575 ### Expression statements
2577 An _expression statement_ is one that evaluates an [expression](#expressions)
2578 and ignores its result. The type of an expression statement `e;` is always
2579 `()`, regardless of the type of `e`. As a rule, an expression statement's
2580 purpose is to trigger the effects of evaluating its expression.
2584 An expression may have two roles: it always produces a *value*, and it may have
2585 *effects* (otherwise known as "side effects"). An expression *evaluates to* a
2586 value, and has effects during *evaluation*. Many expressions contain
2587 sub-expressions (operands). The meaning of each kind of expression dictates
2590 * Whether or not to evaluate the sub-expressions when evaluating the expression
2591 * The order in which to evaluate the sub-expressions
2592 * How to combine the sub-expressions' values to obtain the value of the expression
2594 In this way, the structure of expressions dictates the structure of execution.
2595 Blocks are just another kind of expression, so blocks, statements, expressions,
2596 and blocks again can recursively nest inside each other to an arbitrary depth.
2598 #### Lvalues, rvalues and temporaries
2600 Expressions are divided into two main categories: _lvalues_ and _rvalues_.
2601 Likewise within each expression, sub-expressions may occur in _lvalue context_
2602 or _rvalue context_. The evaluation of an expression depends both on its own
2603 category and the context it occurs within.
2605 An lvalue is an expression that represents a memory location. These expressions
2606 are [paths](#path-expressions) (which refer to local variables, function and
2607 method arguments, or static variables), dereferences (`*expr`), [indexing
2608 expressions](#index-expressions) (`expr[expr]`), and [field
2609 references](#field-expressions) (`expr.f`). All other expressions are rvalues.
2611 The left operand of an [assignment](#assignment-expressions) or
2612 [compound-assignment](#compound-assignment-expressions) expression is an lvalue
2613 context, as is the single operand of a unary
2614 [borrow](#unary-operator-expressions). All other expression contexts are
2617 When an lvalue is evaluated in an _lvalue context_, it denotes a memory
2618 location; when evaluated in an _rvalue context_, it denotes the value held _in_
2619 that memory location.
2621 When an rvalue is used in an lvalue context, a temporary un-named lvalue is
2622 created and used instead. A temporary's lifetime equals the largest lifetime
2623 of any reference that points to it.
2625 #### Moved and copied types
2627 When a [local variable](#variables) is used as an
2628 [rvalue](#lvalues,-rvalues-and-temporaries) the variable will either be moved
2629 or copied, depending on its type. All values whose type implements `Copy` are
2630 copied, all others are moved.
2632 ### Literal expressions
2634 A _literal expression_ consists of one of the [literal](#literals) forms
2635 described earlier. It directly describes a number, character, string, boolean
2636 value, or the unit value.
2640 "hello"; // string type
2641 '5'; // character type
2645 ### Path expressions
2647 A [path](#paths) used as an expression context denotes either a local variable
2648 or an item. Path expressions are [lvalues](#lvalues,-rvalues-and-temporaries).
2650 ### Tuple expressions
2652 Tuples are written by enclosing zero or more comma-separated expressions in
2653 parentheses. They are used to create [tuple-typed](#tuple-types) values.
2661 ### Unit expressions
2663 The expression `()` denotes the _unit value_, the only value of the type with
2666 ### Structure expressions
2669 struct_expr : expr_path '{' ident ':' expr
2670 [ ',' ident ':' expr ] *
2673 [ ',' expr ] * ')' |
2677 There are several forms of structure expressions. A _structure expression_
2678 consists of the [path](#paths) of a [structure item](#structures), followed by
2679 a brace-enclosed list of one or more comma-separated name-value pairs,
2680 providing the field values of a new instance of the structure. A field name
2681 can be any identifier, and is separated from its value expression by a colon.
2682 The location denoted by a structure field is mutable if and only if the
2683 enclosing structure is mutable.
2685 A _tuple structure expression_ consists of the [path](#paths) of a [structure
2686 item](#structures), followed by a parenthesized list of one or more
2687 comma-separated expressions (in other words, the path of a structure item
2688 followed by a tuple expression). The structure item must be a tuple structure
2691 A _unit-like structure expression_ consists only of the [path](#paths) of a
2692 [structure item](#structures).
2694 The following are examples of structure expressions:
2697 # struct Point { x: f64, y: f64 }
2698 # struct TuplePoint(f64, f64);
2699 # mod game { pub struct User<'a> { pub name: &'a str, pub age: u32, pub score: usize } }
2700 # struct Cookie; fn some_fn<T>(t: T) {}
2701 Point {x: 10.0, y: 20.0};
2702 TuplePoint(10.0, 20.0);
2703 let u = game::User {name: "Joe", age: 35, score: 100_000};
2704 some_fn::<Cookie>(Cookie);
2707 A structure expression forms a new value of the named structure type. Note
2708 that for a given *unit-like* structure type, this will always be the same
2711 A structure expression can terminate with the syntax `..` followed by an
2712 expression to denote a functional update. The expression following `..` (the
2713 base) must have the same structure type as the new structure type being formed.
2714 The entire expression denotes the result of constructing a new structure (with
2715 the same type as the base expression) with the given values for the fields that
2716 were explicitly specified and the values in the base expression for all other
2720 # struct Point3d { x: i32, y: i32, z: i32 }
2721 let base = Point3d {x: 1, y: 2, z: 3};
2722 Point3d {y: 0, z: 10, .. base};
2725 ### Block expressions
2728 block_expr : '{' [ stmt ';' | item ] *
2732 A _block expression_ is similar to a module in terms of the declarations that
2733 are possible. Each block conceptually introduces a new namespace scope. Use
2734 items can bring new names into scopes and declared items are in scope for only
2737 A block will execute each statement sequentially, and then execute the
2738 expression (if given). If the block ends in a statement, its value is `()`:
2741 let x: () = { println!("Hello."); };
2744 If it ends in an expression, its value and type are that of the expression:
2747 let x: i32 = { println!("Hello."); 5 };
2752 ### Method-call expressions
2755 method_call_expr : expr '.' ident paren_expr_list ;
2758 A _method call_ consists of an expression followed by a single dot, an
2759 identifier, and a parenthesized expression-list. Method calls are resolved to
2760 methods on specific traits, either statically dispatching to a method if the
2761 exact `self`-type of the left-hand-side is known, or dynamically dispatching if
2762 the left-hand-side expression is an indirect [object type](#object-types).
2764 ### Field expressions
2767 field_expr : expr '.' ident ;
2770 A _field expression_ consists of an expression followed by a single dot and an
2771 identifier, when not immediately followed by a parenthesized expression-list
2772 (the latter is a [method call expression](#method-call-expressions)). A field
2773 expression denotes a field of a [structure](#structure-types).
2778 (Struct {a: 10, b: 20}).a;
2781 A field access is an [lvalue](#lvalues,-rvalues-and-temporaries) referring to
2782 the value of that field. When the type providing the field inherits mutability,
2783 it can be [assigned](#assignment-expressions) to.
2785 Also, if the type of the expression to the left of the dot is a pointer, it is
2786 automatically dereferenced to make the field access possible.
2788 ### Array expressions
2791 array_expr : '[' "mut" ? array_elems? ']' ;
2793 array_elems : [expr [',' expr]*] | [expr ';' expr] ;
2796 An [array](#array,-and-slice-types) _expression_ is written by enclosing zero
2797 or more comma-separated expressions of uniform type in square brackets.
2799 In the `[expr ';' expr]` form, the expression after the `';'` must be a
2800 constant expression that can be evaluated at compile time, such as a
2801 [literal](#literals) or a [static item](#static-items).
2805 ["a", "b", "c", "d"];
2806 [0; 128]; // array with 128 zeros
2807 [0u8, 0u8, 0u8, 0u8];
2810 ### Index expressions
2813 idx_expr : expr '[' expr ']' ;
2816 [Array](#array,-and-slice-types)-typed expressions can be indexed by
2817 writing a square-bracket-enclosed expression (the index) after them. When the
2818 array is mutable, the resulting [lvalue](#lvalues,-rvalues-and-temporaries) can
2821 Indices are zero-based, and may be of any integral type. Vector access is
2822 bounds-checked at run-time. When the check fails, it will put the thread in a
2827 (["a", "b"])[10]; // panics
2830 ### Unary operator expressions
2832 Rust defines three unary operators. They are all written as prefix operators,
2833 before the expression they apply to.
2836 : Negation. May only be applied to numeric types.
2838 : Dereference. When applied to a [pointer](#pointer-types) it denotes the
2839 pointed-to location. For pointers to mutable locations, the resulting
2840 [lvalue](#lvalues,-rvalues-and-temporaries) can be assigned to.
2841 On non-pointer types, it calls the `deref` method of the `std::ops::Deref`
2842 trait, or the `deref_mut` method of the `std::ops::DerefMut` trait (if
2843 implemented by the type and required for an outer expression that will or
2844 could mutate the dereference), and produces the result of dereferencing the
2845 `&` or `&mut` borrowed pointer returned from the overload method.
2848 : Logical negation. On the boolean type, this flips between `true` and
2849 `false`. On integer types, this inverts the individual bits in the
2850 two's complement representation of the value.
2852 ### Binary operator expressions
2855 binop_expr : expr binop expr ;
2858 Binary operators expressions are given in terms of [operator
2859 precedence](#operator-precedence).
2861 #### Arithmetic operators
2863 Binary arithmetic expressions are syntactic sugar for calls to built-in traits,
2864 defined in the `std::ops` module of the `std` library. This means that
2865 arithmetic operators can be overridden for user-defined types. The default
2866 meaning of the operators on standard types is given here.
2869 : Addition and array/string concatenation.
2870 Calls the `add` method on the `std::ops::Add` trait.
2873 Calls the `sub` method on the `std::ops::Sub` trait.
2876 Calls the `mul` method on the `std::ops::Mul` trait.
2879 Calls the `div` method on the `std::ops::Div` trait.
2882 Calls the `rem` method on the `std::ops::Rem` trait.
2884 #### Bitwise operators
2886 Like the [arithmetic operators](#arithmetic-operators), bitwise operators are
2887 syntactic sugar for calls to methods of built-in traits. This means that
2888 bitwise operators can be overridden for user-defined types. The default
2889 meaning of the operators on standard types is given here.
2893 Calls the `bitand` method of the `std::ops::BitAnd` trait.
2896 Calls the `bitor` method of the `std::ops::BitOr` trait.
2899 Calls the `bitxor` method of the `std::ops::BitXor` trait.
2902 Calls the `shl` method of the `std::ops::Shl` trait.
2905 Calls the `shr` method of the `std::ops::Shr` trait.
2907 #### Lazy boolean operators
2909 The operators `||` and `&&` may be applied to operands of boolean type. The
2910 `||` operator denotes logical 'or', and the `&&` operator denotes logical
2911 'and'. They differ from `|` and `&` in that the right-hand operand is only
2912 evaluated when the left-hand operand does not already determine the result of
2913 the expression. That is, `||` only evaluates its right-hand operand when the
2914 left-hand operand evaluates to `false`, and `&&` only when it evaluates to
2917 #### Comparison operators
2919 Comparison operators are, like the [arithmetic
2920 operators](#arithmetic-operators), and [bitwise operators](#bitwise-operators),
2921 syntactic sugar for calls to built-in traits. This means that comparison
2922 operators can be overridden for user-defined types. The default meaning of the
2923 operators on standard types is given here.
2927 Calls the `eq` method on the `std::cmp::PartialEq` trait.
2930 Calls the `ne` method on the `std::cmp::PartialEq` trait.
2933 Calls the `lt` method on the `std::cmp::PartialOrd` trait.
2936 Calls the `gt` method on the `std::cmp::PartialOrd` trait.
2938 : Less than or equal.
2939 Calls the `le` method on the `std::cmp::PartialOrd` trait.
2941 : Greater than or equal.
2942 Calls the `ge` method on the `std::cmp::PartialOrd` trait.
2944 #### Type cast expressions
2946 A type cast expression is denoted with the binary operator `as`.
2948 Executing an `as` expression casts the value on the left-hand side to the type
2949 on the right-hand side.
2951 An example of an `as` expression:
2954 # fn sum(v: &[f64]) -> f64 { 0.0 }
2955 # fn len(v: &[f64]) -> i32 { 0 }
2957 fn avg(v: &[f64]) -> f64 {
2958 let sum: f64 = sum(v);
2959 let sz: f64 = len(v) as f64;
2964 #### Assignment expressions
2966 An _assignment expression_ consists of an
2967 [lvalue](#lvalues,-rvalues-and-temporaries) expression followed by an equals
2968 sign (`=`) and an [rvalue](#lvalues,-rvalues-and-temporaries) expression.
2970 Evaluating an assignment expression [either copies or
2971 moves](#moved-and-copied-types) its right-hand operand to its left-hand
2981 #### Compound assignment expressions
2983 The `+`, `-`, `*`, `/`, `%`, `&`, `|`, `^`, `<<`, and `>>` operators may be
2984 composed with the `=` operator. The expression `lval OP= val` is equivalent to
2985 `lval = lval OP val`. For example, `x = x + 1` may be written as `x += 1`.
2987 Any such expression always has the [`unit`](#primitive-types) type.
2989 #### Operator precedence
2991 The precedence of Rust binary operators is ordered as follows, going from
2994 ```{.text .precedence}
3008 Operators at the same precedence level are evaluated left-to-right. [Unary
3009 operators](#unary-operator-expressions) have the same precedence level and are
3010 stronger than any of the binary operators.
3012 ### Grouped expressions
3014 An expression enclosed in parentheses evaluates to the result of the enclosed
3015 expression. Parentheses can be used to explicitly specify evaluation order
3016 within an expression.
3019 paren_expr : '(' expr ')' ;
3022 An example of a parenthesized expression:
3025 let x: i32 = (2 + 3) * 4;
3029 ### Call expressions
3032 expr_list : [ expr [ ',' expr ]* ] ? ;
3033 paren_expr_list : '(' expr_list ')' ;
3034 call_expr : expr paren_expr_list ;
3037 A _call expression_ invokes a function, providing zero or more input variables
3038 and an optional location to move the function's output into. If the function
3039 eventually returns, then the expression completes.
3041 Some examples of call expressions:
3044 # fn add(x: i32, y: i32) -> i32 { 0 }
3046 let x: i32 = add(1i32, 2i32);
3047 let pi: Result<f32, _> = "3.14".parse();
3050 ### Lambda expressions
3053 ident_list : [ ident [ ',' ident ]* ] ? ;
3054 lambda_expr : '|' ident_list '|' expr ;
3057 A _lambda expression_ (sometimes called an "anonymous function expression")
3058 defines a function and denotes it as a value, in a single expression. A lambda
3059 expression is a pipe-symbol-delimited (`|`) list of identifiers followed by an
3062 A lambda expression denotes a function that maps a list of parameters
3063 (`ident_list`) onto the expression that follows the `ident_list`. The
3064 identifiers in the `ident_list` are the parameters to the function. These
3065 parameters' types need not be specified, as the compiler infers them from
3068 Lambda expressions are most useful when passing functions as arguments to other
3069 functions, as an abbreviation for defining and capturing a separate function.
3071 Significantly, lambda expressions _capture their environment_, which regular
3072 [function definitions](#functions) do not. The exact type of capture depends
3073 on the [function type](#function-types) inferred for the lambda expression. In
3074 the simplest and least-expensive form (analogous to a ```|| { }``` expression),
3075 the lambda expression captures its environment by reference, effectively
3076 borrowing pointers to all outer variables mentioned inside the function.
3077 Alternately, the compiler may infer that a lambda expression should copy or
3078 move values (depending on their type) from the environment into the lambda
3079 expression's captured environment.
3081 In this example, we define a function `ten_times` that takes a higher-order
3082 function argument, and call it with a lambda expression as an argument:
3085 fn ten_times<F>(f: F) where F: Fn(i32) {
3093 ten_times(|j| println!("hello, {}", j));
3099 while_expr : [ lifetime ':' ] "while" no_struct_literal_expr '{' block '}' ;
3102 A `while` loop begins by evaluating the boolean loop conditional expression.
3103 If the loop conditional expression evaluates to `true`, the loop body block
3104 executes and control returns to the loop conditional expression. If the loop
3105 conditional expression evaluates to `false`, the `while` expression completes.
3120 A `loop` expression denotes an infinite loop.
3123 loop_expr : [ lifetime ':' ] "loop" '{' block '}';
3126 A `loop` expression may optionally have a _label_. If a label is present, then
3127 labeled `break` and `continue` expressions nested within this loop may exit out
3128 of this loop or return control to its head. See [Break
3129 expressions](#break-expressions) and [Continue
3130 expressions](#continue-expressions).
3132 ### Break expressions
3135 break_expr : "break" [ lifetime ];
3138 A `break` expression has an optional _label_. If the label is absent, then
3139 executing a `break` expression immediately terminates the innermost loop
3140 enclosing it. It is only permitted in the body of a loop. If the label is
3141 present, then `break foo` terminates the loop with label `foo`, which need not
3142 be the innermost label enclosing the `break` expression, but must enclose it.
3144 ### Continue expressions
3147 continue_expr : "continue" [ lifetime ];
3150 A `continue` expression has an optional _label_. If the label is absent, then
3151 executing a `continue` expression immediately terminates the current iteration
3152 of the innermost loop enclosing it, returning control to the loop *head*. In
3153 the case of a `while` loop, the head is the conditional expression controlling
3154 the loop. In the case of a `for` loop, the head is the call-expression
3155 controlling the loop. If the label is present, then `continue foo` returns
3156 control to the head of the loop with label `foo`, which need not be the
3157 innermost label enclosing the `break` expression, but must enclose it.
3159 A `continue` expression is only permitted in the body of a loop.
3164 for_expr : [ lifetime ':' ] "for" pat "in" no_struct_literal_expr '{' block '}' ;
3167 A `for` expression is a syntactic construct for looping over elements provided
3168 by an implementation of `std::iter::Iterator`.
3170 An example of a for loop over the contents of an array:
3174 # fn bar(f: Foo) { }
3179 let v: &[Foo] = &[a, b, c];
3186 An example of a for loop over a series of integers:
3189 # fn bar(b:usize) { }
3198 if_expr : "if" no_struct_literal_expr '{' block '}'
3201 else_tail : "else" [ if_expr | if_let_expr
3205 An `if` expression is a conditional branch in program control. The form of an
3206 `if` expression is a condition expression, followed by a consequent block, any
3207 number of `else if` conditions and blocks, and an optional trailing `else`
3208 block. The condition expressions must have type `bool`. If a condition
3209 expression evaluates to `true`, the consequent block is executed and any
3210 subsequent `else if` or `else` block is skipped. If a condition expression
3211 evaluates to `false`, the consequent block is skipped and any subsequent `else
3212 if` condition is evaluated. If all `if` and `else if` conditions evaluate to
3213 `false` then any `else` block is executed.
3215 ### Match expressions
3218 match_expr : "match" no_struct_literal_expr '{' match_arm * '}' ;
3220 match_arm : attribute * match_pat "=>" [ expr "," | '{' block '}' ] ;
3222 match_pat : pat [ '|' pat ] * [ "if" expr ] ? ;
3225 A `match` expression branches on a *pattern*. The exact form of matching that
3226 occurs depends on the pattern. Patterns consist of some combination of
3227 literals, destructured arrays or enum constructors, structures and tuples,
3228 variable binding specifications, wildcards (`..`), and placeholders (`_`). A
3229 `match` expression has a *head expression*, which is the value to compare to
3230 the patterns. The type of the patterns must equal the type of the head
3233 In a pattern whose head expression has an `enum` type, a placeholder (`_`)
3234 stands for a *single* data field, whereas a wildcard `..` stands for *all* the
3235 fields of a particular variant.
3237 A `match` behaves differently depending on whether or not the head expression
3238 is an [lvalue or an rvalue](#lvalues,-rvalues-and-temporaries). If the head
3239 expression is an rvalue, it is first evaluated into a temporary location, and
3240 the resulting value is sequentially compared to the patterns in the arms until
3241 a match is found. The first arm with a matching pattern is chosen as the branch
3242 target of the `match`, any variables bound by the pattern are assigned to local
3243 variables in the arm's block, and control enters the block.
3245 When the head expression is an lvalue, the match does not allocate a temporary
3246 location (however, a by-value binding may copy or move from the lvalue). When
3247 possible, it is preferable to match on lvalues, as the lifetime of these
3248 matches inherits the lifetime of the lvalue, rather than being restricted to
3249 the inside of the match.
3251 An example of a `match` expression:
3257 1 => println!("one"),
3258 2 => println!("two"),
3259 3 => println!("three"),
3260 4 => println!("four"),
3261 5 => println!("five"),
3262 _ => println!("something else"),
3266 Patterns that bind variables default to binding to a copy or move of the
3267 matched value (depending on the matched value's type). This can be changed to
3268 bind to a reference by using the `ref` keyword, or to a mutable reference using
3271 Subpatterns can also be bound to variables by the use of the syntax `variable @
3272 subpattern`. For example:
3278 e @ 1 ... 5 => println!("got a range element {}", e),
3279 _ => println!("anything"),
3283 Patterns can also dereference pointers by using the `&`, `&mut` and `box`
3284 symbols, as appropriate. For example, these two matches on `x: &i32` are
3289 let y = match *x { 0 => "zero", _ => "some" };
3290 let z = match x { &0 => "zero", _ => "some" };
3295 A pattern that's just an identifier, like `Nil` in the previous example, could
3296 either refer to an enum variant that's in scope, or bind a new variable. The
3297 compiler resolves this ambiguity by forbidding variable bindings that occur in
3298 `match` patterns from shadowing names of variants that are in scope. For
3299 example, wherever `List` is in scope, a `match` pattern would not be able to
3300 bind `Nil` as a new name. The compiler interprets a variable pattern `x` as a
3301 binding _only_ if there is no variant named `x` in scope. A convention you can
3302 use to avoid conflicts is simply to name variants with upper-case letters, and
3303 local variables with lower-case letters.
3305 Multiple match patterns may be joined with the `|` operator. A range of values
3306 may be specified with `...`. For example:
3311 let message = match x {
3312 0 | 1 => "not many",
3318 Range patterns only work on scalar types (like integers and characters; not
3319 like arrays and structs, which have sub-components). A range pattern may not
3320 be a sub-range of another range pattern inside the same `match`.
3322 Finally, match patterns can accept *pattern guards* to further refine the
3323 criteria for matching a case. Pattern guards appear after the pattern and
3324 consist of a bool-typed expression following the `if` keyword. A pattern guard
3325 may refer to the variables bound within the pattern they follow.
3328 # let maybe_digit = Some(0);
3329 # fn process_digit(i: i32) { }
3330 # fn process_other(i: i32) { }
3332 let message = match maybe_digit {
3333 Some(x) if x < 10 => process_digit(x),
3334 Some(x) => process_other(x),
3339 ### If let expressions
3342 if_let_expr : "if" "let" pat '=' expr '{' block '}'
3344 else_tail : "else" [ if_expr | if_let_expr | '{' block '}' ] ;
3347 An `if let` expression is semantically identical to an `if` expression but in place
3348 of a condition expression it expects a refutable let statement. If the value of the
3349 expression on the right hand side of the let statement matches the pattern, the corresponding
3350 block will execute, otherwise flow proceeds to the first `else` block that follows.
3355 while_let_expr : "while" "let" pat '=' expr '{' block '}' ;
3358 A `while let` loop is semantically identical to a `while` loop but in place of a
3359 condition expression it expects a refutable let statement. If the value of the
3360 expression on the right hand side of the let statement matches the pattern, the
3361 loop body block executes and control returns to the pattern matching statement.
3362 Otherwise, the while expression completes.
3364 ### Return expressions
3367 return_expr : "return" expr ? ;
3370 Return expressions are denoted with the keyword `return`. Evaluating a `return`
3371 expression moves its argument into the designated output location for the
3372 current function call, destroys the current function activation frame, and
3373 transfers control to the caller frame.
3375 An example of a `return` expression:
3378 fn max(a: i32, b: i32) -> i32 {
3390 Every variable, item and value in a Rust program has a type. The _type_ of a
3391 *value* defines the interpretation of the memory holding it.
3393 Built-in types and type-constructors are tightly integrated into the language,
3394 in nontrivial ways that are not possible to emulate in user-defined types.
3395 User-defined types have limited capabilities.
3399 The primitive types are the following:
3401 * The "unit" type `()`, having the single "unit" value `()` (occasionally called
3403 * The boolean type `bool` with values `true` and `false`.
3404 * The machine types.
3405 * The machine-dependent integer and floating-point types.
3407 [^unittype]: The "unit" value `()` is *not* a sentinel "null pointer" value for
3408 reference variables; the "unit" type is the implicit return type from functions
3409 otherwise lacking a return type, and can be used in other contexts (such as
3410 message-sending or type-parametric code) as a zero-size type.]
3414 The machine types are the following:
3416 * The unsigned word types `u8`, `u16`, `u32` and `u64`, with values drawn from
3417 the integer intervals [0, 2^8 - 1], [0, 2^16 - 1], [0, 2^32 - 1] and
3418 [0, 2^64 - 1] respectively.
3420 * The signed two's complement word types `i8`, `i16`, `i32` and `i64`, with
3421 values drawn from the integer intervals [-(2^(7)), 2^7 - 1],
3422 [-(2^(15)), 2^15 - 1], [-(2^(31)), 2^31 - 1], [-(2^(63)), 2^63 - 1]
3425 * The IEEE 754-2008 `binary32` and `binary64` floating-point types: `f32` and
3426 `f64`, respectively.
3428 #### Machine-dependent integer types
3430 The `usize` type is an unsigned integer type with the same number of bits as the
3431 platform's pointer type. It can represent every memory address in the process.
3433 The `isize` type is a signed integer type with the same number of bits as the
3434 platform's pointer type. The theoretical upper bound on object and array size
3435 is the maximum `isize` value. This ensures that `isize` can be used to calculate
3436 differences between pointers into an object or array and can address every byte
3437 within an object along with one byte past the end.
3441 The types `char` and `str` hold textual data.
3443 A value of type `char` is a [Unicode scalar value](
3444 http://www.unicode.org/glossary/#unicode_scalar_value) (i.e. a code point that
3445 is not a surrogate), represented as a 32-bit unsigned word in the 0x0000 to
3446 0xD7FF or 0xE000 to 0x10FFFF range. A `[char]` array is effectively an UCS-4 /
3449 A value of type `str` is a Unicode string, represented as an array of 8-bit
3450 unsigned bytes holding a sequence of UTF-8 codepoints. Since `str` is of
3451 unknown size, it is not a _first-class_ type, but can only be instantiated
3452 through a pointer type, such as `&str` or `String`.
3456 A tuple *type* is a heterogeneous product of other types, called the *elements*
3457 of the tuple. It has no nominal name and is instead structurally typed.
3459 Tuple types and values are denoted by listing the types or values of their
3460 elements, respectively, in a parenthesized, comma-separated list.
3462 Because tuple elements don't have a name, they can only be accessed by
3463 pattern-matching or by using `N` directly as a field to access the
3466 An example of a tuple type and its use:
3469 type Pair<'a> = (i32, &'a str);
3470 let p: Pair<'static> = (10, "hello");
3472 assert!(b != "world");
3476 ### Array, and Slice types
3478 Rust has two different types for a list of items:
3480 * `[T; N]`, an 'array'.
3481 * `&[T]`, a 'slice'.
3483 An array has a fixed size, and can be allocated on either the stack or the
3486 A slice is a 'view' into an array. It doesn't own the data it points
3489 An example of each kind:
3492 let vec: Vec<i32> = vec![1, 2, 3];
3493 let arr: [i32; 3] = [1, 2, 3];
3494 let s: &[i32] = &vec[..];
3497 As you can see, the `vec!` macro allows you to create a `Vec<T>` easily. The
3498 `vec!` macro is also part of the standard library, rather than the language.
3500 All in-bounds elements of arrays, and slices are always initialized, and access
3501 to an array or slice is always bounds-checked.
3505 A `struct` *type* is a heterogeneous product of other types, called the
3506 *fields* of the type.[^structtype]
3508 [^structtype]: `struct` types are analogous `struct` types in C,
3509 the *record* types of the ML family,
3510 or the *structure* types of the Lisp family.
3512 New instances of a `struct` can be constructed with a [struct
3513 expression](#structure-expressions).
3515 The memory layout of a `struct` is undefined by default to allow for compiler
3516 optimizations like field reordering, but it can be fixed with the
3517 `#[repr(...)]` attribute. In either case, fields may be given in any order in
3518 a corresponding struct *expression*; the resulting `struct` value will always
3519 have the same memory layout.
3521 The fields of a `struct` may be qualified by [visibility
3522 modifiers](#re-exporting-and-visibility), to allow access to data in a
3523 structure outside a module.
3525 A _tuple struct_ type is just like a structure type, except that the fields are
3528 A _unit-like struct_ type is like a structure type, except that it has no
3529 fields. The one value constructed by the associated [structure
3530 expression](#structure-expressions) is the only value that inhabits such a
3533 ### Enumerated types
3535 An *enumerated type* is a nominal, heterogeneous disjoint union type, denoted
3536 by the name of an [`enum` item](#enumerations). [^enumtype]
3538 [^enumtype]: The `enum` type is analogous to a `data` constructor declaration in
3539 ML, or a *pick ADT* in Limbo.
3541 An [`enum` item](#enumerations) declares both the type and a number of *variant
3542 constructors*, each of which is independently named and takes an optional tuple
3545 New instances of an `enum` can be constructed by calling one of the variant
3546 constructors, in a [call expression](#call-expressions).
3548 Any `enum` value consumes as much memory as the largest variant constructor for
3549 its corresponding `enum` type.
3551 Enum types cannot be denoted *structurally* as types, but must be denoted by
3552 named reference to an [`enum` item](#enumerations).
3556 Nominal types — [enumerations](#enumerated-types) and
3557 [structures](#structure-types) — may be recursive. That is, each `enum`
3558 constructor or `struct` field may refer, directly or indirectly, to the
3559 enclosing `enum` or `struct` type itself. Such recursion has restrictions:
3561 * Recursive types must include a nominal type in the recursion
3562 (not mere [type definitions](#type-definitions),
3563 or other structural types such as [arrays](#array,-and-slice-types) or [tuples](#tuple-types)).
3564 * A recursive `enum` item must have at least one non-recursive constructor
3565 (in order to give the recursion a basis case).
3566 * The size of a recursive type must be finite;
3567 in other words the recursive fields of the type must be [pointer types](#pointer-types).
3568 * Recursive type definitions can cross module boundaries, but not module *visibility* boundaries,
3569 or crate boundaries (in order to simplify the module system and type checker).
3571 An example of a *recursive* type and its use:
3576 Cons(T, Box<List<T>>)
3579 let a: List<i32> = List::Cons(7, Box::new(List::Cons(13, Box::new(List::Nil))));
3584 All pointers in Rust are explicit first-class values. They can be copied,
3585 stored into data structures, and returned from functions. There are two
3586 varieties of pointer in Rust:
3589 : These point to memory _owned by some other value_.
3590 A reference type is written `&type` for some lifetime-variable `f`,
3591 or just `&'a type` when you need an explicit lifetime.
3592 Copying a reference is a "shallow" operation:
3593 it involves only copying the pointer itself.
3594 Releasing a reference typically has no effect on the value it points to,
3595 with the exception of temporary values, which are released when the last
3596 reference to them is released.
3598 * Raw pointers (`*`)
3599 : Raw pointers are pointers without safety or liveness guarantees.
3600 Raw pointers are written as `*const T` or `*mut T`,
3601 for example `*const int` means a raw pointer to an integer.
3602 Copying or dropping a raw pointer has no effect on the lifecycle of any
3603 other value. Dereferencing a raw pointer or converting it to any other
3604 pointer type is an [`unsafe` operation](#unsafe-functions).
3605 Raw pointers are generally discouraged in Rust code;
3606 they exist to support interoperability with foreign code,
3607 and writing performance-critical or low-level functions.
3609 The standard library contains additional 'smart pointer' types beyond references
3614 The function type constructor `fn` forms new function types. A function type
3615 consists of a possibly-empty set of function-type modifiers (such as `unsafe`
3616 or `extern`), a sequence of input types and an output type.
3618 An example of a `fn` type:
3621 fn add(x: i32, y: i32) -> i32 {
3625 let mut x = add(5,7);
3627 type Binop = fn(i32, i32) -> i32;
3628 let bo: Binop = add;
3634 ```{.ebnf .notation}
3635 closure_type := [ 'unsafe' ] [ '<' lifetime-list '>' ] '|' arg-list '|'
3636 [ ':' bound-list ] [ '->' type ]
3637 lifetime-list := lifetime | lifetime ',' lifetime-list
3638 arg-list := ident ':' type | ident ':' type ',' arg-list
3639 bound-list := bound | bound '+' bound-list
3640 bound := path | lifetime
3643 The type of a closure mapping an input of type `A` to an output of type `B` is
3644 `|A| -> B`. A closure with no arguments or return values has type `||`.
3646 An example of creating and calling a closure:
3649 let captured_var = 10;
3651 let closure_no_args = || println!("captured_var={}", captured_var);
3653 let closure_args = |arg: i32| -> i32 {
3654 println!("captured_var={}, arg={}", captured_var, arg);
3655 arg // Note lack of semicolon after 'arg'
3658 fn call_closure<F: Fn(), G: Fn(i32) -> i32>(c1: F, c2: G) {
3663 call_closure(closure_no_args, closure_args);
3669 Every trait item (see [traits](#traits)) defines a type with the same name as
3670 the trait. This type is called the _object type_ of the trait. Object types
3671 permit "late binding" of methods, dispatched using _virtual method tables_
3672 ("vtables"). Whereas most calls to trait methods are "early bound" (statically
3673 resolved) to specific implementations at compile time, a call to a method on an
3674 object type is only resolved to a vtable entry at compile time. The actual
3675 implementation for each vtable entry can vary on an object-by-object basis.
3677 Given a pointer-typed expression `E` of type `&T` or `Box<T>`, where `T`
3678 implements trait `R`, casting `E` to the corresponding pointer type `&R` or
3679 `Box<R>` results in a value of the _object type_ `R`. This result is
3680 represented as a pair of pointers: the vtable pointer for the `T`
3681 implementation of `R`, and the pointer value of `E`.
3683 An example of an object type:
3687 fn stringify(&self) -> String;
3690 impl Printable for i32 {
3691 fn stringify(&self) -> String { self.to_string() }
3694 fn print(a: Box<Printable>) {
3695 println!("{}", a.stringify());
3699 print(Box::new(10) as Box<Printable>);
3703 In this example, the trait `Printable` occurs as an object type in both the
3704 type signature of `print`, and the cast expression in `main`.
3708 Within the body of an item that has type parameter declarations, the names of
3709 its type parameters are types:
3712 fn map<A: Clone, B: Clone>(f: |A| -> B, xs: &[A]) -> Vec<B> {
3716 let first: B = f(xs[0].clone());
3717 let mut rest: Vec<B> = map(f, xs.slice(1, xs.len()));
3718 rest.insert(0, first);
3723 Here, `first` has type `B`, referring to `map`'s `B` type parameter; and `rest`
3724 has type `Vec<B>`, a vector type with element type `B`.
3728 The special type `self` has a meaning within methods inside an impl item. It
3729 refers to the type of the implicit `self` argument. For example, in:
3733 fn make_string(&self) -> String;
3736 impl Printable for String {
3737 fn make_string(&self) -> String {
3743 `self` refers to the value of type `String` that is the receiver for a call to
3744 the method `make_string`.
3748 Several traits define special evaluation behavior.
3752 The `Copy` trait changes the semantics of a type implementing it. Values whose
3753 type implements `Copy` are copied rather than moved upon assignment.
3755 ## The `Sized` trait
3757 The `Sized` trait indicates that the size of this type is known at compile-time.
3761 The `Drop` trait provides a destructor, to be run whenever a value of this type
3766 A Rust program's memory consists of a static set of *items* and a *heap*.
3767 Immutable portions of the heap may be safely shared between threads, mutable
3768 portions may not be safely shared, but several mechanisms for effectively-safe
3769 sharing of mutable values, built on unsafe code but enforcing a safe locking
3770 discipline, exist in the standard library.
3772 Allocations in the stack consist of *variables*, and allocations in the heap
3775 ### Memory allocation and lifetime
3777 The _items_ of a program are those functions, modules and types that have their
3778 value calculated at compile-time and stored uniquely in the memory image of the
3779 rust process. Items are neither dynamically allocated nor freed.
3781 The _heap_ is a general term that describes boxes. The lifetime of an
3782 allocation in the heap depends on the lifetime of the box values pointing to
3783 it. Since box values may themselves be passed in and out of frames, or stored
3784 in the heap, heap allocations may outlive the frame they are allocated within.
3786 ### Memory ownership
3788 When a stack frame is exited, its local allocations are all released, and its
3789 references to boxes are dropped.
3793 A _variable_ is a component of a stack frame, either a named function parameter,
3794 an anonymous [temporary](#lvalues,-rvalues-and-temporaries), or a named local
3797 A _local variable_ (or *stack-local* allocation) holds a value directly,
3798 allocated within the stack's memory. The value is a part of the stack frame.
3800 Local variables are immutable unless declared otherwise like: `let mut x = ...`.
3802 Function parameters are immutable unless declared with `mut`. The `mut` keyword
3803 applies only to the following parameter (so `|mut x, y|` and `fn f(mut x:
3804 Box<i32>, y: Box<i32>)` declare one mutable variable `x` and one immutable
3807 Methods that take either `self` or `Box<Self>` can optionally place them in a
3808 mutable variable by prefixing them with `mut` (similar to regular arguments):
3812 fn change(mut self) -> Self;
3813 fn modify(mut self: Box<Self>) -> Box<Self>;
3817 Local variables are not initialized when allocated; the entire frame worth of
3818 local variables are allocated at once, on frame-entry, in an uninitialized
3819 state. Subsequent statements within a function may or may not initialize the
3820 local variables. Local variables can be used only after they have been
3821 initialized; this is enforced by the compiler.
3825 The Rust compiler supports various methods to link crates together both
3826 statically and dynamically. This section will explore the various methods to
3827 link Rust crates together, and more information about native libraries can be
3828 found in the [ffi section of the book][ffi].
3830 In one session of compilation, the compiler can generate multiple artifacts
3831 through the usage of either command line flags or the `crate_type` attribute.
3832 If one or more command line flag is specified, all `crate_type` attributes will
3833 be ignored in favor of only building the artifacts specified by command line.
3835 * `--crate-type=bin`, `#[crate_type = "bin"]` - A runnable executable will be
3836 produced. This requires that there is a `main` function in the crate which
3837 will be run when the program begins executing. This will link in all Rust and
3838 native dependencies, producing a distributable binary.
3840 * `--crate-type=lib`, `#[crate_type = "lib"]` - A Rust library will be produced.
3841 This is an ambiguous concept as to what exactly is produced because a library
3842 can manifest itself in several forms. The purpose of this generic `lib` option
3843 is to generate the "compiler recommended" style of library. The output library
3844 will always be usable by rustc, but the actual type of library may change from
3845 time-to-time. The remaining output types are all different flavors of
3846 libraries, and the `lib` type can be seen as an alias for one of them (but the
3847 actual one is compiler-defined).
3849 * `--crate-type=dylib`, `#[crate_type = "dylib"]` - A dynamic Rust library will
3850 be produced. This is different from the `lib` output type in that this forces
3851 dynamic library generation. The resulting dynamic library can be used as a
3852 dependency for other libraries and/or executables. This output type will
3853 create `*.so` files on linux, `*.dylib` files on osx, and `*.dll` files on
3856 * `--crate-type=staticlib`, `#[crate_type = "staticlib"]` - A static system
3857 library will be produced. This is different from other library outputs in that
3858 the Rust compiler will never attempt to link to `staticlib` outputs. The
3859 purpose of this output type is to create a static library containing all of
3860 the local crate's code along with all upstream dependencies. The static
3861 library is actually a `*.a` archive on linux and osx and a `*.lib` file on
3862 windows. This format is recommended for use in situations such as linking
3863 Rust code into an existing non-Rust application because it will not have
3864 dynamic dependencies on other Rust code.
3866 * `--crate-type=rlib`, `#[crate_type = "rlib"]` - A "Rust library" file will be
3867 produced. This is used as an intermediate artifact and can be thought of as a
3868 "static Rust library". These `rlib` files, unlike `staticlib` files, are
3869 interpreted by the Rust compiler in future linkage. This essentially means
3870 that `rustc` will look for metadata in `rlib` files like it looks for metadata
3871 in dynamic libraries. This form of output is used to produce statically linked
3872 executables as well as `staticlib` outputs.
3874 Note that these outputs are stackable in the sense that if multiple are
3875 specified, then the compiler will produce each form of output at once without
3876 having to recompile. However, this only applies for outputs specified by the
3877 same method. If only `crate_type` attributes are specified, then they will all
3878 be built, but if one or more `--crate-type` command line flag is specified,
3879 then only those outputs will be built.
3881 With all these different kinds of outputs, if crate A depends on crate B, then
3882 the compiler could find B in various different forms throughout the system. The
3883 only forms looked for by the compiler, however, are the `rlib` format and the
3884 dynamic library format. With these two options for a dependent library, the
3885 compiler must at some point make a choice between these two formats. With this
3886 in mind, the compiler follows these rules when determining what format of
3887 dependencies will be used:
3889 1. If a static library is being produced, all upstream dependencies are
3890 required to be available in `rlib` formats. This requirement stems from the
3891 reason that a dynamic library cannot be converted into a static format.
3893 Note that it is impossible to link in native dynamic dependencies to a static
3894 library, and in this case warnings will be printed about all unlinked native
3895 dynamic dependencies.
3897 2. If an `rlib` file is being produced, then there are no restrictions on what
3898 format the upstream dependencies are available in. It is simply required that
3899 all upstream dependencies be available for reading metadata from.
3901 The reason for this is that `rlib` files do not contain any of their upstream
3902 dependencies. It wouldn't be very efficient for all `rlib` files to contain a
3903 copy of `libstd.rlib`!
3905 3. If an executable is being produced and the `-C prefer-dynamic` flag is not
3906 specified, then dependencies are first attempted to be found in the `rlib`
3907 format. If some dependencies are not available in an rlib format, then
3908 dynamic linking is attempted (see below).
3910 4. If a dynamic library or an executable that is being dynamically linked is
3911 being produced, then the compiler will attempt to reconcile the available
3912 dependencies in either the rlib or dylib format to create a final product.
3914 A major goal of the compiler is to ensure that a library never appears more
3915 than once in any artifact. For example, if dynamic libraries B and C were
3916 each statically linked to library A, then a crate could not link to B and C
3917 together because there would be two copies of A. The compiler allows mixing
3918 the rlib and dylib formats, but this restriction must be satisfied.
3920 The compiler currently implements no method of hinting what format a library
3921 should be linked with. When dynamically linking, the compiler will attempt to
3922 maximize dynamic dependencies while still allowing some dependencies to be
3923 linked in via an rlib.
3925 For most situations, having all libraries available as a dylib is recommended
3926 if dynamically linking. For other situations, the compiler will emit a
3927 warning if it is unable to determine which formats to link each library with.
3929 In general, `--crate-type=bin` or `--crate-type=lib` should be sufficient for
3930 all compilation needs, and the other options are just available if more
3931 fine-grained control is desired over the output format of a Rust crate.
3933 # Appendix: Rationales and design tradeoffs
3937 # Appendix: Influences
3939 Rust is not a particularly original language, with design elements coming from
3940 a wide range of sources. Some of these are listed below (including elements
3941 that have since been removed):
3943 * SML, OCaml: algebraic datatypes, pattern matching, type inference,
3944 semicolon statement separation
3945 * C++: references, RAII, smart pointers, move semantics, monomorphisation,
3947 * ML Kit, Cyclone: region based memory management
3948 * Haskell (GHC): typeclasses, type families
3949 * Newsqueak, Alef, Limbo: channels, concurrency
3950 * Erlang: message passing, task failure, ~~linked task failure~~,
3951 ~~lightweight concurrency~~
3952 * Swift: optional bindings
3953 * Scheme: hygienic macros
3955 * Ruby: ~~block syntax~~
3956 * NIL, Hermes: ~~typestate~~
3957 * [Unicode Annex #31](http://www.unicode.org/reports/tr31/): identifier and
3960 [ffi]: book/ffi.html
3961 [plugin]: book/plugins.html