1 % The Rust Reference Manual
5 This document is the reference manual for the Rust programming language. It
6 provides three kinds of material:
8 - Chapters that formally define the language grammar and, for each
9 construct, informally describe its semantics and give examples of its
11 - Chapters that informally describe the memory model, concurrency model,
12 runtime services, linkage model and debugging facilities.
13 - Appendix chapters providing rationale and references to languages that
14 influenced the design.
16 This document does not serve as a tutorial introduction to the
17 language. Background familiarity with the language is assumed. A separate
18 [tutorial] document is available to help acquire such background familiarity.
20 This document also does not serve as a reference to the [standard] or [extra]
21 libraries included in the language distribution. Those libraries are
22 documented separately by extracting documentation attributes from their
25 [tutorial]: tutorial.html
26 [standard]: std/index.html
27 [extra]: extra/index.html
31 Rust is a work in progress. The language continues to evolve as the design
32 shifts and is fleshed out in working code. Certain parts work, certain parts
33 do not, certain parts will be removed or changed.
35 This manual is a snapshot written in the present tense. All features described
36 exist in working code unless otherwise noted, but some are quite primitive or
37 remain to be further modified by planned work. Some may be temporary. It is a
38 *draft*, and we ask that you not take anything you read here as final.
40 If you have suggestions to make, please try to focus them on *reductions* to
41 the language: possible features that can be combined or omitted. We aim to
42 keep the size and complexity of the language under control.
44 > **Note:** The grammar for Rust given in this document is rough and
45 > very incomplete; only a modest number of sections have accompanying grammar
46 > rules. Formalizing the grammar accepted by the Rust parser is ongoing work,
47 > but future versions of this document will contain a complete
48 > grammar. Moreover, we hope that this grammar will be extracted and verified
49 > as LL(1) by an automated grammar-analysis tool, and further tested against the
50 > Rust sources. Preliminary versions of this automation exist, but are not yet
55 Rust's grammar is defined over Unicode codepoints, each conventionally
56 denoted `U+XXXX`, for 4 or more hexadecimal digits `X`. _Most_ of Rust's
57 grammar is confined to the ASCII range of Unicode, and is described in this
58 document by a dialect of Extended Backus-Naur Form (EBNF), specifically a
59 dialect of EBNF supported by common automated LL(k) parsing tools such as
60 `llgen`, rather than the dialect given in ISO 14977. The dialect can be
61 defined self-referentially as follows:
63 ~~~~ {.notrust .ebnf .notation}
65 rule : nonterminal ':' productionrule ';' ;
66 productionrule : production [ '|' production ] * ;
68 term : element repeats ;
69 element : LITERAL | IDENTIFIER | '[' productionrule ']' ;
70 repeats : [ '*' | '+' ] NUMBER ? | NUMBER ? | '?' ;
75 - Whitespace in the grammar is ignored.
76 - Square brackets are used to group rules.
77 - `LITERAL` is a single printable ASCII character, or an escaped hexadecimal
78 ASCII code of the form `\xQQ`, in single quotes, denoting the corresponding
79 Unicode codepoint `U+00QQ`.
80 - `IDENTIFIER` is a nonempty string of ASCII letters and underscores.
81 - The `repeat` forms apply to the adjacent `element`, and are as follows:
82 - `?` means zero or one repetition
83 - `*` means zero or more repetitions
84 - `+` means one or more repetitions
85 - NUMBER trailing a repeat symbol gives a maximum repetition count
86 - NUMBER on its own gives an exact repetition count
88 This EBNF dialect should hopefully be familiar to many readers.
90 ## Unicode productions
92 A few productions in Rust's grammar permit Unicode codepoints outside the ASCII range.
93 We define these productions in terms of character properties specified in the Unicode standard,
94 rather than in terms of ASCII-range codepoints.
95 The section [Special Unicode Productions](#special-unicode-productions) lists these productions.
97 ## String table productions
99 Some rules in the grammar -- notably [unary
100 operators](#unary-operator-expressions), [binary
101 operators](#binary-operator-expressions), and [keywords](#keywords) --
102 are given in a simplified form: as a listing of a table of unquoted,
103 printable whitespace-separated strings. These cases form a subset of
104 the rules regarding the [token](#tokens) rule, and are assumed to be
105 the result of a lexical-analysis phase feeding the parser, driven by a
106 DFA, operating over the disjunction of all such string table entries.
108 When such a string enclosed in double-quotes (`"`) occurs inside the
109 grammar, it is an implicit reference to a single member of such a string table
110 production. See [tokens](#tokens) for more information.
116 Rust input is interpreted as a sequence of Unicode codepoints encoded in UTF-8,
117 normalized to Unicode normalization form NFKC.
118 Most Rust grammar rules are defined in terms of printable ASCII-range codepoints,
119 but a small number are defined in terms of Unicode properties or explicit codepoint lists.
120 ^[Substitute definitions for the special Unicode productions are provided to the grammar verifier, restricted to ASCII range, when verifying the grammar in this document.]
122 ## Special Unicode Productions
124 The following productions in the Rust grammar are defined in terms of Unicode properties:
125 `ident`, `non_null`, `non_star`, `non_eol`, `non_slash_or_star`, `non_single_quote` and `non_double_quote`.
129 The `ident` production is any nonempty Unicode string of the following form:
131 - The first character has property `XID_start`
132 - The remaining characters have property `XID_continue`
134 that does _not_ occur in the set of [keywords](#keywords).
136 Note: `XID_start` and `XID_continue` as character properties cover the
137 character ranges used to form the more familiar C and Java language-family
140 ### Delimiter-restricted productions
142 Some productions are defined by exclusion of particular Unicode characters:
144 - `non_null` is any single Unicode character aside from `U+0000` (null)
145 - `non_eol` is `non_null` restricted to exclude `U+000A` (`'\n'`)
146 - `non_star` is `non_null` restricted to exclude `U+002A` (`*`)
147 - `non_slash_or_star` is `non_null` restricted to exclude `U+002F` (`/`) and `U+002A` (`*`)
148 - `non_single_quote` is `non_null` restricted to exclude `U+0027` (`'`)
149 - `non_double_quote` is `non_null` restricted to exclude `U+0022` (`"`)
153 ~~~~ {.notrust .ebnf .gram}
154 comment : block_comment | line_comment ;
155 block_comment : "/*" block_comment_body * '*' + '/' ;
156 block_comment_body : (block_comment | character) * ;
157 line_comment : "//" non_eol * ;
160 Comments in Rust code follow the general C++ style of line and block-comment forms,
161 with no nesting of block-comment delimiters.
163 Line comments beginning with exactly _three_ slashes (`///`), and block
164 comments beginning with a exactly one repeated asterisk in the block-open
165 sequence (`/**`), are interpreted as a special syntax for `doc`
166 [attributes](#attributes). That is, they are equivalent to writing
167 `#[doc="..."]` around the body of the comment (this includes the comment
168 characters themselves, ie `/// Foo` turns into `#[doc="/// Foo"]`).
170 Non-doc comments are interpreted as a form of whitespace.
174 ~~~~ {.notrust .ebnf .gram}
175 whitespace_char : '\x20' | '\x09' | '\x0a' | '\x0d' ;
176 whitespace : [ whitespace_char | comment ] + ;
179 The `whitespace_char` production is any nonempty Unicode string consisting of any
180 of the following Unicode characters: `U+0020` (space, `' '`), `U+0009` (tab,
181 `'\t'`), `U+000A` (LF, `'\n'`), `U+000D` (CR, `'\r'`).
183 Rust is a "free-form" language, meaning that all forms of whitespace serve
184 only to separate _tokens_ in the grammar, and have no semantic significance.
186 A Rust program has identical meaning if each whitespace element is replaced
187 with any other legal whitespace element, such as a single space character.
191 ~~~~ {.notrust .ebnf .gram}
192 simple_token : keyword | unop | binop ;
193 token : simple_token | ident | literal | symbol | whitespace token ;
196 Tokens are primitive productions in the grammar defined by regular
197 (non-recursive) languages. "Simple" tokens are given in [string table
198 production](#string-table-productions) form, and occur in the rest of the
199 grammar as double-quoted strings. Other tokens have exact rules given.
203 The keywords are the following strings:
205 ~~~~ {.notrust .keyword}
216 self static struct super
222 Each of these keywords has special meaning in its grammar,
223 and all of them are excluded from the `ident` rule.
227 A literal is an expression consisting of a single token, rather than a
228 sequence of tokens, that immediately and directly denotes the value it
229 evaluates to, rather than referring to it by name or some other evaluation
230 rule. A literal is a form of constant expression, so is evaluated (primarily)
233 ~~~~ {.notrust .ebnf .gram}
234 literal : string_lit | char_lit | num_lit ;
237 #### Character and string literals
239 ~~~~ {.notrust .ebnf .gram}
240 char_lit : '\x27' char_body '\x27' ;
241 string_lit : '"' string_body * '"' | 'r' raw_string ;
243 char_body : non_single_quote
244 | '\x5c' [ '\x27' | common_escape ] ;
246 string_body : non_double_quote
247 | '\x5c' [ '\x22' | common_escape ] ;
248 raw_string : '"' raw_string_body '"' | '#' raw_string '#' ;
250 common_escape : '\x5c'
251 | 'n' | 'r' | 't' | '0'
256 hex_digit : 'a' | 'b' | 'c' | 'd' | 'e' | 'f'
257 | 'A' | 'B' | 'C' | 'D' | 'E' | 'F'
259 oct_digit : '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' ;
260 dec_digit : '0' | nonzero_dec ;
261 nonzero_dec: '1' | '2' | '3' | '4'
262 | '5' | '6' | '7' | '8' | '9' ;
265 A _character literal_ is a single Unicode character enclosed within two
266 `U+0027` (single-quote) characters, with the exception of `U+0027` itself,
267 which must be _escaped_ by a preceding U+005C character (`\`).
269 A _string literal_ is a sequence of any Unicode characters enclosed within
270 two `U+0022` (double-quote) characters, with the exception of `U+0022`
271 itself, which must be _escaped_ by a preceding `U+005C` character (`\`),
272 or a _raw string literal_.
274 Some additional _escapes_ are available in either character or non-raw string
275 literals. An escape starts with a `U+005C` (`\`) and continues with one of
278 * An _8-bit codepoint escape_ escape starts with `U+0078` (`x`) and is
279 followed by exactly two _hex digits_. It denotes the Unicode codepoint
280 equal to the provided hex value.
281 * A _16-bit codepoint escape_ starts with `U+0075` (`u`) and is followed
282 by exactly four _hex digits_. It denotes the Unicode codepoint equal to
283 the provided hex value.
284 * A _32-bit codepoint escape_ starts with `U+0055` (`U`) and is followed
285 by exactly eight _hex digits_. It denotes the Unicode codepoint equal to
286 the provided hex value.
287 * A _whitespace escape_ is one of the characters `U+006E` (`n`), `U+0072`
288 (`r`), or `U+0074` (`t`), denoting the unicode values `U+000A` (LF),
289 `U+000D` (CR) or `U+0009` (HT) respectively.
290 * The _backslash escape_ is the character `U+005C` (`\`) which must be
291 escaped in order to denote *itself*.
293 Raw string literals do not process any escapes. They start with the character
294 `U+0072` (`r`), followed zero or more of the character `U+0023` (`#`) and a
295 `U+0022` (double-quote) character. The _raw string body_ is not defined in the
296 EBNF grammar above: it can contain any sequence of Unicode characters and is
297 terminated only by another `U+0022` (double-quote) character, followed by the
298 same number of `U+0023` (`#`) characters that preceeded the opening `U+0022`
299 (double-quote) character.
301 All Unicode characters contained in the raw string body represent themselves,
302 the characters `U+0022` (double-quote) (except when followed by at least as
303 many `U+0023` (`#`) characters as were used to start the raw string literal) or
304 `U+005C` (`\`) do not have any special meaning.
306 Examples for string literals:
309 "foo"; r"foo"; // foo
310 "\"foo\""; r#""foo""#; // "foo"
313 r##"foo #"# bar"##; // foo #"# bar
315 "\x52"; "R"; r"R"; // R
316 "\\x52"; r"\x52"; // \x52
321 ~~~~ {.notrust .ebnf .gram}
322 num_lit : nonzero_dec [ dec_digit | '_' ] * num_suffix ?
323 | '0' [ [ dec_digit | '_' ] * num_suffix ?
324 | 'b' [ '1' | '0' | '_' ] + int_suffix ?
325 | 'o' [ oct_digit | '_' ] + int_suffix ?
326 | 'x' [ hex_digit | '_' ] + int_suffix ? ] ;
328 num_suffix : int_suffix | float_suffix ;
330 int_suffix : 'u' int_suffix_size ?
331 | 'i' int_suffix_size ? ;
332 int_suffix_size : [ '8' | '1' '6' | '3' '2' | '6' '4' ] ;
334 float_suffix : [ exponent | '.' dec_lit exponent ? ] ? float_suffix_ty ? ;
335 float_suffix_ty : 'f' [ '3' '2' | '6' '4' ] ;
336 exponent : ['E' | 'e'] ['-' | '+' ] ? dec_lit ;
337 dec_lit : [ dec_digit | '_' ] + ;
340 A _number literal_ is either an _integer literal_ or a _floating-point
341 literal_. The grammar for recognizing the two kinds of literals is mixed,
342 as they are differentiated by suffixes.
344 ##### Integer literals
346 An _integer literal_ has one of four forms:
348 * A _decimal literal_ starts with a *decimal digit* and continues with any
349 mixture of *decimal digits* and _underscores_.
350 * A _hex literal_ starts with the character sequence `U+0030` `U+0078`
351 (`0x`) and continues as any mixture hex digits and underscores.
352 * An _octal literal_ starts with the character sequence `U+0030` `U+006F`
353 (`0o`) and continues as any mixture octal digits and underscores.
354 * A _binary literal_ starts with the character sequence `U+0030` `U+0062`
355 (`0b`) and continues as any mixture binary digits and underscores.
357 An integer literal may be followed (immediately, without any spaces) by an
358 _integer suffix_, which changes the type of the literal. There are two kinds
359 of integer literal suffix:
361 * The `i` and `u` suffixes give the literal type `int` or `uint`,
363 * Each of the signed and unsigned machine types `u8`, `i8`,
364 `u16`, `i16`, `u32`, `i32`, `u64` and `i64`
365 give the literal the corresponding machine type.
367 The type of an _unsuffixed_ integer literal is determined by type inference.
368 If a integer type can be _uniquely_ determined from the surrounding program
369 context, the unsuffixed integer literal has that type. If the program context
370 underconstrains the type, the unsuffixed integer literal's type is `int`; if
371 the program context overconstrains the type, it is considered a static type
374 Examples of integer literals of various forms:
377 123; 0xff00; // type determined by program context
378 // defaults to int in absence of type
384 0o70_i16; // type i16
385 0b1111_1111_1001_0000_i32; // type i32
388 ##### Floating-point literals
390 A _floating-point literal_ has one of two forms:
392 * Two _decimal literals_ separated by a period
393 character `U+002E` (`.`), with an optional _exponent_ trailing after the
394 second decimal literal.
395 * A single _decimal literal_ followed by an _exponent_.
397 By default, a floating-point literal has a generic type, but will fall back to
398 `f64`. A floating-point literal may be followed (immediately, without any
399 spaces) by a _floating-point suffix_, which changes the type of the literal.
400 There are two floating-point suffixes: `f32`, and `f64` (the 32-bit and 64-bit
401 floating point types).
403 Examples of floating-point literals of various forms:
409 12E+99_f64; // type f64
412 ##### Unit and boolean literals
414 The _unit value_, the only value of the type that has the same name, is written as `()`.
415 The two values of the boolean type are written `true` and `false`.
419 ~~~~ {.notrust .ebnf .gram}
421 | '#' | '[' | ']' | '(' | ')' | '{' | '}'
425 Symbols are a general class of printable [token](#tokens) that play structural
426 roles in a variety of grammar productions. They are catalogued here for
427 completeness as the set of remaining miscellaneous printable tokens that do not
428 otherwise appear as [unary operators](#unary-operator-expressions), [binary
429 operators](#binary-operator-expressions), or [keywords](#keywords).
434 ~~~~ {.notrust .ebnf .gram}
435 expr_path : ident [ "::" expr_path_tail ] + ;
436 expr_path_tail : '<' type_expr [ ',' type_expr ] + '>'
439 type_path : ident [ type_path_tail ] + ;
440 type_path_tail : '<' type_expr [ ',' type_expr ] + '>'
444 A _path_ is a sequence of one or more path components _logically_ separated by
445 a namespace qualifier (`::`). If a path consists of only one component, it may
446 refer to either an [item](#items) or a [slot](#memory-slots) in a local
447 control scope. If a path has multiple components, it refers to an item.
449 Every item has a _canonical path_ within its crate, but the path naming an
450 item is only meaningful within a given crate. There is no global namespace
451 across crates; an item's canonical path merely identifies it within the crate.
453 Two examples of simple paths consisting of only identifier components:
460 Path components are usually [identifiers](#identifiers), but the trailing
461 component of a path may be an angle-bracket-enclosed list of type
462 arguments. In [expression](#expressions) context, the type argument list is
463 given after a final (`::`) namespace qualifier in order to disambiguate it
464 from a relational expression involving the less-than symbol (`<`). In type
465 expression context, the final namespace qualifier is omitted.
467 Two examples of paths with type arguments:
470 # struct HashMap<K, V>;
472 # fn id<T>(t: T) -> T { t }
473 type T = HashMap<int,~str>; // Type arguments used in a type expression
474 let x = id::<int>(10); // Type arguments used in a call expression
480 A number of minor features of Rust are not central enough to have their own
481 syntax, and yet are not implementable as functions. Instead, they are given
482 names, and invoked through a consistent syntax: `name!(...)`. Examples
485 * `format!` : format data into a string
486 * `env!` : look up an environment variable's value at compile time
487 * `file!`: return the path to the file being compiled
488 * `stringify!` : pretty-print the Rust expression given as an argument
489 * `include!` : include the Rust expression in the given file
490 * `include_str!` : include the contents of the given file as a string
491 * `include_bin!` : include the contents of the given file as a binary blob
492 * `error!`, `warn!`, `info!`, `debug!` : provide diagnostic information.
494 All of the above extensions are expressions with values.
498 ~~~~ {.notrust .ebnf .gram}
499 expr_macro_rules : "macro_rules" '!' ident '(' macro_rule * ')'
500 macro_rule : '(' matcher * ')' "=>" '(' transcriber * ')' ';'
501 matcher : '(' matcher * ')' | '[' matcher * ']'
502 | '{' matcher * '}' | '$' ident ':' ident
503 | '$' '(' matcher * ')' sep_token? [ '*' | '+' ]
505 transcriber : '(' transcriber * ')' | '[' transcriber * ']'
506 | '{' transcriber * '}' | '$' ident
507 | '$' '(' transcriber * ')' sep_token? [ '*' | '+' ]
511 User-defined syntax extensions are called "macros",
512 and the `macro_rules` syntax extension defines them.
513 Currently, user-defined macros can expand to expressions, statements, or items.
515 (A `sep_token` is any token other than `*` and `+`.
516 A `non_special_token` is any token other than a delimiter or `$`.)
518 The macro expander looks up macro invocations by name,
519 and tries each macro rule in turn.
520 It transcribes the first successful match.
521 Matching and transcription are closely related to each other,
522 and we will describe them together.
526 The macro expander matches and transcribes every token that does not begin with a `$` literally, including delimiters.
527 For parsing reasons, delimiters must be balanced, but they are otherwise not special.
529 In the matcher, `$` _name_ `:` _designator_ matches the nonterminal in the
530 Rust syntax named by _designator_. Valid designators are `item`, `block`,
531 `stmt`, `pat`, `expr`, `ty` (type), `ident`, `path`, `matchers` (lhs of the `=>` in macro rules),
532 `tt` (rhs of the `=>` in macro rules). In the transcriber, the designator is already known, and so only
533 the name of a matched nonterminal comes after the dollar sign.
535 In both the matcher and transcriber, the Kleene star-like operator indicates repetition.
536 The Kleene star operator consists of `$` and parens, optionally followed by a separator token, followed by `*` or `+`.
537 `*` means zero or more repetitions, `+` means at least one repetition.
538 The parens are not matched or transcribed.
539 On the matcher side, a name is bound to _all_ of the names it
540 matches, in a structure that mimics the structure of the repetition
541 encountered on a successful match. The job of the transcriber is to sort that
544 The rules for transcription of these repetitions are called "Macro By Example".
545 Essentially, one "layer" of repetition is discharged at a time, and all of
546 them must be discharged by the time a name is transcribed. Therefore,
547 `( $( $i:ident ),* ) => ( $i )` is an invalid macro, but
548 `( $( $i:ident ),* ) => ( $( $i:ident ),* )` is acceptable (if trivial).
550 When Macro By Example encounters a repetition, it examines all of the `$`
551 _name_ s that occur in its body. At the "current layer", they all must repeat
552 the same number of times, so
553 ` ( $( $i:ident ),* ; $( $j:ident ),* ) => ( $( ($i,$j) ),* )` is valid if
554 given the argument `(a,b,c ; d,e,f)`, but not `(a,b,c ; d,e)`. The repetition
555 walks through the choices at that layer in lockstep, so the former input
556 transcribes to `( (a,d), (b,e), (c,f) )`.
558 Nested repetitions are allowed.
560 ### Parsing limitations
562 The parser used by the macro system is reasonably powerful, but the parsing of
563 Rust syntax is restricted in two ways:
565 1. The parser will always parse as much as possible. If it attempts to match
566 `$i:expr [ , ]` against `8 [ , ]`, it will attempt to parse `i` as an array
567 index operation and fail. Adding a separator can solve this problem.
568 2. The parser must have eliminated all ambiguity by the time it reaches a `$` _name_ `:` _designator_.
569 This requirement most often affects name-designator pairs when they occur at the beginning of, or immediately after, a `$(...)*`; requiring a distinctive token in front can solve the problem.
571 ## Syntax extensions useful for the macro author
573 * `log_syntax!` : print out the arguments at compile time
574 * `trace_macros!` : supply `true` or `false` to enable or disable macro expansion logging
575 * `stringify!` : turn the identifier argument into a string literal
576 * `concat!` : concatenates a comma-separated list of literals
577 * `concat_idents!` : create a new identifier by concatenating the arguments
579 # Crates and source files
581 Rust is a *compiled* language.
582 Its semantics obey a *phase distinction* between compile-time and run-time.
583 Those semantic rules that have a *static interpretation* govern the success or failure of compilation.
584 We refer to these rules as "static semantics".
585 Semantic rules called "dynamic semantics" govern the behavior of programs at run-time.
586 A program that fails to compile due to violation of a compile-time rule has no defined dynamic semantics; the compiler should halt with an error report, and produce no executable artifact.
588 The compilation model centres on artifacts called _crates_.
589 Each compilation processes a single crate in source form, and if successful, produces a single crate in binary form: either an executable or a library.^[A crate is somewhat
590 analogous to an *assembly* in the ECMA-335 CLI model, a *library* in the
591 SML/NJ Compilation Manager, a *unit* in the Owens and Flatt module system,
592 or a *configuration* in Mesa.]
594 A _crate_ is a unit of compilation and linking, as well as versioning, distribution and runtime loading.
595 A crate contains a _tree_ of nested [module](#modules) scopes.
596 The top level of this tree is a module that is anonymous (from the point of view of paths within the module) and any item within a crate has a canonical [module path](#paths) denoting its location within the crate's module tree.
598 The Rust compiler is always invoked with a single source file as input, and always produces a single output crate.
599 The processing of that source file may result in other source files being loaded as modules.
600 Source files have the extension `.rs`.
602 A Rust source file describes a module, the name and
603 location of which -- in the module tree of the current crate -- are defined
604 from outside the source file: either by an explicit `mod_item` in
605 a referencing source file, or by the name of the crate itself.
607 Each source file contains a sequence of zero or more `item` definitions,
608 and may optionally begin with any number of `attributes` that apply to the containing module.
609 Attributes on the anonymous crate module define important metadata that influences
610 the behavior of the compiler.
614 #[ crate_id = "projx#2.5" ];
616 // Additional metadata attributes
617 #[ desc = "Project X" ];
618 #[ license = "BSD" ];
619 #[ comment = "This is a comment on Project X." ];
621 // Specify the output type
622 #[ crate_type = "lib" ];
625 #[ warn(non_camel_case_types) ];
628 A crate that contains a `main` function can be compiled to an executable.
629 If a `main` function is present, its return type must be [`unit`](#primitive-types) and it must take no arguments.
631 # Items and attributes
633 Crates contain [items](#items),
634 each of which may have some number of [attributes](#attributes) attached to it.
638 ~~~~ {.notrust .ebnf .gram}
639 item : mod_item | fn_item | type_item | struct_item | enum_item
640 | static_item | trait_item | impl_item | extern_block ;
643 An _item_ is a component of a crate; some module items can be defined in crate
644 files, but most are defined in source files. Items are organized within a
645 crate by a nested set of [modules](#modules). Every crate has a single
646 "outermost" anonymous module; all further items within the crate have
647 [paths](#paths) within the module tree of the crate.
649 Items are entirely determined at compile-time, generally remain fixed during
650 execution, and may reside in read-only memory.
652 There are several kinds of item:
654 * [modules](#modules)
655 * [functions](#functions)
656 * [type definitions](#type-definitions)
657 * [structures](#structures)
658 * [enumerations](#enumerations)
659 * [static items](#static-items)
661 * [implementations](#implementations)
663 Some items form an implicit scope for the declaration of sub-items. In other
664 words, within a function or module, declarations of items can (in many cases)
665 be mixed with the statements, control blocks, and similar artifacts that
666 otherwise compose the item body. The meaning of these scoped items is the same
667 as if the item was declared outside the scope -- it is still a static item --
668 except that the item's *path name* within the module namespace is qualified by
669 the name of the enclosing item, or is private to the enclosing item (in the
671 The grammar specifies the exact locations in which sub-item declarations may appear.
675 All items except modules may be *parameterized* by type. Type parameters are
676 given as a comma-separated list of identifiers enclosed in angle brackets
677 (`<...>`), after the name of the item and before its definition.
678 The type parameters of an item are considered "part of the name", not part of the type of the item.
679 A referencing [path](#paths) must (in principle) provide type arguments as a list of comma-separated types enclosed within angle brackets, in order to refer to the type-parameterized item.
680 In practice, the type-inference system can usually infer such argument types from context.
681 There are no general type-parametric types, only type-parametric items.
682 That is, Rust has no notion of type abstraction: there are no first-class "forall" types.
686 ~~~~ {.notrust .ebnf .gram}
687 mod_item : "mod" ident ( ';' | '{' mod '}' );
688 mod : [ view_item | item ] * ;
691 A module is a container for zero or more [view items](#view-items) and zero or
692 more [items](#items). The view items manage the visibility of the items
693 defined within the module, as well as the visibility of names from outside the
694 module when referenced from inside the module.
696 A _module item_ is a module, surrounded in braces, named, and prefixed with
697 the keyword `mod`. A module item introduces a new, named module into the tree
698 of modules making up a crate. Modules can nest arbitrarily.
700 An example of a module:
704 type Complex = (f64, f64);
705 fn sin(f: f64) -> f64 {
709 fn cos(f: f64) -> f64 {
713 fn tan(f: f64) -> f64 {
720 Modules and types share the same namespace.
721 Declaring a named type that has the same name as a module in scope is forbidden:
722 that is, a type definition, trait, struct, enumeration, or type parameter
723 can't shadow the name of a module in scope, or vice versa.
725 A module without a body is loaded from an external file, by default with the same
726 name as the module, plus the `.rs` extension.
727 When a nested submodule is loaded from an external file,
728 it is loaded from a subdirectory path that mirrors the module hierarchy.
731 // Load the `vec` module from `vec.rs`
735 // Load the `local_data` module from `task/local_data.rs`
740 The directories and files used for loading external file modules can be influenced
741 with the `path` attribute.
744 #[path = "task_files"]
746 // Load the `local_data` module from `task_files/tls.rs`
754 ~~~~ {.notrust .ebnf .gram}
755 view_item : extern_crate_decl | use_decl ;
758 A view item manages the namespace of a module.
759 View items do not define new items, but rather, simply change other items' visibility.
760 There are several kinds of view item:
762 * [`extern crate` declarations](#extern-crate-declarations)
763 * [`use` declarations](#use-declarations)
765 ##### Extern crate declarations
767 ~~~~ {.notrust .ebnf .gram}
768 extern_crate_decl : "extern" "crate" ident [ '(' link_attrs ')' ] ? [ '=' string_lit ] ? ;
769 link_attrs : link_attr [ ',' link_attrs ] + ;
770 link_attr : ident '=' literal ;
773 An _`extern crate` declaration_ specifies a dependency on an external crate.
774 The external crate is then bound into the declaring scope as the `ident` provided
775 in the `extern_crate_decl`.
777 The external crate is resolved to a specific `soname` at compile time, and a
778 runtime linkage requirement to that `soname` is passed to the linker for
779 loading at runtime. The `soname` is resolved at compile time by scanning the
780 compiler's library path and matching the optional `crateid` provided as a string literal
781 against the `crateid` attributes that were declared on the external crate when
782 it was compiled. If no `crateid` is provided, a default `name` attribute is
783 assumed, equal to the `ident` given in the `extern_crate_decl`.
785 Four examples of `extern crate` declarations:
790 extern crate std; // equivalent to: extern crate std = "std";
792 extern crate ruststd = "std"; // linking to 'std' under another name
794 extern crate foo = "some/where/rust-foo#foo:1.0"; // a full package ID for external tools
797 ##### Use declarations
799 ~~~~ {.notrust .ebnf .gram}
800 use_decl : "pub" ? "use" ident [ '=' path
803 path_glob : ident [ "::" path_glob ] ?
805 | '{' ident [ ',' ident ] * '}'
808 A _use declaration_ creates one or more local name bindings synonymous
809 with some other [path](#paths).
810 Usually a `use` declaration is used to shorten the path required to refer to a
811 module item. These declarations may appear at the top of [modules](#modules) and
814 *Note*: Unlike in many languages,
815 `use` declarations in Rust do *not* declare linkage dependency with external crates.
816 Rather, [`extern crate` declarations](#extern-crate-declarations) declare linkage dependencies.
818 Use declarations support a number of convenient shortcuts:
820 * Rebinding the target name as a new local name, using the syntax `use x = p::q::r;`.
821 * Simultaneously binding a list of paths differing only in their final element,
822 using the glob-like brace syntax `use a::b::{c,d,e,f};`
823 * Binding all paths matching a given prefix, using the asterisk wildcard syntax `use a::b::*;`
825 An example of `use` declarations:
829 use std::option::{Some, None};
834 // Equivalent to 'std::num::sin(1.0);'
837 // Equivalent to 'foo(~[std::option::Some(1.0), std::option::None]);'
838 foo(~[Some(1.0), None]);
842 Like items, `use` declarations are private to the containing module, by default.
843 Also like items, a `use` declaration can be public, if qualified by the `pub` keyword.
844 Such a `use` declaration serves to _re-export_ a name.
845 A public `use` declaration can therefore _redirect_ some public name to a different target definition:
846 even a definition with a private canonical path, inside a different module.
847 If a sequence of such redirections form a cycle or cannot be resolved unambiguously,
848 they represent a compile-time error.
850 An example of re-exporting:
855 pub use quux::foo::*;
864 In this example, the module `quux` re-exports all of the public names defined in `foo`.
866 Also note that the paths contained in `use` items are relative to the crate root.
867 So, in the previous example, the `use` refers to `quux::foo::*`, and not simply to `foo::*`.
868 This also means that top-level module declarations should be at the crate root if direct usage
869 of the declared modules within `use` items is desired. It is also possible to use `self` and `super`
870 at the beginning of a `use` item to refer to the current and direct parent modules respectively.
871 All rules regarding accessing declared modules in `use` declarations applies to both module declarations
872 and `extern crate` declarations.
874 An example of what will and will not work for `use` items:
877 # #[allow(unused_imports)];
878 use foo::native::start; // good: foo is at the root of the crate
879 use foo::baz::foobaz; // good: foo is at the root of the crate
884 use foo::native::start; // good: foo is at crate root
885 // use native::start; // bad: native is not at the crate root
886 use self::baz::foobaz; // good: self refers to module 'foo'
887 use foo::bar::foobar; // good: foo is at crate root
894 use super::bar::foobar; // good: super refers to module 'foo'
904 A _function item_ defines a sequence of [statements](#statements) and an optional final [expression](#expressions), along with a name and a set of parameters.
905 Functions are declared with the keyword `fn`.
906 Functions declare a set of *input* [*slots*](#memory-slots) as parameters, through which the caller passes arguments into the function, and an *output* [*slot*](#memory-slots) through which the function passes results back to the caller.
908 A function may also be copied into a first class *value*, in which case the
909 value has the corresponding [*function type*](#function-types), and can be
910 used otherwise exactly as a function item (with a minor additional cost of
911 calling the function indirectly).
913 Every control path in a function logically ends with a `return` expression or a
914 diverging expression. If the outermost block of a function has a
915 value-producing expression in its final-expression position, that expression
916 is interpreted as an implicit `return` expression applied to the
919 An example of a function:
922 fn add(x: int, y: int) -> int {
927 As with `let` bindings, function arguments are irrefutable patterns,
928 so any pattern that is valid in a let binding is also valid as an argument.
931 fn first((value, _): (int, int)) -> int { value }
935 #### Generic functions
937 A _generic function_ allows one or more _parameterized types_ to
938 appear in its signature. Each type parameter must be explicitly
939 declared, in an angle-bracket-enclosed, comma-separated list following
943 fn iter<T>(seq: &[T], f: |T|) {
944 for elt in seq.iter() { f(elt); }
946 fn map<T, U>(seq: &[T], f: |T| -> U) -> ~[U] {
948 for elt in seq.iter() { acc.push(f(elt)); }
953 Inside the function signature and body, the name of the type parameter
954 can be used as a type name.
956 When a generic function is referenced, its type is instantiated based
957 on the context of the reference. For example, calling the `iter`
958 function defined above on `[1, 2]` will instantiate type parameter `T`
959 with `int`, and require the closure parameter to have type
962 The type parameters can also be explicitly supplied in a trailing
963 [path](#paths) component after the function name. This might be necessary
964 if there is not sufficient context to determine the type parameters. For
965 example, `mem::size_of::<u32>() == 4`.
967 Since a parameter type is opaque to the generic function, the set of
968 operations that can be performed on it is limited. Values of parameter
969 type can only be moved, not copied.
972 fn id<T>(x: T) -> T { x }
975 Similarly, [trait](#traits) bounds can be specified for type
976 parameters to allow methods with that trait to be called on values
982 Unsafe operations are those that potentially violate the memory-safety guarantees of Rust's static semantics.
984 The following language level features cannot be used in the safe subset of Rust:
986 - Dereferencing a [raw pointer](#pointer-types).
987 - Calling an unsafe function (including an intrinsic or foreign function).
989 ##### Unsafe functions
991 Unsafe functions are functions that are not safe in all contexts and/or for all possible inputs.
992 Such a function must be prefixed with the keyword `unsafe`.
996 A block of code can also be prefixed with the `unsafe` keyword, to permit calling `unsafe` functions
997 or dereferencing raw pointers within a safe function.
999 When a programmer has sufficient conviction that a sequence of potentially unsafe operations is
1000 actually safe, they can encapsulate that sequence (taken as a whole) within an `unsafe` block. The
1001 compiler will consider uses of such code safe, in the surrounding context.
1003 Unsafe blocks are used to wrap foreign libraries, make direct use of hardware or implement features
1004 not directly present in the language. For example, Rust provides the language features necessary to
1005 implement memory-safe concurrency in the language but the implementation of tasks and message
1006 passing is in the standard library.
1008 Rust's type system is a conservative approximation of the dynamic safety requirements, so in some
1009 cases there is a performance cost to using safe code. For example, a doubly-linked list is not a
1010 tree structure and can only be represented with managed or reference-counted pointers in safe code.
1011 By using `unsafe` blocks to represent the reverse links as raw pointers, it can be implemented with
1012 only owned pointers.
1014 ##### Behavior considered unsafe
1016 This is a list of behavior which is forbidden in all Rust code. Type checking provides the guarantee
1017 that these issues are never caused by safe code. An `unsafe` block or function is responsible for
1018 never invoking this behaviour or exposing an API making it possible for it to occur in safe code.
1021 * Dereferencing a null/dangling raw pointer
1022 * Mutating an immutable value/reference
1023 * Reads of [undef](http://llvm.org/docs/LangRef.html#undefined-values) (uninitialized) memory
1024 * Breaking the [pointer aliasing rules](http://llvm.org/docs/LangRef.html#pointer-aliasing-rules)
1025 with raw pointers (a subset of the rules used by C)
1026 * Invoking undefined behavior via compiler intrinsics:
1027 * Indexing outside of the bounds of an object with `std::ptr::offset` (`offset` intrinsic), with
1028 the exception of one byte past the end which is permitted.
1029 * Using `std::ptr::copy_nonoverlapping_memory` (`memcpy32`/`memcpy64` instrinsics) on
1031 * Invalid values in primitive types, even in private fields/locals:
1032 * Dangling/null pointers in non-raw pointers, or slices
1033 * A value other than `false` (0) or `true` (1) in a `bool`
1034 * A discriminant in an `enum` not included in the type definition
1035 * A value in a `char` which is a surrogate or above `char::MAX`
1036 * non-UTF-8 byte sequences in a `str`
1038 ##### Behaviour not considered unsafe
1040 This is a list of behaviour not considered *unsafe* in Rust terms, but that may be undesired.
1043 * Reading data from private fields (`std::repr`, `format!("{:?}", x)`)
1044 * Leaks due to reference count cycles, even in the global heap
1045 * Exiting without calling destructors
1047 * Accessing/modifying the file system
1048 * Unsigned integer overflow (well-defined as wrapping)
1049 * Signed integer overflow (well-defined as two's complement representation wrapping)
1051 #### Diverging functions
1053 A special kind of function can be declared with a `!` character where the
1054 output slot type would normally be. For example:
1057 fn my_err(s: &str) -> ! {
1063 We call such functions "diverging" because they never return a value to the
1064 caller. Every control path in a diverging function must end with a
1065 `fail!()` or a call to another diverging function on every
1066 control path. The `!` annotation does *not* denote a type. Rather, the result
1067 type of a diverging function is a special type called $\bot$ ("bottom") that
1068 unifies with any type. Rust has no syntax for $\bot$.
1070 It might be necessary to declare a diverging function because as mentioned
1071 previously, the typechecker checks that every control path in a function ends
1072 with a [`return`](#return-expressions) or diverging expression. So, if `my_err`
1073 were declared without the `!` annotation, the following code would not
1077 # fn my_err(s: &str) -> ! { fail!() }
1079 fn f(i: int) -> int {
1084 my_err("Bad number!");
1089 This will not compile without the `!` annotation on `my_err`,
1090 since the `else` branch of the conditional in `f` does not return an `int`,
1091 as required by the signature of `f`.
1092 Adding the `!` annotation to `my_err` informs the typechecker that,
1093 should control ever enter `my_err`, no further type judgments about `f` need to hold,
1094 since control will never resume in any context that relies on those judgments.
1095 Thus the return type on `f` only needs to reflect the `if` branch of the conditional.
1098 #### Extern functions
1100 Extern functions are part of Rust's foreign function interface,
1101 providing the opposite functionality to [external blocks](#external-blocks).
1102 Whereas external blocks allow Rust code to call foreign code,
1103 extern functions with bodies defined in Rust code _can be called by foreign
1104 code_. They are defined in the same way as any other Rust function,
1105 except that they have the `extern` modifier.
1108 // Declares an extern fn, the ABI defaults to "C"
1109 extern fn new_vec() -> ~[int] { ~[] }
1111 // Declares an extern fn with "stdcall" ABI
1112 extern "stdcall" fn new_vec_stdcall() -> ~[int] { ~[] }
1115 Unlike normal functions, extern fns have an `extern "ABI" fn()`.
1116 This is the same type as the functions declared in an extern
1120 # extern fn new_vec() -> ~[int] { ~[] }
1121 let fptr: extern "C" fn() -> ~[int] = new_vec;
1124 Extern functions may be called directly from Rust code as Rust uses large,
1125 contiguous stack segments like C.
1127 ### Type definitions
1129 A _type definition_ defines a new name for an existing [type](#types). Type
1130 definitions are declared with the keyword `type`. Every value has a single,
1131 specific type; the type-specified aspects of a value include:
1133 * Whether the value is composed of sub-values or is indivisible.
1134 * Whether the value represents textual or numerical information.
1135 * Whether the value represents integral or floating-point information.
1136 * The sequence of memory operations required to access the value.
1137 * The [kind](#type-kinds) of the type.
1139 For example, the type `(u8, u8)` defines the set of immutable values that are composite pairs,
1140 each containing two unsigned 8-bit integers accessed by pattern-matching and laid out in memory with the `x` component preceding the `y` component.
1144 A _structure_ is a nominal [structure type](#structure-types) defined with the keyword `struct`.
1146 An example of a `struct` item and its use:
1149 struct Point {x: int, y: int}
1150 let p = Point {x: 10, y: 11};
1154 A _tuple structure_ is a nominal [tuple type](#tuple-types), also defined with the keyword `struct`.
1158 struct Point(int, int);
1159 let p = Point(10, 11);
1160 let px: int = match p { Point(x, _) => x };
1163 A _unit-like struct_ is a structure without any fields, defined by leaving off the list of fields entirely.
1164 Such types will have a single value, just like the [unit value `()`](#unit-and-boolean-literals) of the unit type.
1169 let c = [Cookie, Cookie, Cookie, Cookie];
1174 An _enumeration_ is a simultaneous definition of a nominal [enumerated type](#enumerated-types) as well as a set of *constructors*,
1175 that can be used to create or pattern-match values of the corresponding enumerated type.
1177 Enumerations are declared with the keyword `enum`.
1179 An example of an `enum` item and its use:
1187 let mut a: Animal = Dog;
1191 Enumeration constructors can have either named or unnamed fields:
1196 Cat { name: ~str, weight: f64 }
1199 let mut a: Animal = Dog(~"Cocoa", 37.2);
1200 a = Cat{ name: ~"Spotty", weight: 2.7 };
1203 In this example, `Cat` is a _struct-like enum variant_,
1204 whereas `Dog` is simply called an enum variant.
1208 ~~~~ {.notrust .ebnf .gram}
1209 static_item : "static" ident ':' type '=' expr ';' ;
1212 A *static item* is a named _constant value_ stored in the global data section of a crate.
1213 Immutable static items are stored in the read-only data section.
1214 The constant value bound to a static item is, like all constant values, evaluated at compile time.
1215 Static items have the `static` lifetime, which outlives all other lifetimes in a Rust program.
1216 Static items are declared with the `static` keyword.
1217 A static item must have a _constant expression_ giving its definition.
1219 Static items must be explicitly typed.
1220 The type may be ```bool```, ```char```, a number, or a type derived from those primitive types.
1221 The derived types are references with the `static` lifetime,
1222 fixed-size arrays, tuples, and structs.
1225 static BIT1: uint = 1 << 0;
1226 static BIT2: uint = 1 << 1;
1228 static BITS: [uint, ..2] = [BIT1, BIT2];
1229 static STRING: &'static str = "bitstring";
1231 struct BitsNStrings<'a> {
1232 mybits: [uint, ..2],
1236 static bits_n_strings: BitsNStrings<'static> = BitsNStrings {
1242 #### Mutable statics
1244 If a static item is declared with the ```mut``` keyword, then it is allowed to
1245 be modified by the program. One of Rust's goals is to make concurrency bugs hard
1246 to run into, and this is obviously a very large source of race conditions or
1247 other bugs. For this reason, an ```unsafe``` block is required when either
1248 reading or writing a mutable static variable. Care should be taken to ensure
1249 that modifications to a mutable static are safe with respect to other tasks
1250 running in the same process.
1252 Mutable statics are still very useful, however. They can be used with C
1253 libraries and can also be bound from C libraries (in an ```extern``` block).
1256 # fn atomic_add(_: &mut uint, _: uint) -> uint { 2 }
1258 static mut LEVELS: uint = 0;
1260 // This violates the idea of no shared state, and this doesn't internally
1261 // protect against races, so this function is `unsafe`
1262 unsafe fn bump_levels_unsafe1() -> uint {
1268 // Assuming that we have an atomic_add function which returns the old value,
1269 // this function is "safe" but the meaning of the return value may not be what
1270 // callers expect, so it's still marked as `unsafe`
1271 unsafe fn bump_levels_unsafe2() -> uint {
1272 return atomic_add(&mut LEVELS, 1);
1278 A _trait_ describes a set of method types.
1280 Traits can include default implementations of methods,
1281 written in terms of some unknown [`self` type](#self-types);
1282 the `self` type may either be completely unspecified,
1283 or constrained by some other trait.
1285 Traits are implemented for specific types through separate [implementations](#implementations).
1288 # type Surface = int;
1289 # type BoundingBox = int;
1292 fn draw(&self, Surface);
1293 fn bounding_box(&self) -> BoundingBox;
1297 This defines a trait with two methods.
1298 All values that have [implementations](#implementations) of this trait in scope can have their `draw` and `bounding_box` methods called,
1299 using `value.bounding_box()` [syntax](#method-call-expressions).
1301 Type parameters can be specified for a trait to make it generic.
1302 These appear after the trait name, using the same syntax used in [generic functions](#generic-functions).
1306 fn len(&self) -> uint;
1307 fn elt_at(&self, n: uint) -> T;
1308 fn iter(&self, |T|);
1312 Generic functions may use traits as _bounds_ on their type parameters.
1313 This will have two effects: only types that have the trait may instantiate the parameter,
1314 and within the generic function,
1315 the methods of the trait can be called on values that have the parameter's type.
1319 # type Surface = int;
1320 # trait Shape { fn draw(&self, Surface); }
1322 fn draw_twice<T: Shape>(surface: Surface, sh: T) {
1328 Traits also define an [object type](#object-types) with the same name as the trait.
1329 Values of this type are created by [casting](#type-cast-expressions) pointer values
1330 (pointing to a type for which an implementation of the given trait is in scope)
1331 to pointers to the trait name, used as a type.
1335 # impl Shape for int { }
1338 let myshape: ~Shape = ~mycircle as ~Shape;
1341 The resulting value is a managed box containing the value that was cast,
1342 along with information that identifies the methods of the implementation that was used.
1343 Values with a trait type can have [methods called](#method-call-expressions) on them,
1344 for any method in the trait,
1345 and can be used to instantiate type parameters that are bounded by the trait.
1347 Trait methods may be static,
1348 which means that they lack a `self` argument.
1349 This means that they can only be called with function call syntax (`f(x)`)
1350 and not method call syntax (`obj.f()`).
1351 The way to refer to the name of a static method is to qualify it with the trait name,
1352 treating the trait name like a module.
1357 fn from_int(n: int) -> Self;
1360 fn from_int(n: int) -> f64 { n as f64 }
1362 let x: f64 = Num::from_int(42);
1365 Traits may inherit from other traits. For example, in
1368 trait Shape { fn area() -> f64; }
1369 trait Circle : Shape { fn radius() -> f64; }
1372 the syntax `Circle : Shape` means that types that implement `Circle` must also have an implementation for `Shape`.
1373 Multiple supertraits are separated by spaces, `trait Circle : Shape Eq { }`.
1374 In an implementation of `Circle` for a given type `T`, methods can refer to `Shape` methods,
1375 since the typechecker checks that any type with an implementation of `Circle` also has an implementation of `Shape`.
1377 In type-parameterized functions,
1378 methods of the supertrait may be called on values of subtrait-bound type parameters.
1379 Referring to the previous example of `trait Circle : Shape`:
1382 # trait Shape { fn area(&self) -> f64; }
1383 # trait Circle : Shape { fn radius(&self) -> f64; }
1384 fn radius_times_area<T: Circle>(c: T) -> f64 {
1385 // `c` is both a Circle and a Shape
1386 c.radius() * c.area()
1390 Likewise, supertrait methods may also be called on trait objects.
1393 # trait Shape { fn area(&self) -> f64; }
1394 # trait Circle : Shape { fn radius(&self) -> f64; }
1395 # impl Shape for int { fn area(&self) -> f64 { 0.0 } }
1396 # impl Circle for int { fn radius(&self) -> f64 { 0.0 } }
1399 let mycircle: Circle = ~mycircle as ~Circle;
1400 let nonsense = mycircle.radius() * mycircle.area();
1405 An _implementation_ is an item that implements a [trait](#traits) for a specific type.
1407 Implementations are defined with the keyword `impl`.
1410 # struct Point {x: f64, y: f64};
1411 # type Surface = int;
1412 # struct BoundingBox {x: f64, y: f64, width: f64, height: f64};
1413 # trait Shape { fn draw(&self, Surface); fn bounding_box(&self) -> BoundingBox; }
1414 # fn do_draw_circle(s: Surface, c: Circle) { }
1421 impl Shape for Circle {
1422 fn draw(&self, s: Surface) { do_draw_circle(s, *self); }
1423 fn bounding_box(&self) -> BoundingBox {
1424 let r = self.radius;
1425 BoundingBox{x: self.center.x - r, y: self.center.y - r,
1426 width: 2.0 * r, height: 2.0 * r}
1431 It is possible to define an implementation without referring to a trait.
1432 The methods in such an implementation can only be used
1433 as direct calls on the values of the type that the implementation targets.
1434 In such an implementation, the trait type and `for` after `impl` are omitted.
1435 Such implementations are limited to nominal types (enums, structs),
1436 and the implementation must appear in the same module or a sub-module as the `self` type.
1438 When a trait _is_ specified in an `impl`,
1439 all methods declared as part of the trait must be implemented,
1440 with matching types and type parameter counts.
1442 An implementation can take type parameters,
1443 which can be different from the type parameters taken by the trait it implements.
1444 Implementation parameters are written after the `impl` keyword.
1449 impl<T> Seq<T> for ~[T] {
1452 impl Seq<bool> for u32 {
1453 /* Treat the integer as a sequence of bits */
1459 ~~~~ {.notrust .ebnf .gram}
1460 extern_block_item : "extern" '{' extern_block '} ;
1461 extern_block : [ foreign_fn ] * ;
1464 External blocks form the basis for Rust's foreign function interface.
1465 Declarations in an external block describe symbols
1466 in external, non-Rust libraries.
1468 Functions within external blocks
1469 are declared in the same way as other Rust functions,
1470 with the exception that they may not have a body
1471 and are instead terminated by a semicolon.
1474 # use std::libc::{c_char, FILE};
1477 fn fopen(filename: *c_char, mode: *c_char) -> *FILE;
1481 Functions within external blocks may be called by Rust code,
1482 just like functions defined in Rust.
1483 The Rust compiler automatically translates
1484 between the Rust ABI and the foreign ABI.
1486 A number of [attributes](#attributes) control the behavior of external
1489 By default external blocks assume that the library they are calling
1490 uses the standard C "cdecl" ABI. Other ABIs may be specified using
1491 an `abi` string, as shown here:
1494 // Interface to the Windows API
1495 extern "stdcall" { }
1498 The `link` attribute allows the name of the library to be specified. When
1499 specified the compiler will attempt to link against the native library of the
1503 #[link(name = "crypto")]
1507 The type of a function
1508 declared in an extern block
1509 is `extern "abi" fn(A1, ..., An) -> R`,
1510 where `A1...An` are the declared types of its arguments
1511 and `R` is the decalred return type.
1513 ## Visibility and Privacy
1515 These two terms are often used interchangeably, and what they are attempting to
1516 convey is the answer to the question "Can this item be used at this location?"
1518 Rust's name resolution operates on a global hierarchy of namespaces. Each level
1519 in the hierarchy can be thought of as some item. The items are one of those
1520 mentioned above, but also include external crates. Declaring or defining a new
1521 module can be thought of as inserting a new tree into the hierarchy at the
1522 location of the definition.
1524 To control whether interfaces can be used across modules, Rust checks each use
1525 of an item to see whether it should be allowed or not. This is where privacy
1526 warnings are generated, or otherwise "you used a private item of another module
1527 and weren't allowed to."
1529 By default, everything in rust is *private*, with two exceptions. The first
1530 exception is that struct fields are public by default (but the struct itself is
1531 still private by default), and the remaining exception is that enum variants in
1532 a `pub` enum are the default visibility of the enum container itself.. You are
1533 allowed to alter this default visibility with the `pub` keyword (or `priv`
1534 keyword for struct fields and enum variants). When an item is declared as `pub`,
1535 it can be thought of as being accessible to the outside world. For example:
1539 // Declare a private struct
1542 // Declare a public struct with a private field
1547 // Declare a public enum with public and private variants
1549 PubliclyAccessibleState,
1550 priv PrivatelyAccessibleState
1554 With the notion of an item being either public or private, Rust allows item
1555 accesses in two cases:
1557 1. If an item is public, then it can be used externally through any of its
1559 2. If an item is private, it may be accessed by the current module and its
1562 These two cases are surprisingly powerful for creating module hierarchies
1563 exposing public APIs while hiding internal implementation details. To help
1564 explain, here's a few use cases and what they would entail.
1566 * A library developer needs to expose functionality to crates which link against
1567 their library. As a consequence of the first case, this means that anything
1568 which is usable externally must be `pub` from the root down to the destination
1569 item. Any private item in the chain will disallow external accesses.
1571 * A crate needs a global available "helper module" to itself, but it doesn't
1572 want to expose the helper module as a public API. To accomplish this, the root
1573 of the crate's hierarchy would have a private module which then internally has
1574 a "public api". Because the entire crate is a descendant of the root, then the
1575 entire local crate can access this private module through the second case.
1577 * When writing unit tests for a module, it's often a common idiom to have an
1578 immediate child of the module to-be-tested named `mod test`. This module could
1579 access any items of the parent module through the second case, meaning that
1580 internal implementation details could also be seamlessly tested from the child
1583 In the second case, it mentions that a private item "can be accessed" by the
1584 current module and its descendants, but the exact meaning of accessing an item
1585 depends on what the item is. Accessing a module, for example, would mean looking
1586 inside of it (to import more items). On the other hand, accessing a function
1587 would mean that it is invoked. Additionally, path expressions and import
1588 statements are considered to access an item in the sense that the
1589 import/expression is only valid if the destination is in the current visibility
1592 Here's an example of a program which exemplifies the three cases outlined above.
1595 // This module is private, meaning that no external crate can access this
1596 // module. Because it is private at the root of this current crate, however, any
1597 // module in the crate may access any publicly visible item in this module.
1598 mod crate_helper_module {
1600 // This function can be used by anything in the current crate
1601 pub fn crate_helper() {}
1603 // This function *cannot* be used by anything else in the crate. It is not
1604 // publicly visible outside of the `crate_helper_module`, so only this
1605 // current module and its descendants may access it.
1606 fn implementation_detail() {}
1609 // This function is "public to the root" meaning that it's available to external
1610 // crates linking against this one.
1611 pub fn public_api() {}
1613 // Similarly to 'public_api', this module is public so external crates may look
1616 use crate_helper_module;
1618 pub fn my_method() {
1619 // Any item in the local crate may invoke the helper module's public
1620 // interface through a combination of the two rules above.
1621 crate_helper_module::crate_helper();
1624 // This function is hidden to any module which is not a descendant of
1626 fn my_implementation() {}
1632 fn test_my_implementation() {
1633 // Because this module is a descendant of `submodule`, it's allowed
1634 // to access private items inside of `submodule` without a privacy
1636 super::my_implementation();
1644 For a rust program to pass the privacy checking pass, all paths must be valid
1645 accesses given the two rules above. This includes all use statements,
1646 expressions, types, etc.
1648 ### Re-exporting and Visibility
1650 Rust allows publicly re-exporting items through a `pub use` directive. Because
1651 this is a public directive, this allows the item to be used in the current
1652 module through the rules above. It essentially allows public access into the
1653 re-exported item. For example, this program is valid:
1656 pub use api = self::implementation;
1658 mod implementation {
1665 This means that any external crate referencing `implementation::f` would receive
1666 a privacy violation, while the path `api::f` would be allowed.
1668 When re-exporting a private item, it can be thought of as allowing the "privacy
1669 chain" being short-circuited through the reexport instead of passing through the
1670 namespace hierarchy as it normally would.
1672 ### Glob imports and Visibility
1674 Currently glob imports are considered an "experimental" language feature. For
1675 sanity purpose along with helping the implementation, glob imports will only
1676 import public items from their destination, not private items.
1678 > **Note:** This is subject to change, glob exports may be removed entirely or
1679 > they could possibly import private items for a privacy error to later be
1680 > issued if the item is used.
1684 ~~~~ {.notrust .ebnf .gram}
1685 attribute : '#' '[' attr_list ']' ;
1686 attr_list : attr [ ',' attr_list ]*
1687 attr : ident [ '=' literal
1688 | '(' attr_list ')' ] ? ;
1691 Static entities in Rust -- crates, modules and items -- may have _attributes_
1692 applied to them. ^[Attributes in Rust are modeled on Attributes in ECMA-335,
1694 An attribute is a general, free-form metadatum that is interpreted according to name, convention, and language and compiler version.
1695 Attributes may appear as any of
1697 * A single identifier, the attribute name
1698 * An identifier followed by the equals sign '=' and a literal, providing a key/value pair
1699 * An identifier followed by a parenthesized list of sub-attribute arguments
1701 Attributes terminated by a semi-colon apply to the entity that the attribute is declared
1702 within. Attributes that are not terminated by a semi-colon apply to the next entity.
1704 An example of attributes:
1707 // General metadata applied to the enclosing module or crate.
1710 // A function marked as a unit test
1716 // A conditionally-compiled module
1717 #[cfg(target_os="linux")]
1722 // A lint attribute used to suppress a warning/error
1723 #[allow(non_camel_case_types)]
1724 pub type int8_t = i8;
1727 > **Note:** In future versions of Rust, user-provided extensions to the compiler
1728 > will be able to interpret attributes. When this facility is provided, the
1729 > compiler will distinguish between language-reserved and user-available
1732 At present, only the Rust compiler interprets attributes, so all attribute names
1733 are effectively reserved. Some significant attributes include:
1735 * The `doc` attribute, for documenting code in-place.
1736 * The `cfg` attribute, for conditional-compilation by build-configuration (see
1737 [Conditional compilation](#conditional-compilation)).
1738 * The `crate_id` attribute, for describing the package ID of a crate.
1739 * The `lang` attribute, for custom definitions of traits and functions that are
1740 known to the Rust compiler (see [Language items](#language-items)).
1741 * The `link` attribute, for describing linkage metadata for a extern blocks.
1742 * The `test` attribute, for marking functions as unit tests.
1743 * The `allow`, `warn`, `forbid`, and `deny` attributes, for
1744 controlling lint checks (see [Lint check attributes](#lint-check-attributes)).
1745 * The `deriving` attribute, for automatically generating implementations of
1747 * The `inline` attribute, for expanding functions at caller location (see
1748 [Inline attributes](#inline-attributes)).
1749 * The `static_assert` attribute, for asserting that a static bool is true at
1751 * The `thread_local` attribute, for defining a `static mut` as a thread-local.
1752 Note that this is only a low-level building block, and is not local to a
1753 *task*, nor does it provide safety.
1755 Other attributes may be added or removed during development of the language.
1757 ### Conditional compilation
1759 Sometimes one wants to have different compiler outputs from the same code,
1760 depending on build target, such as targeted operating system, or to enable
1763 There are two kinds of configuration options, one that is either defined or not
1764 (`#[cfg(foo)]`), and the other that contains a string that can be checked
1765 against (`#[cfg(bar = "baz")]` (currently only compiler-defined configuration
1766 options can have the latter form).
1769 // The function is only included in the build when compiling for OSX
1770 #[cfg(target_os = "macos")]
1775 // This function is only included when either foo or bar is defined
1778 fn needs_foo_or_bar() {
1782 // This function is only included when compiling for a unixish OS with a 32-bit
1784 #[cfg(unix, target_word_size = "32")]
1785 fn on_32bit_unix() {
1790 This illustrates some conditional compilation can be achieved using the
1791 `#[cfg(...)]` attribute. Note that `#[cfg(foo, bar)]` is a condition that needs
1792 both `foo` and `bar` to be defined while `#[cfg(foo)] #[cfg(bar)]` only needs
1793 one of `foo` and `bar` to be defined (this resembles in the disjunctive normal
1794 form). Additionally, one can reverse a condition by enclosing it in a
1795 `not(...)`, like e. g. `#[cfg(not(target_os = "win32"))]`.
1797 To pass a configuration option which triggers a `#[cfg(identifier)]` one can use
1798 `rustc --cfg identifier`. In addition to that, the following configurations are
1799 pre-defined by the compiler:
1801 * `target_arch = "..."`. Target CPU architecture, such as `"x86"`, `"x86_64"`
1802 `"mips"`, or `"arm"`.
1803 * `target_endian = "..."`. Endianness of the target CPU, either `"little"` or
1805 * `target_family = "..."`. Operating system family of the target, e. g.
1806 `"unix"` or `"windows"`. The value of this configuration option is defined as
1807 a configuration itself, like `unix` or `windows`.
1808 * `target_os = "..."`. Operating system of the target, examples include
1809 `"win32"`, `"macos"`, `"linux"`, `"android"` or `"freebsd"`.
1810 * `target_word_size = "..."`. Target word size in bits. This is set to `"32"`
1811 for 32-bit CPU targets, and likewise set to `"64"` for 64-bit CPU targets.
1812 * `test`. Only set in test builds (`rustc --test`).
1813 * `unix`. See `target_family`.
1814 * `windows`. See `target_family`.
1816 ### Lint check attributes
1818 A lint check names a potentially undesirable coding pattern, such as
1819 unreachable code or omitted documentation, for the static entity to
1820 which the attribute applies.
1822 For any lint check `C`:
1824 * `warn(C)` warns about violations of `C` but continues compilation,
1825 * `deny(C)` signals an error after encountering a violation of `C`,
1826 * `allow(C)` overrides the check for `C` so that violations will go
1828 * `forbid(C)` is the same as `deny(C)`, but also forbids uses of
1829 `allow(C)` within the entity.
1831 The lint checks supported by the compiler can be found via `rustc -W help`,
1832 along with their default settings.
1836 // Missing documentation is ignored here
1837 #[allow(missing_doc)]
1838 pub fn undocumented_one() -> int { 1 }
1840 // Missing documentation signals a warning here
1841 #[warn(missing_doc)]
1842 pub fn undocumented_too() -> int { 2 }
1844 // Missing documentation signals an error here
1845 #[deny(missing_doc)]
1846 pub fn undocumented_end() -> int { 3 }
1850 This example shows how one can use `allow` and `warn` to toggle
1851 a particular check on and off.
1854 #[warn(missing_doc)]
1856 #[allow(missing_doc)]
1858 // Missing documentation is ignored here
1859 pub fn undocumented_one() -> int { 1 }
1861 // Missing documentation signals a warning here,
1862 // despite the allow above.
1863 #[warn(missing_doc)]
1864 pub fn undocumented_two() -> int { 2 }
1867 // Missing documentation signals a warning here
1868 pub fn undocumented_too() -> int { 3 }
1872 This example shows how one can use `forbid` to disallow uses
1873 of `allow` for that lint check.
1876 #[forbid(missing_doc)]
1878 // Attempting to toggle warning signals an error here
1879 #[allow(missing_doc)]
1881 pub fn undocumented_too() -> int { 2 }
1887 Some primitive Rust operations are defined in Rust code,
1888 rather than being implemented directly in C or assembly language.
1889 The definitions of these operations have to be easy for the compiler to find.
1890 The `lang` attribute makes it possible to declare these operations.
1891 For example, the `str` module in the Rust standard library defines the string equality function:
1895 pub fn eq_slice(a: &str, b: &str) -> bool {
1900 The name `str_eq` has a special meaning to the Rust compiler,
1901 and the presence of this definition means that it will use this definition
1902 when generating calls to the string equality function.
1904 A complete list of the built-in language items follows:
1909 : Cannot be mutated.
1911 : Are uniquely owned.
1913 : Contain references.
1917 : Elements can be added (for example, integers and floats).
1919 : Elements can be subtracted.
1921 : Elements can be multiplied.
1923 : Elements have a division operation.
1925 : Elements have a remainder operation.
1927 : Elements can be negated arithmetically.
1929 : Elements can be negated logically.
1931 : Elements have an exclusive-or operation.
1933 : Elements have a bitwise `and` operation.
1935 : Elements have a bitwise `or` operation.
1937 : Elements have a left shift operation.
1939 : Elements have a right shift operation.
1941 : Elements can be indexed.
1943 : Elements can be compared for equality.
1945 : Elements have a partial ordering.
1950 : Compare two strings for equality.
1952 : Compare two owned strings for equality.
1954 : Destroy a box before freeing it.
1956 : Generically print a string representation of any type.
1958 : Abort the program with an error.
1960 : Abort the program with a bounds check error.
1962 : Allocate memory on the exchange heap.
1964 : Free memory that was allocated on the exchange heap.
1966 : Allocate memory on the managed heap.
1968 : Free memory that was allocated on the managed heap.
1970 : Create an immutable reference to a mutable value.
1972 : Release a reference created with `return_to_mut`
1973 `check_not_borrowed`
1974 : Fail if a value has existing references to it.
1976 : Return a new unique string
1977 containing a copy of the contents of a unique string.
1979 > **Note:** This list is likely to become out of date. We should auto-generate it
1980 > from `librustc/middle/lang_items.rs`.
1982 ### Inline attributes
1984 The inline attribute is used to suggest to the compiler to perform an inline
1985 expansion and place a copy of the function in the caller rather than generating
1986 code to call the function where it is defined.
1988 The compiler automatically inlines functions based on internal heuristics.
1989 Incorrectly inlining functions can actually making the program slower, so it
1990 should be used with care.
1992 `#[inline]` and `#[inline(always)]` always causes the function to be serialized
1993 into crate metadata to allow cross-crate inlining.
1995 There are three different types of inline attributes:
1997 * `#[inline]` hints the compiler to perform an inline expansion.
1998 * `#[inline(always)]` asks the compiler to always perform an inline expansion.
1999 * `#[inline(never)]` asks the compiler to never perform an inline expansion.
2003 The `deriving` attribute allows certain traits to be automatically
2004 implemented for data structures. For example, the following will
2005 create an `impl` for the `Eq` and `Clone` traits for `Foo`, the type
2006 parameter `T` will be given the `Eq` or `Clone` constraints for the
2010 #[deriving(Eq, Clone)]
2017 The generated `impl` for `Eq` is equivalent to
2020 # struct Foo<T> { a: int, b: T }
2021 impl<T: Eq> Eq for Foo<T> {
2022 fn eq(&self, other: &Foo<T>) -> bool {
2023 self.a == other.a && self.b == other.b
2026 fn ne(&self, other: &Foo<T>) -> bool {
2027 self.a != other.a || self.b != other.b
2032 Supported traits for `deriving` are:
2034 * Comparison traits: `Eq`, `TotalEq`, `Ord`, `TotalOrd`.
2035 * Serialization: `Encodable`, `Decodable`. These require `serialize`.
2036 * `Clone`, to create `T` from `&T` via a copy.
2037 * `Hash`, to iterate over the bytes in a data type.
2038 * `Rand`, to create a random instance of a data type.
2039 * `Default`, to create an empty instance of a data type.
2040 * `Zero`, to create an zero instance of a numeric data type.
2041 * `FromPrimitive`, to create an instance from a numeric primitive.
2042 * `Show`, to format a value using the `{}` formatter.
2045 One can indicate the stability of an API using the following attributes:
2047 * `deprecated`: This item should no longer be used, e.g. it has been
2048 replaced. No guarantee of backwards-compatibility.
2049 * `experimental`: This item was only recently introduced or is
2050 otherwise in a state of flux. It may change significantly, or even
2051 be removed. No guarantee of backwards-compatibility.
2052 * `unstable`: This item is still under development, but requires more
2053 testing to be considered stable. No guarantee of backwards-compatibility.
2054 * `stable`: This item is considered stable, and will not change
2055 significantly. Guarantee of backwards-compatibility.
2056 * `frozen`: This item is very stable, and is unlikely to
2057 change. Guarantee of backwards-compatibility.
2058 * `locked`: This item will never change unless a serious bug is
2059 found. Guarantee of backwards-compatibility.
2061 These levels are directly inspired by
2062 [Node.js' "stability index"](http://nodejs.org/api/documentation.html).
2064 There are lints for disallowing items marked with certain levels:
2065 `deprecated`, `experimental` and `unstable`; the first two will warn
2066 by default. Items with not marked with a stability are considered to
2067 be unstable for the purposes of the lint. One can give an optional
2068 string that will be displayed when the lint flags the use of an item.
2073 #[deprecated="replaced by `best`"]
2075 // delete everything
2079 // delete fewer things
2088 bad(); // "warning: use of deprecated item: replaced by `best`"
2090 better(); // "warning: use of unmarked item"
2092 best(); // no warning
2096 > **Note:** Currently these are only checked when applied to
2097 > individual functions, structs, methods and enum variants, *not* to
2098 > entire modules, traits, impls or enums themselves.
2100 ### Compiler Features
2102 Certain aspects of Rust may be implemented in the compiler, but they're not
2103 necessarily ready for every-day use. These features are often of "prototype
2104 quality" or "almost production ready", but may not be stable enough to be
2105 considered a full-fleged language feature.
2107 For this reason, rust recognizes a special crate-level attribute of the form:
2110 #[feature(feature1, feature2, feature3)]
2113 This directive informs the compiler that the feature list: `feature1`,
2114 `feature2`, and `feature3` should all be enabled. This is only recognized at a
2115 crate-level, not at a module-level. Without this directive, all features are
2116 considered off, and using the features will result in a compiler error.
2118 The currently implemented features of the compiler are:
2120 * `macro_rules` - The definition of new macros. This does not encompass
2121 macro-invocation, that is always enabled by default, this only
2122 covers the definition of new macros. There are currently
2123 various problems with invoking macros, how they interact with
2124 their environment, and possibly how they are used outside of
2125 location in which they are defined. Macro definitions are
2126 likely to change slightly in the future, so they are currently
2127 hidden behind this feature.
2129 * `globs` - Importing everything in a module through `*`. This is currently a
2130 large source of bugs in name resolution for Rust, and it's not clear
2131 whether this will continue as a feature or not. For these reasons,
2132 the glob import statement has been hidden behind this feature flag.
2134 * `struct_variant` - Structural enum variants (those with named fields). It is
2135 currently unknown whether this style of enum variant is as
2136 fully supported as the tuple-forms, and it's not certain
2137 that this style of variant should remain in the language.
2138 For now this style of variant is hidden behind a feature
2141 * `once_fns` - Onceness guarantees a closure is only executed once. Defining a
2142 closure as `once` is unlikely to be supported going forward. So
2143 they are hidden behind this feature until they are to be removed.
2145 * `managed_boxes` - Usage of `@` pointers is gated due to many
2146 planned changes to this feature. In the past, this has meant
2147 "a GC pointer", but the current implementation uses
2148 reference counting and will likely change drastically over
2149 time. Additionally, the `@` syntax will no longer be used to
2152 * `asm` - The `asm!` macro provides a means for inline assembly. This is often
2153 useful, but the exact syntax for this feature along with its semantics
2154 are likely to change, so this macro usage must be opted into.
2156 * `non_ascii_idents` - The compiler supports the use of non-ascii identifiers,
2157 but the implementation is a little rough around the
2158 edges, so this can be seen as an experimental feature for
2159 now until the specification of identifiers is fully
2162 * `thread_local` - The usage of the `#[thread_local]` attribute is experimental
2163 and should be seen as unstable. This attribute is used to
2164 declare a `static` as being unique per-thread leveraging
2165 LLVM's implementation which works in concert with the kernel
2166 loader and dynamic linker. This is not necessarily available
2167 on all platforms, and usage of it is discouraged (rust
2168 focuses more on task-local data instead of thread-local
2171 * `link_args` - This attribute is used to specify custom flags to the linker,
2172 but usage is strongly discouraged. The compiler's usage of the
2173 system linker is not guaranteed to continue in the future, and
2174 if the system linker is not used then specifying custom flags
2175 doesn't have much meaning.
2177 If a feature is promoted to a language feature, then all existing programs will
2178 start to receive compilation warnings about #[feature] directives which enabled
2179 the new feature (because the directive is no longer necessary). However, if
2180 a feature is decided to be removed from the language, errors will be issued (if
2181 there isn't a parser error first). The directive in this case is no longer
2182 necessary, and it's likely that existing code will break if the feature isn't
2185 If a unknown feature is found in a directive, it results in a compiler error. An
2186 unknown feature is one which has never been recognized by the compiler.
2188 # Statements and expressions
2190 Rust is _primarily_ an expression language. This means that most forms of
2191 value-producing or effect-causing evaluation are directed by the uniform
2192 syntax category of _expressions_. Each kind of expression can typically _nest_
2193 within each other kind of expression, and rules for evaluation of expressions
2194 involve specifying both the value produced by the expression and the order in
2195 which its sub-expressions are themselves evaluated.
2197 In contrast, statements in Rust serve _mostly_ to contain and explicitly
2198 sequence expression evaluation.
2202 A _statement_ is a component of a block, which is in turn a component of an
2203 outer [expression](#expressions) or [function](#functions).
2205 Rust has two kinds of statement:
2206 [declaration statements](#declaration-statements) and
2207 [expression statements](#expression-statements).
2209 ### Declaration statements
2211 A _declaration statement_ is one that introduces one or more *names* into the enclosing statement block.
2212 The declared names may denote new slots or new items.
2214 #### Item declarations
2216 An _item declaration statement_ has a syntactic form identical to an
2217 [item](#items) declaration within a module. Declaring an item -- a function,
2218 enumeration, structure, type, static, trait, implementation or module -- locally
2219 within a statement block is simply a way of restricting its scope to a narrow
2220 region containing all of its uses; it is otherwise identical in meaning to
2221 declaring the item outside the statement block.
2223 Note: there is no implicit capture of the function's dynamic environment when
2224 declaring a function-local item.
2226 #### Slot declarations
2228 ~~~~ {.notrust .ebnf .gram}
2229 let_decl : "let" pat [':' type ] ? [ init ] ? ';' ;
2230 init : [ '=' ] expr ;
2233 A _slot declaration_ introduces a new set of slots, given by a pattern.
2234 The pattern may be followed by a type annotation, and/or an initializer expression.
2235 When no type annotation is given, the compiler will infer the type,
2236 or signal an error if insufficient type information is available for definite inference.
2237 Any slots introduced by a slot declaration are visible from the point of declaration until the end of the enclosing block scope.
2239 ### Expression statements
2241 An _expression statement_ is one that evaluates an [expression](#expressions)
2242 and ignores its result.
2243 The type of an expression statement `e;` is always `()`, regardless of the type of `e`.
2244 As a rule, an expression statement's purpose is to trigger the effects of evaluating its expression.
2248 An expression may have two roles: it always produces a *value*, and it may have *effects*
2249 (otherwise known as "side effects").
2250 An expression *evaluates to* a value, and has effects during *evaluation*.
2251 Many expressions contain sub-expressions (operands).
2252 The meaning of each kind of expression dictates several things:
2253 * Whether or not to evaluate the sub-expressions when evaluating the expression
2254 * The order in which to evaluate the sub-expressions
2255 * How to combine the sub-expressions' values to obtain the value of the expression.
2257 In this way, the structure of expressions dictates the structure of execution.
2258 Blocks are just another kind of expression,
2259 so blocks, statements, expressions, and blocks again can recursively nest inside each other
2260 to an arbitrary depth.
2262 #### Lvalues, rvalues and temporaries
2264 Expressions are divided into two main categories: _lvalues_ and _rvalues_.
2265 Likewise within each expression, sub-expressions may occur in _lvalue context_ or _rvalue context_.
2266 The evaluation of an expression depends both on its own category and the context it occurs within.
2268 An lvalue is an expression that represents a memory location. These
2269 expressions are [paths](#path-expressions) (which refer to local
2270 variables, function and method arguments, or static variables),
2271 dereferences (`*expr`), [indexing expressions](#index-expressions)
2272 (`expr[expr]`), and [field references](#field-expressions) (`expr.f`).
2273 All other expressions are rvalues.
2275 The left operand of an [assignment](#assignment-expressions) or
2276 [compound-assignment](#compound-assignment-expressions) expression is an lvalue context,
2277 as is the single operand of a unary [borrow](#unary-operator-expressions).
2278 All other expression contexts are rvalue contexts.
2280 When an lvalue is evaluated in an _lvalue context_, it denotes a memory location;
2281 when evaluated in an _rvalue context_, it denotes the value held _in_ that memory location.
2283 When an rvalue is used in lvalue context, a temporary un-named lvalue is created and used instead.
2284 A temporary's lifetime equals the largest lifetime of any reference that points to it.
2286 #### Moved and copied types
2288 When a [local variable](#memory-slots) is used
2289 as an [rvalue](#lvalues-rvalues-and-temporaries)
2290 the variable will either be moved or copied, depending on its type.
2291 For types that contain [owning pointers](#pointer-types)
2292 or values that implement the special trait `Drop`,
2293 the variable is moved.
2294 All other types are copied.
2296 ### Literal expressions
2298 A _literal expression_ consists of one of the [literal](#literals)
2299 forms described earlier. It directly describes a number, character,
2300 string, boolean value, or the unit value.
2304 "hello"; // string type
2305 '5'; // character type
2309 ### Path expressions
2311 A [path](#paths) used as an expression context denotes either a local variable or an item.
2312 Path expressions are [lvalues](#lvalues-rvalues-and-temporaries).
2314 ### Tuple expressions
2316 Tuples are written by enclosing one or more comma-separated
2317 expressions in parentheses. They are used to create [tuple-typed](#tuple-types)
2326 ### Structure expressions
2328 ~~~~ {.notrust .ebnf .gram}
2329 struct_expr : expr_path '{' ident ':' expr
2330 [ ',' ident ':' expr ] *
2333 [ ',' expr ] * ')' |
2337 There are several forms of structure expressions.
2338 A _structure expression_ consists of the [path](#paths) of a [structure item](#structures),
2339 followed by a brace-enclosed list of one or more comma-separated name-value pairs,
2340 providing the field values of a new instance of the structure.
2341 A field name can be any identifier, and is separated from its value expression by a colon.
2342 The location denoted by a structure field is mutable if and only if the enclosing structure is mutable.
2344 A _tuple structure expression_ consists of the [path](#paths) of a [structure item](#structures),
2345 followed by a parenthesized list of one or more comma-separated expressions
2346 (in other words, the path of a structure item followed by a tuple expression).
2347 The structure item must be a tuple structure item.
2349 A _unit-like structure expression_ consists only of the [path](#paths) of a [structure item](#structures).
2351 The following are examples of structure expressions:
2354 # struct Point { x: f64, y: f64 }
2355 # struct TuplePoint(f64, f64);
2356 # mod game { pub struct User<'a> { name: &'a str, age: uint, score: uint } }
2357 # struct Cookie; fn some_fn<T>(t: T) {}
2358 Point {x: 10.0, y: 20.0};
2359 TuplePoint(10.0, 20.0);
2360 let u = game::User {name: "Joe", age: 35, score: 100_000};
2361 some_fn::<Cookie>(Cookie);
2364 A structure expression forms a new value of the named structure type.
2365 Note that for a given *unit-like* structure type, this will always be the same value.
2367 A structure expression can terminate with the syntax `..` followed by an expression to denote a functional update.
2368 The expression following `..` (the base) must have the same structure type as the new structure type being formed.
2369 The entire expression denotes the result of allocating a new structure
2370 (with the same type as the base expression)
2371 with the given values for the fields that were explicitly specified
2372 and the values in the base record for all other fields.
2375 # struct Point3d { x: int, y: int, z: int }
2376 let base = Point3d {x: 1, y: 2, z: 3};
2377 Point3d {y: 0, z: 10, .. base};
2380 ### Block expressions
2382 ~~~~ {.notrust .ebnf .gram}
2383 block_expr : '{' [ view_item ] *
2384 [ stmt ';' | item ] *
2388 A _block expression_ is similar to a module in terms of the declarations that
2389 are possible. Each block conceptually introduces a new namespace scope. View
2390 items can bring new names into scopes and declared items are in scope for only
2393 A block will execute each statement sequentially, and then execute the
2394 expression (if given). If the final expression is omitted, the type and return
2395 value of the block are `()`, but if it is provided, the type and return value
2396 of the block are that of the expression itself.
2398 ### Method-call expressions
2400 ~~~~ {.notrust .ebnf .gram}
2401 method_call_expr : expr '.' ident paren_expr_list ;
2404 A _method call_ consists of an expression followed by a single dot, an identifier, and a parenthesized expression-list.
2405 Method calls are resolved to methods on specific traits,
2406 either statically dispatching to a method if the exact `self`-type of the left-hand-side is known,
2407 or dynamically dispatching if the left-hand-side expression is an indirect [object type](#object-types).
2409 ### Field expressions
2411 ~~~~ {.notrust .ebnf .gram}
2412 field_expr : expr '.' ident
2415 A _field expression_ consists of an expression followed by a single dot and an identifier,
2416 when not immediately followed by a parenthesized expression-list (the latter is a [method call expression](#method-call-expressions)).
2417 A field expression denotes a field of a [structure](#structure-types).
2419 ~~~~ {.ignore .field}
2422 (Struct {a: 10, b: 20}).a;
2425 A field access on a record is an [lvalue](#lvalues-rvalues-and-temporaries) referring to the value of that field.
2426 When the field is mutable, it can be [assigned](#assignment-expressions) to.
2428 When the type of the expression to the left of the dot is a pointer to a record or structure,
2429 it is automatically dereferenced to make the field access possible.
2431 ### Vector expressions
2433 ~~~~ {.notrust .ebnf .gram}
2434 vec_expr : '[' "mut" ? vec_elems? ']'
2436 vec_elems : [expr [',' expr]*] | [expr ',' ".." expr]
2439 A [_vector_](#vector-types) _expression_ is written by enclosing zero or
2440 more comma-separated expressions of uniform type in square brackets.
2442 In the `[expr ',' ".." expr]` form, the expression after the `".."`
2443 must be a constant expression that can be evaluated at compile time, such
2444 as a [literal](#literals) or a [static item](#static-items).
2448 ["a", "b", "c", "d"];
2449 [0, ..128]; // vector with 128 zeros
2450 [0u8, 0u8, 0u8, 0u8];
2453 ### Index expressions
2455 ~~~~ {.notrust .ebnf .gram}
2456 idx_expr : expr '[' expr ']'
2459 [Vector](#vector-types)-typed expressions can be indexed by writing a
2460 square-bracket-enclosed expression (the index) after them. When the
2461 vector is mutable, the resulting [lvalue](#lvalues-rvalues-and-temporaries) can be assigned to.
2463 Indices are zero-based, and may be of any integral type. Vector access
2464 is bounds-checked at run-time. When the check fails, it will put the
2465 task in a _failing state_.
2472 (["a", "b"])[10]; // fails
2477 ### Unary operator expressions
2479 Rust defines six symbolic unary operators.
2480 They are all written as prefix operators,
2481 before the expression they apply to.
2484 : Negation. May only be applied to numeric types.
2486 : Dereference. When applied to a [pointer](#pointer-types) it denotes the pointed-to location.
2487 For pointers to mutable locations, the resulting [lvalue](#lvalues-rvalues-and-temporaries) can be assigned to.
2488 On non-pointer types, it calls the `deref` method of the `std::ops::Deref` trait, or the
2489 `deref_mut` method of the `std::ops::DerefMut` trait (if implemented by the type and required
2490 for an outer expression that will or could mutate the dereference), and produces the
2491 result of dereferencing the `&` or `&mut` borrowed pointer returned from the overload method.
2494 : Logical negation. On the boolean type, this flips between `true` and
2495 `false`. On integer types, this inverts the individual bits in the
2496 two's complement representation of the value.
2498 : [Boxing](#pointer-types) operators. Allocate a box to hold the value they are applied to,
2499 and store the value in it. `~` creates an owned box.
2501 : Borrow operator. Returns a reference, pointing to its operand.
2502 The operand of a borrow is statically proven to outlive the resulting pointer.
2503 If the borrow-checker cannot prove this, it is a compilation error.
2505 ### Binary operator expressions
2507 ~~~~ {.notrust .ebnf .gram}
2508 binop_expr : expr binop expr ;
2511 Binary operators expressions are given in terms of
2512 [operator precedence](#operator-precedence).
2514 #### Arithmetic operators
2516 Binary arithmetic expressions are syntactic sugar for calls to built-in traits,
2517 defined in the `std::ops` module of the `std` library.
2518 This means that arithmetic operators can be overridden for user-defined types.
2519 The default meaning of the operators on standard types is given here.
2522 : Addition and vector/string concatenation.
2523 Calls the `add` method on the `std::ops::Add` trait.
2526 Calls the `sub` method on the `std::ops::Sub` trait.
2529 Calls the `mul` method on the `std::ops::Mul` trait.
2532 Calls the `div` method on the `std::ops::Div` trait.
2535 Calls the `rem` method on the `std::ops::Rem` trait.
2537 #### Bitwise operators
2539 Like the [arithmetic operators](#arithmetic-operators), bitwise operators
2540 are syntactic sugar for calls to methods of built-in traits.
2541 This means that bitwise operators can be overridden for user-defined types.
2542 The default meaning of the operators on standard types is given here.
2546 Calls the `bitand` method of the `std::ops::BitAnd` trait.
2549 Calls the `bitor` method of the `std::ops::BitOr` trait.
2552 Calls the `bitxor` method of the `std::ops::BitXor` trait.
2554 : Logical left shift.
2555 Calls the `shl` method of the `std::ops::Shl` trait.
2557 : Logical right shift.
2558 Calls the `shr` method of the `std::ops::Shr` trait.
2560 #### Lazy boolean operators
2562 The operators `||` and `&&` may be applied to operands of boolean type.
2563 The `||` operator denotes logical 'or', and the `&&` operator denotes logical 'and'.
2564 They differ from `|` and `&` in that the right-hand operand is only evaluated
2565 when the left-hand operand does not already determine the result of the expression.
2566 That is, `||` only evaluates its right-hand operand
2567 when the left-hand operand evaluates to `false`, and `&&` only when it evaluates to `true`.
2569 #### Comparison operators
2571 Comparison operators are, like the [arithmetic operators](#arithmetic-operators),
2572 and [bitwise operators](#bitwise-operators),
2573 syntactic sugar for calls to built-in traits.
2574 This means that comparison operators can be overridden for user-defined types.
2575 The default meaning of the operators on standard types is given here.
2579 Calls the `eq` method on the `std::cmp::Eq` trait.
2582 Calls the `ne` method on the `std::cmp::Eq` trait.
2585 Calls the `lt` method on the `std::cmp::Ord` trait.
2588 Calls the `gt` method on the `std::cmp::Ord` trait.
2590 : Less than or equal.
2591 Calls the `le` method on the `std::cmp::Ord` trait.
2593 : Greater than or equal.
2594 Calls the `ge` method on the `std::cmp::Ord` trait.
2596 #### Type cast expressions
2598 A type cast expression is denoted with the binary operator `as`.
2600 Executing an `as` expression casts the value on the left-hand side to the type
2601 on the right-hand side.
2603 A numeric value can be cast to any numeric type.
2604 A raw pointer value can be cast to or from any integral type or raw pointer type.
2605 Any other cast is unsupported and will fail to compile.
2607 An example of an `as` expression:
2610 # fn sum(v: &[f64]) -> f64 { 0.0 }
2611 # fn len(v: &[f64]) -> int { 0 }
2613 fn avg(v: &[f64]) -> f64 {
2614 let sum: f64 = sum(v);
2615 let sz: f64 = len(v) as f64;
2620 #### Assignment expressions
2622 An _assignment expression_ consists of an [lvalue](#lvalues-rvalues-and-temporaries) expression followed by an
2623 equals sign (`=`) and an [rvalue](#lvalues-rvalues-and-temporaries) expression.
2625 Evaluating an assignment expression [either copies or moves](#moved-and-copied-types) its right-hand operand to its left-hand operand.
2634 #### Compound assignment expressions
2636 The `+`, `-`, `*`, `/`, `%`, `&`, `|`, `^`, `<<`, and `>>`
2637 operators may be composed with the `=` operator. The expression `lval
2638 OP= val` is equivalent to `lval = lval OP val`. For example, `x = x +
2639 1` may be written as `x += 1`.
2641 Any such expression always has the [`unit`](#primitive-types) type.
2643 #### Operator precedence
2645 The precedence of Rust binary operators is ordered as follows, going
2646 from strong to weak:
2648 ~~~~ {.notrust .precedence}
2663 Operators at the same precedence level are evaluated left-to-right. [Unary operators](#unary-operator-expressions)
2664 have the same precedence level and it is stronger than any of the binary operators'.
2666 ### Grouped expressions
2668 An expression enclosed in parentheses evaluates to the result of the enclosed
2669 expression. Parentheses can be used to explicitly specify evaluation order
2670 within an expression.
2672 ~~~~ {.notrust .ebnf .gram}
2673 paren_expr : '(' expr ')' ;
2676 An example of a parenthesized expression:
2679 let x = (2 + 3) * 4;
2683 ### Call expressions
2685 ~~~~ {.notrust .ebnf .gram}
2686 expr_list : [ expr [ ',' expr ]* ] ? ;
2687 paren_expr_list : '(' expr_list ')' ;
2688 call_expr : expr paren_expr_list ;
2691 A _call expression_ invokes a function, providing zero or more input slots and
2692 an optional reference slot to serve as the function's output, bound to the
2693 `lval` on the right hand side of the call. If the function eventually returns,
2694 then the expression completes.
2696 Some examples of call expressions:
2699 # use std::from_str::FromStr;
2700 # fn add(x: int, y: int) -> int { 0 }
2702 let x: int = add(1, 2);
2703 let pi: Option<f32> = FromStr::from_str("3.14");
2706 ### Lambda expressions
2708 ~~~~ {.notrust .ebnf .gram}
2709 ident_list : [ ident [ ',' ident ]* ] ? ;
2710 lambda_expr : '|' ident_list '|' expr ;
2713 A _lambda expression_ (sometimes called an "anonymous function expression") defines a function and denotes it as a value,
2714 in a single expression.
2715 A lambda expression is a pipe-symbol-delimited (`|`) list of identifiers followed by an expression.
2717 A lambda expression denotes a function that maps a list of parameters (`ident_list`)
2718 onto the expression that follows the `ident_list`.
2719 The identifiers in the `ident_list` are the parameters to the function.
2720 These parameters' types need not be specified, as the compiler infers them from context.
2722 Lambda expressions are most useful when passing functions as arguments to other functions,
2723 as an abbreviation for defining and capturing a separate function.
2725 Significantly, lambda expressions _capture their environment_,
2726 which regular [function definitions](#functions) do not.
2727 The exact type of capture depends on the [function type](#function-types) inferred for the lambda expression.
2728 In the simplest and least-expensive form (analogous to a ```|| { }``` expression),
2729 the lambda expression captures its environment by reference,
2730 effectively borrowing pointers to all outer variables mentioned inside the function.
2731 Alternately, the compiler may infer that a lambda expression should copy or move values (depending on their type.)
2732 from the environment into the lambda expression's captured environment.
2734 In this example, we define a function `ten_times` that takes a higher-order function argument,
2735 and call it with a lambda expression as an argument.
2738 fn ten_times(f: |int|) {
2746 ten_times(|j| println!("hello, {}", j));
2751 ~~~~ {.notrust .ebnf .gram}
2752 while_expr : "while" expr '{' block '}' ;
2755 A `while` loop begins by evaluating the boolean loop conditional expression.
2756 If the loop conditional expression evaluates to `true`, the loop body block
2757 executes and control returns to the loop conditional expression. If the loop
2758 conditional expression evaluates to `false`, the `while` expression completes.
2773 The keyword `loop` in Rust appears both in _loop expressions_ and in _continue expressions_.
2774 A loop expression denotes an infinite loop;
2775 see [Continue expressions](#continue-expressions) for continue expressions.
2777 ~~~~ {.notrust .ebnf .gram}
2778 loop_expr : [ lifetime ':' ] "loop" '{' block '}';
2781 A `loop` expression may optionally have a _label_.
2782 If a label is present,
2783 then labeled `break` and `loop` expressions nested within this loop may exit out of this loop or return control to its head.
2784 See [Break expressions](#break-expressions).
2786 ### Break expressions
2788 ~~~~ {.notrust .ebnf .gram}
2789 break_expr : "break" [ lifetime ];
2792 A `break` expression has an optional `label`.
2793 If the label is absent, then executing a `break` expression immediately terminates the innermost loop enclosing it.
2794 It is only permitted in the body of a loop.
2795 If the label is present, then `break foo` terminates the loop with label `foo`,
2796 which need not be the innermost label enclosing the `break` expression,
2797 but must enclose it.
2799 ### Continue expressions
2801 ~~~~ {.notrust .ebnf .gram}
2802 continue_expr : "loop" [ lifetime ];
2805 A continue expression, written `loop`, also has an optional `label`.
2806 If the label is absent,
2807 then executing a `loop` expression immediately terminates the current iteration of the innermost loop enclosing it,
2808 returning control to the loop *head*.
2809 In the case of a `while` loop,
2810 the head is the conditional expression controlling the loop.
2811 In the case of a `for` loop, the head is the call-expression controlling the loop.
2812 If the label is present, then `loop foo` returns control to the head of the loop with label `foo`,
2813 which need not be the innermost label enclosing the `break` expression,
2814 but must enclose it.
2816 A `loop` expression is only permitted in the body of a loop.
2820 ~~~~ {.notrust .ebnf .gram}
2821 for_expr : "for" pat "in" expr '{' block '}' ;
2824 A `for` expression is a syntactic construct for looping over elements
2825 provided by an implementation of `std::iter::Iterator`.
2827 An example of a for loop over the contents of a vector:
2831 # fn bar(f: Foo) { }
2836 let v: &[Foo] = &[a, b, c];
2843 An example of a for loop over a series of integers:
2846 # fn bar(b:uint) { }
2847 for i in range(0u, 256) {
2854 ~~~~ {.notrust .ebnf .gram}
2855 if_expr : "if" expr '{' block '}'
2858 else_tail : "else" [ if_expr
2862 An `if` expression is a conditional branch in program control. The form of
2863 an `if` expression is a condition expression, followed by a consequent
2864 block, any number of `else if` conditions and blocks, and an optional
2865 trailing `else` block. The condition expressions must have type
2866 `bool`. If a condition expression evaluates to `true`, the
2867 consequent block is executed and any subsequent `else if` or `else`
2868 block is skipped. If a condition expression evaluates to `false`, the
2869 consequent block is skipped and any subsequent `else if` condition is
2870 evaluated. If all `if` and `else if` conditions evaluate to `false`
2871 then any `else` block is executed.
2873 ### Match expressions
2875 ~~~~ {.notrust .ebnf .gram}
2876 match_expr : "match" expr '{' match_arm [ '|' match_arm ] * '}' ;
2878 match_arm : match_pat '=>' [ expr "," | '{' block '}' ] ;
2880 match_pat : pat [ ".." pat ] ? [ "if" expr ] ;
2883 A `match` expression branches on a *pattern*. The exact form of matching that
2884 occurs depends on the pattern. Patterns consist of some combination of
2885 literals, destructured vectors or enum constructors, structures, records and
2886 tuples, variable binding specifications, wildcards (`..`), and placeholders
2887 (`_`). A `match` expression has a *head expression*, which is the value to
2888 compare to the patterns. The type of the patterns must equal the type of the
2891 In a pattern whose head expression has an `enum` type, a placeholder (`_`)
2892 stands for a *single* data field, whereas a wildcard `..` stands for *all* the
2893 fields of a particular variant. For example:
2896 enum List<X> { Nil, Cons(X, ~List<X>) }
2898 let x: List<int> = Cons(10, ~Cons(11, ~Nil));
2901 Cons(_, ~Nil) => fail!("singleton list"),
2903 Nil => fail!("empty list")
2907 The first pattern matches lists constructed by applying `Cons` to any head
2908 value, and a tail value of `~Nil`. The second pattern matches _any_ list
2909 constructed with `Cons`, ignoring the values of its arguments. The difference
2910 between `_` and `..` is that the pattern `C(_)` is only type-correct if `C` has
2911 exactly one argument, while the pattern `C(..)` is type-correct for any enum
2912 variant `C`, regardless of how many arguments `C` has.
2914 Used inside a vector pattern, `..` stands for any number of elements. This
2915 wildcard can be used at most once for a given vector, which implies that it
2916 cannot be used to specifically match elements that are at an unknown distance
2917 from both ends of a vector, like `[.., 42, ..]`. If followed by a variable name,
2918 it will bind the corresponding slice to the variable. Example:
2921 fn is_symmetric(list: &[uint]) -> bool {
2924 [x, ..inside, y] if x == y => is_symmetric(inside),
2930 let sym = &[0, 1, 4, 2, 4, 1, 0];
2931 let not_sym = &[0, 1, 7, 2, 4, 1, 0];
2932 assert!(is_symmetric(sym));
2933 assert!(!is_symmetric(not_sym));
2937 A `match` behaves differently depending on whether or not the head expression
2938 is an [lvalue or an rvalue](#lvalues-rvalues-and-temporaries).
2939 If the head expression is an rvalue, it is
2940 first evaluated into a temporary location, and the resulting value
2941 is sequentially compared to the patterns in the arms until a match
2942 is found. The first arm with a matching pattern is chosen as the branch target
2943 of the `match`, any variables bound by the pattern are assigned to local
2944 variables in the arm's block, and control enters the block.
2946 When the head expression is an lvalue, the match does not allocate a
2947 temporary location (however, a by-value binding may copy or move from
2948 the lvalue). When possible, it is preferable to match on lvalues, as the
2949 lifetime of these matches inherits the lifetime of the lvalue, rather
2950 than being restricted to the inside of the match.
2952 An example of a `match` expression:
2955 # fn process_pair(a: int, b: int) { }
2956 # fn process_ten() { }
2958 enum List<X> { Nil, Cons(X, ~List<X>) }
2960 let x: List<int> = Cons(10, ~Cons(11, ~Nil));
2963 Cons(a, ~Cons(b, _)) => {
2978 Patterns that bind variables
2979 default to binding to a copy or move of the matched value
2980 (depending on the matched value's type).
2981 This can be changed to bind to a reference by
2982 using the `ref` keyword,
2983 or to a mutable reference using `ref mut`.
2985 Subpatterns can also be bound to variables by the use of the syntax
2986 `variable @ pattern`.
2990 enum List { Nil, Cons(uint, ~List) }
2992 fn is_sorted(list: &List) -> bool {
2994 Nil | Cons(_, ~Nil) => true,
2995 Cons(x, ref r @ ~Cons(y, _)) => (x <= y) && is_sorted(*r)
3000 let a = Cons(6, ~Cons(7, ~Cons(42, ~Nil)));
3001 assert!(is_sorted(&a));
3006 Patterns can also dereference pointers by using the `&`,
3007 `~` or `@` symbols, as appropriate. For example, these two matches
3008 on `x: &int` are equivalent:
3012 let y = match *x { 0 => "zero", _ => "some" };
3013 let z = match x { &0 => "zero", _ => "some" };
3018 A pattern that's just an identifier, like `Nil` in the previous example,
3019 could either refer to an enum variant that's in scope, or bind a new variable.
3020 The compiler resolves this ambiguity by forbidding variable bindings that occur
3021 in `match` patterns from shadowing names of variants that are in scope.
3022 For example, wherever `List` is in scope,
3023 a `match` pattern would not be able to bind `Nil` as a new name.
3024 The compiler interprets a variable pattern `x` as a binding _only_ if there is
3025 no variant named `x` in scope.
3026 A convention you can use to avoid conflicts is simply to name variants with
3027 upper-case letters, and local variables with lower-case letters.
3029 Multiple match patterns may be joined with the `|` operator.
3030 A range of values may be specified with `..`.
3036 let message = match x {
3037 0 | 1 => "not many",
3043 Range patterns only work on scalar types
3044 (like integers and characters; not like vectors and structs, which have sub-components).
3045 A range pattern may not be a sub-range of another range pattern inside the same `match`.
3047 Finally, match patterns can accept *pattern guards* to further refine the
3048 criteria for matching a case. Pattern guards appear after the pattern and
3049 consist of a bool-typed expression following the `if` keyword. A pattern
3050 guard may refer to the variables bound within the pattern they follow.
3053 # let maybe_digit = Some(0);
3054 # fn process_digit(i: int) { }
3055 # fn process_other(i: int) { }
3057 let message = match maybe_digit {
3058 Some(x) if x < 10 => process_digit(x),
3059 Some(x) => process_other(x),
3064 ### Return expressions
3066 ~~~~ {.notrust .ebnf .gram}
3067 return_expr : "return" expr ? ;
3070 Return expressions are denoted with the keyword `return`. Evaluating a `return`
3071 expression moves its argument into the output slot of the current
3072 function, destroys the current function activation frame, and transfers
3073 control to the caller frame.
3075 An example of a `return` expression:
3078 fn max(a: int, b: int) -> int {
3090 Every slot, item and value in a Rust program has a type. The _type_ of a *value*
3091 defines the interpretation of the memory holding it.
3093 Built-in types and type-constructors are tightly integrated into the language,
3094 in nontrivial ways that are not possible to emulate in user-defined
3095 types. User-defined types have limited capabilities.
3099 The primitive types are the following:
3101 * The "unit" type `()`, having the single "unit" value `()` (occasionally called "nil").
3102 ^[The "unit" value `()` is *not* a sentinel "null pointer" value for reference slots; the "unit" type is the implicit return type from functions otherwise lacking a return type, and can be used in other contexts (such as message-sending or type-parametric code) as a zero-size type.]
3103 * The boolean type `bool` with values `true` and `false`.
3104 * The machine types.
3105 * The machine-dependent integer and floating-point types.
3109 The machine types are the following:
3111 * The unsigned word types `u8`, `u16`, `u32` and `u64`, with values drawn from
3112 the integer intervals $[0, 2^8 - 1]$, $[0, 2^{16} - 1]$, $[0, 2^{32} - 1]$ and
3113 $[0, 2^{64} - 1]$ respectively.
3115 * The signed two's complement word types `i8`, `i16`, `i32` and `i64`, with
3116 values drawn from the integer intervals $[-(2^7), 2^7 - 1]$,
3117 $[-(2^{15}), 2^{15} - 1]$, $[-(2^{31}), 2^{31} - 1]$, $[-(2^{63}), 2^{63} - 1]$
3120 * The IEEE 754-2008 `binary32` and `binary64` floating-point types: `f32` and
3121 `f64`, respectively.
3123 #### Machine-dependent integer types
3125 The Rust type `uint`^[A Rust `uint` is analogous to a C99 `uintptr_t`.] is an
3126 unsigned integer type with target-machine-dependent size. Its size, in
3127 bits, is equal to the number of bits required to hold any memory address on
3130 The Rust type `int`^[A Rust `int` is analogous to a C99 `intptr_t`.] is a
3131 two's complement signed integer type with target-machine-dependent size. Its
3132 size, in bits, is equal to the size of the rust type `uint` on the same target
3137 The types `char` and `str` hold textual data.
3139 A value of type `char` is a [Unicode scalar value](
3140 http://www.unicode.org/glossary/#unicode_scalar_value)
3141 (ie. a code point that is not a surrogate),
3142 represented as a 32-bit unsigned word in the 0x0000 to 0xD7FF
3143 or 0xE000 to 0x10FFFF range.
3144 A `[char]` vector is effectively an UCS-4 / UTF-32 string.
3146 A value of type `str` is a Unicode string,
3147 represented as a vector of 8-bit unsigned bytes holding a sequence of UTF-8 codepoints.
3148 Since `str` is of unknown size, it is not a _first class_ type,
3149 but can only be instantiated through a pointer type,
3150 such as `&str` or `~str`.
3154 The tuple type-constructor forms a new heterogeneous product of values similar
3155 to the record type-constructor. The differences are as follows:
3157 * tuple elements cannot be mutable, unlike record fields
3158 * tuple elements are not named and can be accessed only by pattern-matching
3160 Tuple types and values are denoted by listing the types or values of their
3161 elements, respectively, in a parenthesized, comma-separated
3164 The members of a tuple are laid out in memory contiguously, like a record, in
3165 order specified by the tuple type.
3167 An example of a tuple type and its use:
3170 type Pair<'a> = (int,&'a str);
3171 let p: Pair<'static> = (10,"hello");
3173 assert!(b != "world");
3178 The vector type constructor represents a homogeneous array of values of a given type.
3179 A vector has a fixed size.
3180 (Operations like `vec.push` operate solely on owned vectors.)
3181 A vector type can be annotated with a _definite_ size, such as `[int, ..10]`.
3182 Such a definite-sized vector type is a first-class type, since its size is known statically.
3183 A vector without such a size is said to be of _indefinite_ size,
3184 and is therefore not a _first-class_ type.
3185 An indefinite-size vector can only be instantiated through a pointer type,
3186 such as `&[T]` or `~[T]`.
3187 The kind of a vector type depends on the kind of its element type,
3188 as with other simple structural types.
3190 Expressions producing vectors of definite size cannot be evaluated in a
3191 context expecting a vector of indefinite size; one must copy the
3192 definite-sized vector contents into a distinct vector of indefinite size.
3194 An example of a vector type and its use:
3197 let v: &[int] = &[7, 5, 3];
3202 All in-bounds elements of a vector are always initialized,
3203 and access to a vector is always bounds-checked.
3207 A `struct` *type* is a heterogeneous product of other types, called the *fields* of the type.
3208 ^[`struct` types are analogous `struct` types in C,
3209 the *record* types of the ML family,
3210 or the *structure* types of the Lisp family.]
3212 New instances of a `struct` can be constructed with a [struct expression](#structure-expressions).
3214 The memory order of fields in a `struct` is given by the item defining it.
3215 Fields may be given in any order in a corresponding struct *expression*;
3216 the resulting `struct` value will always be laid out in memory in the order specified by the corresponding *item*.
3218 The fields of a `struct` may be qualified by [visibility modifiers](#re-exporting-and-visibility),
3219 to restrict access to implementation-private data in a structure.
3221 A _tuple struct_ type is just like a structure type, except that the fields are anonymous.
3223 A _unit-like struct_ type is like a structure type, except that it has no fields.
3224 The one value constructed by the associated [structure expression](#structure-expressions)
3225 is the only value that inhabits such a type.
3227 ### Enumerated types
3229 An *enumerated type* is a nominal, heterogeneous disjoint union type,
3230 denoted by the name of an [`enum` item](#enumerations).
3231 ^[The `enum` type is analogous to a `data` constructor declaration in ML,
3232 or a *pick ADT* in Limbo.]
3234 An [`enum` item](#enumerations) declares both the type and a number of *variant constructors*,
3235 each of which is independently named and takes an optional tuple of arguments.
3237 New instances of an `enum` can be constructed by calling one of the variant constructors,
3238 in a [call expression](#call-expressions).
3240 Any `enum` value consumes as much memory as the largest variant constructor for its corresponding `enum` type.
3242 Enum types cannot be denoted *structurally* as types,
3243 but must be denoted by named reference to an [`enum` item](#enumerations).
3247 Nominal types -- [enumerations](#enumerated-types) and [structures](#structure-types) -- may be recursive.
3248 That is, each `enum` constructor or `struct` field may refer, directly or indirectly, to the enclosing `enum` or `struct` type itself.
3249 Such recursion has restrictions:
3251 * Recursive types must include a nominal type in the recursion
3252 (not mere [type definitions](#type-definitions),
3253 or other structural types such as [vectors](#vector-types) or [tuples](#tuple-types)).
3254 * A recursive `enum` item must have at least one non-recursive constructor
3255 (in order to give the recursion a basis case).
3256 * The size of a recursive type must be finite;
3257 in other words the recursive fields of the type must be [pointer types](#pointer-types).
3258 * Recursive type definitions can cross module boundaries, but not module *visibility* boundaries,
3259 or crate boundaries (in order to simplify the module system and type checker).
3261 An example of a *recursive* type and its use:
3269 let a: List<int> = Cons(7, ~Cons(13, ~Nil));
3274 All pointers in Rust are explicit first-class values.
3275 They can be copied, stored into data structures, and returned from functions.
3276 There are four varieties of pointer in Rust:
3278 Owning pointers (`~`)
3279 : These point to owned heap allocations (or "boxes") in the shared, inter-task heap.
3280 Each owned box has a single owning pointer; pointer and pointee retain a 1:1 relationship at all times.
3281 Owning pointers are written `~content`,
3282 for example `~int` means an owning pointer to an owned box containing an integer.
3283 Copying an owned box is a "deep" operation:
3284 it involves allocating a new owned box and copying the contents of the old box into the new box.
3285 Releasing an owning pointer immediately releases its corresponding owned box.
3288 : These point to memory _owned by some other value_.
3289 References arise by (automatic) conversion from owning pointers, managed pointers,
3290 or by applying the borrowing operator `&` to some other value,
3291 including [lvalues, rvalues or temporaries](#lvalues-rvalues-and-temporaries).
3292 References are written `&content`, or in some cases `&'f content` for some lifetime-variable `f`,
3293 for example `&int` means a reference to an integer.
3294 Copying a reference is a "shallow" operation:
3295 it involves only copying the pointer itself.
3296 Releasing a reference typically has no effect on the value it points to,
3297 with the exception of temporary values,
3298 which are released when the last reference to them is released.
3301 : Raw pointers are pointers without safety or liveness guarantees.
3302 Raw pointers are written `*content`,
3303 for example `*int` means a raw pointer to an integer.
3304 Copying or dropping a raw pointer has no effect on the lifecycle of any other value.
3305 Dereferencing a raw pointer or converting it to any other pointer type is an [`unsafe` operation](#unsafe-functions).
3306 Raw pointers are generally discouraged in Rust code;
3307 they exist to support interoperability with foreign code,
3308 and writing performance-critical or low-level functions.
3312 The function type constructor `fn` forms new function types.
3313 A function type consists of a possibly-empty set of function-type modifiers
3314 (such as `unsafe` or `extern`), a sequence of input types and an output type.
3316 An example of a `fn` type:
3319 fn add(x: int, y: int) -> int {
3323 let mut x = add(5,7);
3325 type Binop<'a> = 'a |int,int| -> int;
3326 let bo: Binop = add;
3332 The type of a closure mapping an input of type `A` to an output of type `B` is `|A| -> B`. A closure with no arguments or return values has type `||`.
3335 An example of creating and calling a closure:
3338 let captured_var = 10;
3340 let closure_no_args = || println!("captured_var={}", captured_var);
3342 let closure_args = |arg: int| -> int {
3343 println!("captured_var={}, arg={}", captured_var, arg);
3344 arg // Note lack of semicolon after 'arg'
3347 fn call_closure(c1: ||, c2: |int| -> int) {
3352 call_closure(closure_no_args, closure_args);
3358 Every trait item (see [traits](#traits)) defines a type with the same name as the trait.
3359 This type is called the _object type_ of the trait.
3360 Object types permit "late binding" of methods, dispatched using _virtual method tables_ ("vtables").
3361 Whereas most calls to trait methods are "early bound" (statically resolved) to specific implementations at compile time,
3362 a call to a method on an object type is only resolved to a vtable entry at compile time.
3363 The actual implementation for each vtable entry can vary on an object-by-object basis.
3365 Given a pointer-typed expression `E` of type `&T` or `~T`, where `T` implements trait `R`,
3366 casting `E` to the corresponding pointer type `&R` or `~R` results in a value of the _object type_ `R`.
3367 This result is represented as a pair of pointers:
3368 the vtable pointer for the `T` implementation of `R`, and the pointer value of `E`.
3370 An example of an object type:
3374 fn to_string(&self) -> ~str;
3377 impl Printable for int {
3378 fn to_string(&self) -> ~str { self.to_str() }
3381 fn print(a: ~Printable) {
3382 println!("{}", a.to_string());
3386 print(~10 as ~Printable);
3390 In this example, the trait `Printable` occurs as an object type in both the type signature of `print`,
3391 and the cast expression in `main`.
3395 Within the body of an item that has type parameter declarations, the names of its type parameters are types:
3398 fn map<A: Clone, B: Clone>(f: |A| -> B, xs: &[A]) -> ~[B] {
3402 let first: B = f(xs[0].clone());
3403 let rest: ~[B] = map(f, xs.slice(1, xs.len()));
3404 return ~[first] + rest;
3408 Here, `first` has type `B`, referring to `map`'s `B` type parameter;
3409 and `rest` has type `~[B]`, a vector type with element type `B`.
3413 The special type `self` has a meaning within methods inside an
3414 impl item. It refers to the type of the implicit `self` argument. For
3419 fn make_string(&self) -> ~str;
3422 impl Printable for ~str {
3423 fn make_string(&self) -> ~str {
3429 `self` refers to the value of type `~str` that is the receiver for a
3430 call to the method `make_string`.
3434 Types in Rust are categorized into kinds, based on various properties of the components of the type.
3438 : Types of this kind can be safely sent between tasks.
3439 This kind includes scalars, owning pointers, owned closures, and
3440 structural types containing only other owned types.
3441 All `Send` types are `'static`.
3443 : Types of this kind consist of "Plain Old Data"
3444 which can be copied by simply moving bits.
3445 All values of this kind can be implicitly copied.
3446 This kind includes scalars and immutable references,
3447 as well as structural types containing other `Pod` types.
3449 : Types of this kind do not contain any references (except for
3450 references with the `static` lifetime, which are allowed).
3451 This can be a useful guarantee for code
3452 that breaks borrowing assumptions
3453 using [`unsafe` operations](#unsafe-functions).
3455 : This is not strictly a kind,
3456 but its presence interacts with kinds:
3457 the `Drop` trait provides a single method `drop`
3458 that takes no parameters,
3459 and is run when values of the type are dropped.
3460 Such a method is called a "destructor",
3461 and are always executed in "top-down" order:
3462 a value is completely destroyed
3463 before any of the values it owns run their destructors.
3464 Only `Send` types can implement `Drop`.
3467 : Types with destructors, closure environments,
3468 and various other _non-first-class_ types,
3469 are not copyable at all.
3470 Such types can usually only be accessed through pointers,
3471 or in some cases, moved between mutable locations.
3473 Kinds can be supplied as _bounds_ on type parameters, like traits,
3474 in which case the parameter is constrained to types satisfying that kind.
3476 By default, type parameters do not carry any assumed kind-bounds at all.
3477 When instantiating a type parameter,
3478 the kind bounds on the parameter are checked
3479 to be the same or narrower than the kind
3480 of the type that it is instantiated with.
3482 Sending operations are not part of the Rust language,
3483 but are implemented in the library.
3484 Generic functions that send values
3485 bound the kind of these values to sendable.
3487 # Memory and concurrency models
3489 Rust has a memory model centered around concurrently-executing _tasks_. Thus
3490 its memory model and its concurrency model are best discussed simultaneously,
3491 as parts of each only make sense when considered from the perspective of the
3494 When reading about the memory model, keep in mind that it is partitioned in
3495 order to support tasks; and when reading about tasks, keep in mind that their
3496 isolation and communication mechanisms are only possible due to the ownership
3497 and lifetime semantics of the memory model.
3501 A Rust program's memory consists of a static set of *items*, a set of
3502 [tasks](#tasks) each with its own *stack*, and a *heap*. Immutable portions of
3503 the heap may be shared between tasks, mutable portions may not.
3505 Allocations in the stack consist of *slots*, and allocations in the heap
3508 ### Memory allocation and lifetime
3510 The _items_ of a program are those functions, modules and types
3511 that have their value calculated at compile-time and stored uniquely in the
3512 memory image of the rust process. Items are neither dynamically allocated nor
3515 A task's _stack_ consists of activation frames automatically allocated on
3516 entry to each function as the task executes. A stack allocation is reclaimed
3517 when control leaves the frame containing it.
3519 The _heap_ is a general term that describes two separate sets of boxes:
3520 managed boxes -- which may be subject to garbage collection -- and owned
3521 boxes. The lifetime of an allocation in the heap depends on the lifetime of
3522 the box values pointing to it. Since box values may themselves be passed in
3523 and out of frames, or stored in the heap, heap allocations may outlive the
3524 frame they are allocated within.
3526 ### Memory ownership
3528 A task owns all memory it can *safely* reach through local variables,
3529 as well as managed, owned boxes and references.
3531 When a task sends a value that has the `Send` trait to another task,
3532 it loses ownership of the value sent and can no longer refer to it.
3533 This is statically guaranteed by the combined use of "move semantics",
3534 and the compiler-checked _meaning_ of the `Send` trait:
3535 it is only instantiated for (transitively) sendable kinds of data constructor and pointers,
3536 never including managed boxes or references.
3538 When a stack frame is exited, its local allocations are all released, and its
3539 references to boxes (both managed and owned) are dropped.
3541 A managed box may (in the case of a recursive, mutable managed type) be cyclic;
3542 in this case the release of memory inside the managed structure may be deferred
3543 until task-local garbage collection can reclaim it. Code can ensure no such
3544 delayed deallocation occurs by restricting itself to owned boxes and similar
3545 unmanaged kinds of data.
3547 When a task finishes, its stack is necessarily empty and it therefore has no
3548 references to any boxes; the remainder of its heap is immediately freed.
3552 A task's stack contains slots.
3554 A _slot_ is a component of a stack frame, either a function parameter,
3555 a [temporary](#lvalues-rvalues-and-temporaries), or a local variable.
3557 A _local variable_ (or *stack-local* allocation) holds a value directly,
3558 allocated within the stack's memory. The value is a part of the stack frame.
3560 Local variables are immutable unless declared otherwise like: `let mut x = ...`.
3562 Function parameters are immutable unless declared with `mut`. The
3563 `mut` keyword applies only to the following parameter (so `|mut x, y|`
3564 and `fn f(mut x: ~int, y: ~int)` declare one mutable variable `x` and
3565 one immutable variable `y`).
3567 Methods that take either `self` or `~self` can optionally place them in a
3568 mutable slot by prefixing them with `mut` (similar to regular arguments):
3572 fn change(mut self) -> Self;
3573 fn modify(mut ~self) -> ~Self;
3577 Local variables are not initialized when allocated; the entire frame worth of
3578 local variables are allocated at once, on frame-entry, in an uninitialized
3579 state. Subsequent statements within a function may or may not initialize the
3580 local variables. Local variables can be used only after they have been
3581 initialized; this is enforced by the compiler.
3585 An _owned box_ is a reference to a heap allocation holding another value, which is constructed
3586 by the prefix *tilde* sigil `~`
3588 An example of an owned box type and value:
3594 Owned box values exist in 1:1 correspondence with their heap allocation
3595 copying an owned box value makes a shallow copy of the pointer
3596 Rust will consider a shallow copy of an owned box to move ownership of the value. After a value has been moved, the source location cannot be used unless it is reinitialized.
3601 // attempting to use `x` will result in an error here
3608 An executing Rust program consists of a tree of tasks.
3609 A Rust _task_ consists of an entry function, a stack,
3610 a set of outgoing communication channels and incoming communication ports,
3611 and ownership of some portion of the heap of a single operating-system process.
3612 (We expect that many programs will not use channels and ports directly,
3613 but will instead use higher-level abstractions provided in standard libraries,
3616 Multiple Rust tasks may coexist in a single operating-system process.
3617 The runtime scheduler maps tasks to a certain number of operating-system threads.
3618 By default, the scheduler chooses the number of threads based on
3619 the number of concurrent physical CPUs detected at startup.
3620 It's also possible to override this choice at runtime.
3621 When the number of tasks exceeds the number of threads -- which is likely --
3622 the scheduler multiplexes the tasks onto threads.^[
3623 This is an M:N scheduler,
3624 which is known to give suboptimal results for CPU-bound concurrency problems.
3625 In such cases, running with the same number of threads and tasks can yield better results.
3626 Rust has M:N scheduling in order to support very large numbers of tasks
3627 in contexts where threads are too resource-intensive to use in large number.
3628 The cost of threads varies substantially per operating system, and is sometimes quite low,
3629 so this flexibility is not always worth exploiting.]
3631 ### Communication between tasks
3633 Rust tasks are isolated and generally unable to interfere with one another's memory directly,
3634 except through [`unsafe` code](#unsafe-functions).
3635 All contact between tasks is mediated by safe forms of ownership transfer,
3636 and data races on memory are prohibited by the type system.
3638 Inter-task communication and co-ordination facilities are provided in the standard library.
3641 - synchronous and asynchronous communication channels with various communication topologies
3642 - read-only and read-write shared variables with various safe mutual exclusion patterns
3643 - simple locks and semaphores
3645 When such facilities carry values, the values are restricted to the [`Send` type-kind](#type-kinds).
3646 Restricting communication interfaces to this kind ensures that no references or managed pointers move between tasks.
3647 Thus access to an entire data structure can be mediated through its owning "root" value;
3648 no further locking or copying is required to avoid data races within the substructure of such a value.
3652 The _lifecycle_ of a task consists of a finite set of states and events
3653 that cause transitions between the states. The lifecycle states of a task are:
3660 A task begins its lifecycle -- once it has been spawned -- in the *running*
3661 state. In this state it executes the statements of its entry function, and any
3662 functions called by the entry function.
3664 A task may transition from the *running* state to the *blocked*
3665 state any time it makes a blocking communication call. When the
3666 call can be completed -- when a message arrives at a sender, or a
3667 buffer opens to receive a message -- then the blocked task will
3668 unblock and transition back to *running*.
3670 A task may transition to the *failing* state at any time, due being
3671 killed by some external event or internally, from the evaluation of a
3672 `fail!()` macro. Once *failing*, a task unwinds its stack and
3673 transitions to the *dead* state. Unwinding the stack of a task is done by
3674 the task itself, on its own control stack. If a value with a destructor is
3675 freed during unwinding, the code for the destructor is run, also on the task's
3676 control stack. Running the destructor code causes a temporary transition to a
3677 *running* state, and allows the destructor code to cause any subsequent
3678 state transitions. The original task of unwinding and failing thereby may
3679 suspend temporarily, and may involve (recursive) unwinding of the stack of a
3680 failed destructor. Nonetheless, the outermost unwinding activity will continue
3681 until the stack is unwound and the task transitions to the *dead*
3682 state. There is no way to "recover" from task failure. Once a task has
3683 temporarily suspended its unwinding in the *failing* state, failure
3684 occurring from within this destructor results in *hard* failure.
3685 A hard failure currently results in the process aborting.
3687 A task in the *dead* state cannot transition to other states; it exists
3688 only to have its termination status inspected by other tasks, and/or to await
3689 reclamation when the last reference to it drops.
3693 The currently scheduled task is given a finite *time slice* in which to
3694 execute, after which it is *descheduled* at a loop-edge or similar
3695 preemption point, and another task within is scheduled, pseudo-randomly.
3697 An executing task can yield control at any time, by making a library call to
3698 `std::task::yield`, which deschedules it immediately. Entering any other
3699 non-executing state (blocked, dead) similarly deschedules the task.
3701 # Runtime services, linkage and debugging
3703 The Rust _runtime_ is a relatively compact collection of C++ and Rust code
3704 that provides fundamental services and datatypes to all Rust tasks at
3705 run-time. It is smaller and simpler than many modern language runtimes. It is
3706 tightly integrated into the language's execution model of memory, tasks,
3707 communication and logging.
3709 > **Note:** The runtime library will merge with the `std` library in future versions of Rust.
3711 ### Memory allocation
3713 The runtime memory-management system is based on a _service-provider interface_,
3714 through which the runtime requests blocks of memory from its environment
3715 and releases them back to its environment when they are no longer needed.
3716 The default implementation of the service-provider interface
3717 consists of the C runtime functions `malloc` and `free`.
3719 The runtime memory-management system, in turn, supplies Rust tasks with
3720 facilities for allocating releasing stacks, as well as allocating and freeing
3725 The runtime provides C and Rust code to assist with various built-in types,
3726 such as vectors, strings, and the low level communication system (ports,
3729 Support for other built-in types such as simple types, tuples, records, and
3730 enums is open-coded by the Rust compiler.
3732 ### Task scheduling and communication
3734 The runtime provides code to manage inter-task communication. This includes
3735 the system of task-lifecycle state transitions depending on the contents of
3736 queues, as well as code to copy values between queues and their recipients and
3737 to serialize values for transmission over operating-system inter-process
3738 communication facilities.
3742 The Rust compiler supports various methods to link crates together both
3743 statically and dynamically. This section will explore the various methods to
3744 link Rust crates together, and more information about native libraries can be
3745 found in the [ffi tutorial][ffi].
3747 In one session of compilation, the compiler can generate multiple artifacts
3748 through the usage of command line flags and the `crate_type` attribute.
3750 * `--crate-type=bin`, `#[crate_type = "bin"]` - A runnable executable will be
3751 produced. This requires that there is a `main` function in the crate which
3752 will be run when the program begins executing. This will link in all Rust and
3753 native dependencies, producing a distributable binary.
3755 * `--crate-type=lib`, `#[crate_type = "lib"]` - A Rust library will be produced.
3756 This is an ambiguous concept as to what exactly is produced because a library
3757 can manifest itself in several forms. The purpose of this generic `lib` option
3758 is to generate the "compiler recommended" style of library. The output library
3759 will always be usable by rustc, but the actual type of library may change from
3760 time-to-time. The remaining output types are all different flavors of
3761 libraries, and the `lib` type can be seen as an alias for one of them (but the
3762 actual one is compiler-defined).
3764 * `--crate-type=dylib`, `#[crate_type = "dylib"]` - A dynamic Rust library will
3765 be produced. This is different from the `lib` output type in that this forces
3766 dynamic library generation. The resulting dynamic library can be used as a
3767 dependency for other libraries and/or executables. This output type will
3768 create `*.so` files on linux, `*.dylib` files on osx, and `*.dll` files on
3771 * `--crate-type=staticlib`, `#[crate_type = "staticlib"]` - A static system
3772 library will be produced. This is different from other library outputs in that
3773 the Rust compiler will never attempt to link to `staticlib` outputs. The
3774 purpose of this output type is to create a static library containing all of
3775 the local crate's code along with all upstream dependencies. The static
3776 library is actually a `*.a` archive on linux and osx and a `*.lib` file on
3777 windows. This format is recommended for use in situtations such as linking
3778 Rust code into an existing non-Rust application because it will not have
3779 dynamic dependencies on other Rust code.
3781 * `--crate-type=rlib`, `#[crate_type = "rlib"]` - A "Rust library" file will be
3782 produced. This is used as an intermediate artifact and can be thought of as a
3783 "static Rust library". These `rlib` files, unlike `staticlib` files, are
3784 interpreted by the Rust compiler in future linkage. This essentially means
3785 that `rustc` will look for metadata in `rlib` files like it looks for metadata
3786 in dynamic libraries. This form of output is used to produce statically linked
3787 executables as well as `staticlib` outputs.
3789 Note that these outputs are stackable in the sense that if multiple are
3790 specified, then the compiler will produce each form of output at once without
3791 having to recompile.
3793 With all these different kinds of outputs, if crate A depends on crate B, then
3794 the compiler could find B in various different forms throughout the system. The
3795 only forms looked for by the compiler, however, are the `rlib` format and the
3796 dynamic library format. With these two options for a dependent library, the
3797 compiler must at some point make a choice between these two formats. With this
3798 in mind, the compiler follows these rules when determining what format of
3799 dependencies will be used:
3801 1. If a dynamic library is being produced, then it is required for all upstream
3802 Rust dependencies to also be dynamic. This is a limitation of the current
3803 implementation of the linkage model. The reason behind this limitation is to
3804 prevent multiple copies of the same upstream library from showing up, and in
3805 the future it is planned to support a mixture of dynamic and static linking.
3807 When producing a dynamic library, the compiler will generate an error if an
3808 upstream dependency could not be found, and also if an upstream dependency
3809 could only be found in an `rlib` format. Remember that `staticlib` formats
3810 are always ignored by `rustc` for crate-linking purposes.
3812 2. If a static library is being produced, all upstream dependecies are
3813 required to be available in `rlib` formats. This requirement stems from the
3814 same reasons that a dynamic library must have all dynamic dependencies.
3816 Note that it is impossible to link in native dynamic dependencies to a static
3817 library, and in this case warnings will be printed about all unlinked native
3818 dynamic dependencies.
3820 3. If an `rlib` file is being produced, then there are no restrictions on what
3821 format the upstream dependencies are available in. It is simply required that
3822 all upstream dependencies be available for reading metadata from.
3824 The reason for this is that `rlib` files do not contain any of their upstream
3825 dependencies. It wouldn't be very efficient for all `rlib` files to contain a
3826 copy of `libstd.rlib`!
3828 4. If an executable is being produced, then things get a little interesting. As
3829 with the above limitations in dynamic and static libraries, it is required
3830 for all upstream dependencies to be in the same format. The next question is
3831 whether to prefer a dynamic or a static format. The compiler currently favors
3832 static linking over dynamic linking, but this can be inverted with the `-C
3833 prefer-dynamic` flag to the compiler.
3835 What this means is that first the compiler will attempt to find all upstream
3836 dependencies as `rlib` files, and if successful, it will create a statically
3837 linked executable. If an upstream dependency is missing as an `rlib` file,
3838 then the compiler will force all dependencies to be dynamic and will generate
3839 errors if dynamic versions could not be found.
3841 In general, `--crate-type=bin` or `--crate-type=lib` should be sufficient for
3842 all compilation needs, and the other options are just available if more
3843 fine-grained control is desired over the output format of a Rust crate.
3847 The runtime contains a system for directing [logging
3848 expressions](#logging-expressions) to a logging console and/or internal logging
3849 buffers. Logging can be enabled per module.
3851 Logging output is enabled by setting the `RUST_LOG` environment
3852 variable. `RUST_LOG` accepts a logging specification made up of a
3853 comma-separated list of paths, with optional log levels. For each
3854 module containing log expressions, if `RUST_LOG` contains the path to
3855 that module or a parent of that module, then logs of the appropriate
3856 level will be output to the console.
3858 The path to a module consists of the crate name, any parent modules,
3859 then the module itself, all separated by double colons (`::`). The
3860 optional log level can be appended to the module path with an equals
3861 sign (`=`) followed by the log level, from 1 to 4, inclusive. Level 1
3862 is the error level, 2 is warning, 3 info, and 4 debug. You can also
3863 use the symbolic constants `error`, `warn`, `info`, and `debug`. Any
3864 logs less than or equal to the specified level will be output. If not
3865 specified then log level 4 is assumed. Debug messages can be omitted
3866 by passing `--cfg ndebug` to `rustc`.
3868 As an example, to see all the logs generated by the compiler, you would set
3869 `RUST_LOG` to `rustc`, which is the crate name (as specified in its `crate_id`
3870 [attribute](#attributes)). To narrow down the logs to just crate resolution,
3871 you would set it to `rustc::metadata::creader`. To see just error logging
3874 Note that when compiling source files that don't specify a
3875 crate name the crate is given a default name that matches the source file,
3876 with the extension removed. In that case, to turn on logging for a program
3877 compiled from, e.g. `helloworld.rs`, `RUST_LOG` should be set to `helloworld`.
3879 As a convenience, the logging spec can also be set to a special pseudo-crate,
3880 `::help`. In this case, when the application starts, the runtime will
3881 simply output a list of loaded modules containing log expressions, then exit.
3883 #### Logging Expressions
3885 Rust provides several macros to log information. Here's a simple Rust program
3886 that demonstrates all four of them:
3890 #[phase(syntax, link)] extern crate log;
3893 error!("This is an error log")
3894 warn!("This is a warn log")
3895 info!("this is an info log")
3896 debug!("This is a debug log")
3900 These four log levels correspond to levels 1-4, as controlled by `RUST_LOG`:
3902 ``` {.bash .notrust}
3903 $ RUST_LOG=rust=3 ./rust
3904 This is an error log
3909 # Appendix: Rationales and design tradeoffs
3913 # Appendix: Influences and further references
3917 > The essential problem that must be solved in making a fault-tolerant
3918 > software system is therefore that of fault-isolation. Different programmers
3919 > will write different modules, some modules will be correct, others will have
3920 > errors. We do not want the errors in one module to adversely affect the
3921 > behaviour of a module which does not have any errors.
3923 > — Joe Armstrong
3925 > In our approach, all data is private to some process, and processes can
3926 > only communicate through communications channels. *Security*, as used
3927 > in this paper, is the property which guarantees that processes in a system
3928 > cannot affect each other except by explicit communication.
3930 > When security is absent, nothing which can be proven about a single module
3931 > in isolation can be guaranteed to hold when that module is embedded in a
3934 > — Robert Strom and Shaula Yemini
3936 > Concurrent and applicative programming complement each other. The
3937 > ability to send messages on channels provides I/O without side effects,
3938 > while the avoidance of shared data helps keep concurrent processes from
3943 Rust is not a particularly original language. It may however appear unusual
3944 by contemporary standards, as its design elements are drawn from a number of
3945 "historical" languages that have, with a few exceptions, fallen out of
3946 favour. Five prominent lineages contribute the most, though their influences
3947 have come and gone during the course of Rust's development:
3949 * The NIL (1981) and Hermes (1990) family. These languages were developed by
3950 Robert Strom, Shaula Yemini, David Bacon and others in their group at IBM
3951 Watson Research Center (Yorktown Heights, NY, USA).
3953 * The Erlang (1987) language, developed by Joe Armstrong, Robert Virding, Claes
3954 Wikström, Mike Williams and others in their group at the Ericsson Computer
3955 Science Laboratory (Älvsjö, Stockholm, Sweden) .
3957 * The Sather (1990) language, developed by Stephen Omohundro, Chu-Cheow Lim,
3958 Heinz Schmidt and others in their group at The International Computer
3959 Science Institute of the University of California, Berkeley (Berkeley, CA,
3962 * The Newsqueak (1988), Alef (1995), and Limbo (1996) family. These
3963 languages were developed by Rob Pike, Phil Winterbottom, Sean Dorward and
3964 others in their group at Bell Labs Computing Sciences Research Center
3965 (Murray Hill, NJ, USA).
3967 * The Napier (1985) and Napier88 (1988) family. These languages were
3968 developed by Malcolm Atkinson, Ron Morrison and others in their group at
3969 the University of St. Andrews (St. Andrews, Fife, UK).
3971 Additional specific influences can be seen from the following languages:
3973 * The structural algebraic types and compilation manager of SML.
3974 * The attribute and assembly systems of C#.
3975 * The references and deterministic destructor system of C++.
3976 * The memory region systems of the ML Kit and Cyclone.
3977 * The typeclass system of Haskell.
3978 * The lexical identifier rule of Python.
3979 * The block syntax of Ruby.
3981 [ffi]: guide-ffi.html