rust/doc/rust.md

% Rust Reference Manual

# Introduction

This document is the reference manual for the Rust programming language. It
provides three kinds of material:

  - Chapters that formally define the language grammar and, for each
    construct, informally describe its semantics and give examples of its
    use.
  - Chapters that informally describe the memory model, concurrency model,
    runtime services, linkage model and debugging facilities.
  - Appendix chapters providing rationale and references to languages that
    influenced the design.

This document does not serve as a tutorial introduction to the
language. Background familiarity with the language is assumed. A separate
[tutorial] document is available to help acquire such background familiarity.

This document also does not serve as a reference to the [core] or [standard]
libraries included in the language distribution. Those libraries are
documented separately by extracting documentation attributes from their
source code.

[tutorial]: tutorial.html
[core]: core/index.html
[standard]: std/index.html

## Disclaimer

Rust is a work in progress. The language continues to evolve as the design
shifts and is fleshed out in working code. Certain parts work, certain parts
do not, certain parts will be removed or changed.

This manual is a snapshot written in the present tense. All features described
exist in working code unless otherwise noted, but some are quite primitive or
remain to be further modified by planned work. Some may be temporary. It is a
*draft*, and we ask that you not take anything you read here as final.

If you have suggestions to make, please try to focus them on *reductions* to
the language: possible features that can be combined or omitted. We aim to
keep the size and complexity of the language under control.

> **Note:** The grammar for Rust given in this document is rough and
> very incomplete; only a modest number of sections have accompanying grammar
> rules. Formalizing the grammar accepted by the Rust parser is ongoing work,
> but future versions of this document will contain a complete
> grammar. Moreover, we hope that this grammar will be extracted and verified
> as LL(1) by an automated grammar-analysis tool, and further tested against the
> Rust sources. Preliminary versions of this automation exist, but are not yet
> complete.

# Notation

Rust's grammar is defined over Unicode codepoints, each conventionally
denoted `U+XXXX`, for 4 or more hexadecimal digits `X`. _Most_ of Rust's
grammar is confined to the ASCII range of Unicode, and is described in this
document by a dialect of Extended Backus-Naur Form (EBNF), specifically a
dialect of EBNF supported by common automated LL(k) parsing tools such as
`llgen`, rather than the dialect given in ISO 14977. The dialect can be
defined self-referentially as follows:

~~~~~~~~ {.ebnf .notation}

grammar : rule + ;
rule    : nonterminal ':' productionrule ';' ;
productionrule : production [ '|' production ] * ;
production : term * ;
term : element repeats ;
element : LITERAL | IDENTIFIER | '[' productionrule ']' ;
repeats : [ '*' | '+' ] NUMBER ? | NUMBER ? | '?' ;

~~~~~~~~

Where:

  - Whitespace in the grammar is ignored.
  - Square brackets are used to group rules.
  - `LITERAL` is a single printable ASCII character, or an escaped hexadecimal
     ASCII code of the form `\xQQ`, in single quotes, denoting the corresponding
     Unicode codepoint `U+00QQ`.
  - `IDENTIFIER` is a nonempty string of ASCII letters and underscores.
  - The `repeat` forms apply to the adjacent `element`, and are as follows:
    - `?` means zero or one repetition
    - `*` means zero or more repetitions
    - `+` means one or more repetitions
    - NUMBER trailing a repeat symbol gives a maximum repetition count
    - NUMBER on its own gives an exact repetition count

This EBNF dialect should hopefully be familiar to many readers.

## Unicode productions

A small number of productions in Rust's grammar permit Unicode codepoints
outside the ASCII range; these productions are defined in terms of character
properties given by the Unicode standard, rather than ASCII-range
codepoints. These are given in the section [Special Unicode
Productions](#special-unicode-productions).

## String table productions

Some rules in the grammar -- notably [unary
operators](#unary-operator-expressions), [binary
operators](#binary-operator-expressions), and [keywords](#keywords) --
are given in a simplified form: as a listing of a table of unquoted,
printable whitespace-separated strings. These cases form a subset of
the rules regarding the [token](#tokens) rule, and are assumed to be
the result of a lexical-analysis phase feeding the parser, driven by a
DFA, operating over the disjunction of all such string table entries.

When such a string enclosed in double-quotes (`"`) occurs inside the
grammar, it is an implicit reference to a single member of such a string table
production. See [tokens](#tokens) for more information.


# Lexical structure

## Input format

Rust input is interpreted as a sequence of Unicode codepoints encoded in UTF-8,
normalized to Unicode normalization form NFKC.
Most Rust grammar rules are defined in terms of printable ASCII-range codepoints,
but a small number are defined in terms of Unicode properties or explicit codepoint lists.
^[Substitute definitions for the special Unicode productions are provided to the grammar verifier, restricted to ASCII range, when verifying the grammar in this document.]

## Special Unicode Productions

The following productions in the Rust grammar are defined in terms of Unicode properties:
`ident`, `non_null`, `non_star`, `non_eol`, `non_slash`, `non_single_quote` and `non_double_quote`.

### Identifiers

The `ident` production is any nonempty Unicode string of the following form:

   - The first character has property `XID_start`
   - The remaining characters have property `XID_continue`

that does _not_ occur in the set of [keywords](#keywords).

Note: `XID_start` and `XID_continue` as character properties cover the
character ranges used to form the more familiar C and Java language-family
identifiers.

### Delimiter-restricted productions

Some productions are defined by exclusion of particular Unicode characters:

  - `non_null` is any single Unicode character aside from `U+0000` (null)
  - `non_eol` is `non_null` restricted to exclude `U+000A` (`'\n'`)
  - `non_star` is `non_null` restricted to exclude `U+002A` (`*`)
  - `non_slash` is `non_null` restricted to exclude `U+002F` (`/`)
  - `non_single_quote` is `non_null` restricted to exclude `U+0027`  (`'`)
  - `non_double_quote` is `non_null` restricted to exclude `U+0022` (`"`)

## Comments

~~~~~~~~ {.ebnf .gram}
comment : block_comment | line_comment ;
block_comment : "/*" block_comment_body * "*/" ;
block_comment_body : non_star * | '*' non_slash ;
line_comment : "//" non_eol * ;
~~~~~~~~

Comments in Rust code follow the general C++ style of line and block-comment forms,
with no nesting of block-comment delimiters.

Line comments beginning with _three_ slashes (`///`),
and block comments beginning with a repeated asterisk in the block-open sequence (`/**`),
are interpreted as a special syntax for `doc` [attributes](#attributes).
That is, they are equivalent to writing `#[doc "..."]` around the comment's text.

Non-doc comments are interpreted as a form of whitespace.

## Whitespace

~~~~~~~~ {.ebnf .gram}
whitespace_char : '\x20' | '\x09' | '\x0a' | '\x0d' ;
whitespace : [ whitespace_char | comment ] + ;
~~~~~~~~

The `whitespace_char` production is any nonempty Unicode string consisting of any
of the following Unicode characters: `U+0020` (space, `' '`), `U+0009` (tab,
`'\t'`), `U+000A` (LF, `'\n'`), `U+000D` (CR, `'\r'`).

Rust is a "free-form" language, meaning that all forms of whitespace serve
only to separate _tokens_ in the grammar, and have no semantic significance.

A Rust program has identical meaning if each whitespace element is replaced
with any other legal whitespace element, such as a single space character.

## Tokens

~~~~~~~~ {.ebnf .gram}
simple_token : keyword | unop | binop ;
token : simple_token | ident | literal | symbol | whitespace token ;
~~~~~~~~

Tokens are primitive productions in the grammar defined by regular
(non-recursive) languages. "Simple" tokens are given in [string table
production](#string-table-productions) form, and occur in the rest of the
grammar as double-quoted strings. Other tokens have exact rules given.

### Keywords

The keywords in [crate files](#crate-files) are the following strings:

~~~~~~~~ {.keyword}
mod priv pub use
~~~~~~~~

The keywords in [source files](#source-files) are the following strings:

~~~~~~~~ {.keyword}
as assert
break
const copy
do drop
else enum extern
false fn for
if impl
let log loop
match mod move mut
priv pub pure
ref return
self static struct super
true trait type
unsafe use
while
~~~~~~~~

Any of these have special meaning in their respective grammars, and are
excluded from the `ident` rule.

### Literals

A literal is an expression consisting of a single token, rather than a
sequence of tokens, that immediately and directly denotes the value it
evaluates to, rather than referring to it by name or some other evaluation
rule. A literal is a form of constant expression, so is evaluated (primarily)
at compile time.

~~~~~~~~ {.ebnf .gram}
literal : string_lit | char_lit | num_lit ;
~~~~~~~~

#### Character and string literals

~~~~~~~~ {.ebnf .gram}
char_lit : '\x27' char_body '\x27' ;
string_lit : '"' string_body * '"' ;

char_body : non_single_quote
          | '\x5c' [ '\x27' | common_escape ] ;

string_body : non_double_quote
            | '\x5c' [ '\x22' | common_escape ] ;

common_escape : '\x5c'
              | 'n' | 'r' | 't'
              | 'x' hex_digit 2
              | 'u' hex_digit 4
              | 'U' hex_digit 8 ;

hex_digit : 'a' | 'b' | 'c' | 'd' | 'e' | 'f'
          | 'A' | 'B' | 'C' | 'D' | 'E' | 'F'
          | dec_digit ;
dec_digit : '0' | nonzero_dec ;
nonzero_dec: '1' | '2' | '3' | '4'
           | '5' | '6' | '7' | '8' | '9' ;
~~~~~~~~

A _character literal_ is a single Unicode character enclosed within two
`U+0027` (single-quote) characters, with the exception of `U+0027` itself,
which must be _escaped_ by a preceding U+005C character (`\`).

A _string literal_ is a sequence of any Unicode characters enclosed within
two `U+0022` (double-quote) characters, with the exception of `U+0022`
itself, which must be _escaped_ by a preceding `U+005C` character (`\`).

Some additional _escapes_ are available in either character or string
literals. An escape starts with a `U+005C` (`\`) and continues with one of
the following forms:

  * An _8-bit codepoint escape_ escape starts with `U+0078` (`x`) and is
    followed by exactly two _hex digits_. It denotes the Unicode codepoint
    equal to the provided hex value.
  * A _16-bit codepoint escape_ starts with `U+0075` (`u`) and is followed
    by exactly four _hex digits_. It denotes the Unicode codepoint equal to
    the provided hex value.
  * A _32-bit codepoint escape_ starts with `U+0055` (`U`) and is followed
    by exactly eight _hex digits_. It denotes the Unicode codepoint equal to
    the provided hex value.
  * A _whitespace escape_ is one of the characters `U+006E` (`n`), `U+0072`
    (`r`), or `U+0074` (`t`), denoting the unicode values `U+000A` (LF),
    `U+000D` (CR) or `U+0009` (HT) respectively.
  * The _backslash escape_ is the character U+005C (`\`) which must be
    escaped in order to denote *itself*.

#### Number literals

~~~~~~~~ {.ebnf .gram}

num_lit : nonzero_dec [ dec_digit | '_' ] * num_suffix ?
        | '0' [       [ dec_digit | '_' ] + num_suffix ?
              | 'b'   [ '1' | '0' | '_' ] + int_suffix ?
              | 'x'   [ hex_digit | '-' ] + int_suffix ? ] ;

num_suffix : int_suffix | float_suffix ;

int_suffix : 'u' int_suffix_size ?
           | 'i' int_suffix_size ;
int_suffix_size : [ '8' | '1' '6' | '3' '2' | '6' '4' ] ;

float_suffix : [ exponent | '.' dec_lit exponent ? ] float_suffix_ty ? ;
float_suffix_ty : 'f' [ '3' '2' | '6' '4' ] ;
exponent : ['E' | 'e'] ['-' | '+' ] ? dec_lit ;
dec_lit : [ dec_digit | '_' ] + ;
~~~~~~~~

A _number literal_ is either an _integer literal_ or a _floating-point
literal_. The grammar for recognizing the two kinds of literals is mixed,
as they are differentiated by suffixes.

##### Integer literals

An _integer literal_ has one of three forms:

  * A _decimal literal_ starts with a *decimal digit* and continues with any
    mixture of *decimal digits* and _underscores_.
  * A _hex literal_ starts with the character sequence `U+0030` `U+0078`
    (`0x`) and continues as any mixture hex digits and underscores.
  * A _binary literal_ starts with the character sequence `U+0030` `U+0062`
    (`0b`) and continues as any mixture binary digits and underscores.

An integer literal may be followed (immediately, without any spaces) by an
_integer suffix_, which changes the type of the literal. There are two kinds
of integer literal suffix:

  * The `i` and `u` suffixes give the literal type `int` or `uint`,
    respectively.
  * Each of the signed and unsigned machine types `u8`, `i8`,
    `u16`, `i16`, `u32`, `i32`, `u64` and `i64`
    give the literal the corresponding machine type.

The type of an _unsuffixed_ integer literal is determined by type inference.
If a integer type can be _uniquely_ determined from the surrounding program
context, the unsuffixed integer literal has that type.  If the program context
underconstrains the type, the unsuffixed integer literal's type is `int`; if
the program context overconstrains the type, it is considered a static type
error.

Examples of integer literals of various forms:

~~~~
123; 0xff00;                       // type determined by program context
                                   // defaults to int in absence of type
                                   // information

123u;                              // type uint
123_u;                             // type uint
0xff_u8;                           // type u8
0b1111_1111_1001_0000_i32;         // type i32
~~~~

##### Floating-point literals

A _floating-point literal_ has one of two forms:

* Two _decimal literals_ separated by a period
  character `U+002E` (`.`), with an optional _exponent_ trailing after the
  second decimal literal.
* A single _decimal literal_ followed by an _exponent_.

By default, a floating-point literal is of type `float`. A
floating-point literal may be followed (immediately, without any
spaces) by a _floating-point suffix_, which changes the type of the
literal. There are three floating-point suffixes: `f` (for the base
`float` type), `f32`, and `f64` (the 32-bit and 64-bit floating point
types).

Examples of floating-point literals of various forms:

~~~~
123.0;                             // type float
0.1;                               // type float
3f;                                // type float
0.1f32;                            // type f32
12E+99_f64;                        // type f64
~~~~

##### Unit and boolean literals

The _unit value_, the only value of the type that has the same name, is written as `()`.
The two values of the boolean type are written `true` and `false`.

### Symbols

~~~~~~~~ {.ebnf .gram}
symbol : "::" "->"
       | '#' | '[' | ']' | '(' | ')' | '{' | '}'
       | ',' | ';' ;
~~~~~~~~

Symbols are a general class of printable [token](#tokens) that play structural
roles in a variety of grammar productions. They are catalogued here for
completeness as the set of remaining miscellaneous printable tokens that do not
otherwise appear as [unary operators](#unary-operator-expressions), [binary
operators](#binary-operator-expressions), or [keywords](#keywords).


## Paths

~~~~~~~~ {.ebnf .gram}

expr_path : ident [ "::" expr_path_tail ] + ;
expr_path_tail : '<' type_expr [ ',' type_expr ] + '>'
               | expr_path ;

type_path : ident [ type_path_tail ] + ;
type_path_tail : '<' type_expr [ ',' type_expr ] + '>'
               | "::" type_path ;

~~~~~~~~

A _path_ is a sequence of one or more path components _logically_ separated by
a namespace qualifier (`::`). If a path consists of only one component, it may
refer to either an [item](#items) or a [slot](#memory-slots) in a local
control scope. If a path has multiple components, it refers to an item.

Every item has a _canonical path_ within its crate, but the path naming an
item is only meaningful within a given crate. There is no global namespace
across crates; an item's canonical path merely identifies it within the crate.

Two examples of simple paths consisting of only identifier components:

~~~~{.ignore}
x;
x::y::z;
~~~~

Path components are usually [identifiers](#identifiers), but the trailing
component of a path may be an angle-bracket-enclosed list of type
arguments. In [expression](#expressions) context, the type argument list is
given after a final (`::`) namespace qualifier in order to disambiguate it
from a relational expression involving the less-than symbol (`<`). In type
expression context, the final namespace qualifier is omitted.

Two examples of paths with type arguments:

~~~~
# use std::oldmap;
# fn f() {
# fn id<T:Copy>(t: T) -> T { t }
type t = oldmap::HashMap<int,~str>;  // Type arguments used in a type expression
let x = id::<int>(10);           // Type arguments used in a call expression
# }
~~~~

# Syntax extensions

A number of minor features of Rust are not central enough to have their own
syntax, and yet are not implementable as functions. Instead, they are given
names, and invoked through a consistent syntax: `name!(...)`. Examples
include:

* `fmt!` : format data into a string
* `env!` : look up an environment variable's value at compile time
* `stringify!` : pretty-print the Rust expression given as an argument
* `proto!` : define a protocol for inter-task communication
* `include!` : include the Rust expression in the given file
* `include_str!` : include the contents of the given file as a string
* `include_bin!` : include the contents of the given file as a binary blob
* `error!`, `warn!`, `info!`, `debug!` : provide diagnostic information.

All of the above extensions, with the exception of `proto!`, are expressions
with values. `proto!` is an item, defining a new name.

## Macros

~~~~~~~~ {.ebnf .gram}

expr_macro_rules : "macro_rules" '!' ident '(' macro_rule * ')'
macro_rule : '(' matcher * ')' "=>" '(' transcriber * ')' ';'
matcher : '(' matcher * ')' | '[' matcher * ']'
        | '{' matcher * '}' | '$' ident ':' ident
        | '$' '(' matcher * ')' sep_token? [ '*' | '+' ]
        | non_special_token
transcriber : '(' transcriber * ')' | '[' transcriber * ']'
            | '{' transcriber * '}' | '$' ident
            | '$' '(' transcriber * ')' sep_token? [ '*' | '+' ]
            | non_special_token

~~~~~~~~

User-defined syntax extensions are called "macros", and they can be defined
with the `macro_rules!` syntax extension. User-defined macros can currently
be invoked as expressions, statements, or items.

(A `sep_token` is any token other than `*` and `+`. A `non_special_token` is
any token other than a delimiter or `$`.)

Macro invocations are looked up by name, and each macro rule is tried in turn;
the first successful match is transcribed. The matching and transcription
processes are closely related, and will be described together:

### Macro By Example

The macro expander matches and transcribes every token that does not begin with a `$` literally, including delimiters.
For parsing reasons, delimiters must be balanced, but they are otherwise not special.

In the matcher, `$` _name_ `:` _designator_ matches the nonterminal in the
Rust syntax named by _designator_. Valid designators are `item`, `block`,
`stmt`, `pat`, `expr`, `ty` (type), `ident`, `path`, `matchers` (lhs of the `=>` in macro rules),
`tt` (rhs of the `=>` in macro rules). In the transcriber, the designator is already known, and so only
the name of a matched nonterminal comes after the dollar sign.

In both the matcher and transcriber, the Kleene star-like operator indicates repetition.
The Kleene star operator consists of `$` and parens, optionally followed by a separator token, followed by `*` or `+`.
`*` means zero or more repetitions, `+` means at least one repetition.
The parens are not matched or transcribed.
On the matcher side, a name is bound to _all_ of the names it
matches, in a structure that mimics the structure of the repetition
encountered on a successful match. The job of the transcriber is to sort that
structure out.

The rules for transcription of these repetitions are called "Macro By Example".
Essentially, one "layer" of repetition is discharged at a time, and all of
them must be discharged by the time a name is transcribed. Therefore,
`( $( $i:ident ),* ) => ( $i )` is an invalid macro, but
`( $( $i:ident ),* ) => ( $( $i:ident ),*  )` is acceptable (if trivial).

When Macro By Example encounters a repetition, it examines all of the `$`
_name_ s that occur in its body. At the "current layer", they all must repeat
the same number of times, so
` ( $( $i:ident ),* ; $( $j:ident ),* ) => ( $( ($i,$j) ),* )` is valid if
given the argument `(a,b,c ; d,e,f)`, but not `(a,b,c ; d,e)`. The repetition
walks through the choices at that layer in lockstep, so the former input
transcribes to `( (a,d), (b,e), (c,f) )`.

Nested repetitions are allowed.

### Parsing limitations

The parser used by the macro system is reasonably powerful, but the parsing of
Rust syntax is restricted in two ways:

1. The parser will always parse as much as possible. If it attempts to match
`$i:expr [ , ]` against `8 [ , ]`, it will attempt to parse `i` as an array
index operation and fail. Adding a separator can solve this problem.
2. The parser must have eliminated all ambiguity by the time it reaches a `$` _name_ `:` _designator_.
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.


## Syntax extensions useful for the macro author

* `log_syntax!` : print out the arguments at compile time
* `trace_macros!` : supply `true` or `false` to enable or disable printing of the macro expansion process.
* `ident_to_str!` : turn the identifier argument into a string literal
* `concat_idents!` : create a new identifier by concatenating the arguments



# Crates and source files

Rust is a *compiled* language.
Its semantics obey a *phase distinction* between compile-time and run-time.
Those semantic rules that have a *static interpretation* govern the success or failure of compilation.
We refer to these rules as "static semantics".
Semantic rules called "dynamic semantics" govern the behavior of programs at run-time.
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.

The compilation model centres on artifacts called _crates_.
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
analogous to an *assembly* in the ECMA-335 CLI model, a *library* in the
SML/NJ Compilation Manager, a *unit* in the Owens and Flatt module system,
or a *configuration* in Mesa.]

A _crate_ is a unit of compilation and linking, as well as versioning, distribution and runtime loading.
A crate contains a _tree_ of nested [module](#modules) scopes.
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.

The Rust compiler is always invoked with a single source file as input, and always produces a single output crate.
The processing of that source file may result in other source files being loaded as modules.
Source files typically have the extension `.rs` but, by convention,
source files that represent crates have the extension `.rc`, called *crate files*.

A Rust source file describes a module, the name and
location of which -- in the module tree of the current crate -- are defined
from outside the source file: either by an explicit `mod_item` in
a referencing source file, or by the name of the crate itself.

Each source file contains a sequence of zero or more `item` definitions,
and may optionally begin with any number of `attributes` that apply to the containing module.
Atributes on the anonymous crate module define important metadata that influences
the behavior of the compiler.

~~~~~~~~
// Linkage attributes
#[ link(name = "projx",
        vers = "2.5",
        uuid = "9cccc5d5-aceb-4af5-8285-811211826b82") ];

// Additional metadata attributes
#[ desc = "Project X" ];
#[ license = "BSD" ];
#[ author = "Jane Doe" ];

// Specify the output type
#[ crate_type = "lib" ];

// Turn on a warning
#[ warn(non_camel_case_types) ];
~~~~~~~~

A crate that contains a `main` function can be compiled to an executable.
If a `main` function is present, its return type must be [`unit`](#primitive-types) and it must take no arguments.


# Items and attributes

Crates contain [items](#items),
each of which may have some number of [attributes](#attributes) attached to it.

## Items

~~~~~~~~ {.ebnf .gram}
item : mod_item | fn_item | type_item | enum_item
     | const_item | trait_item | impl_item | foreign_mod_item ;
~~~~~~~~

An _item_ is a component of a crate; some module items can be defined in crate
files, but most are defined in source files. Items are organized within a
crate by a nested set of [modules](#modules). Every crate has a single
"outermost" anonymous module; all further items within the crate have
[paths](#paths) within the module tree of the crate.

Items are entirely determined at compile-time, remain constant during
execution, and may reside in read-only memory.

There are several kinds of item:

  * [modules](#modules)
  * [functions](#functions)
  * [type definitions](#type-definitions)
  * [structures](#structures)
  * [enumerations](#enumerations)
  * [constants](#constants)
  * [traits](#traits)
  * [implementations](#implementations)

Some items form an implicit scope for the declaration of sub-items. In other
words, within a function or module, declarations of items can (in many cases)
be mixed with the statements, control blocks, and similar artifacts that
otherwise compose the item body. The meaning of these scoped items is the same
as if the item was declared outside the scope -- it is still a static item --
except that the item's *path name* within the module namespace is qualified by
the name of the enclosing item, or is private to the enclosing item (in the
case of functions).
The grammar specifies the exact locations in which sub-item declarations may appear.

### Type Parameters

All items except modules may be *parameterized* by type. Type parameters are
given as a comma-separated list of identifiers enclosed in angle brackets
(`<...>`), after the name of the item and before its definition.
The type parameters of an item are considered "part of the name", not part of the type of the item.
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.
In practice, the type-inference system can usually infer such argument types from context.
There are no general type-parametric types, only type-parametric items.
That is, Rust has no notion of type abstraction: there are no first-class "forall" types.

### Modules

~~~~~~~~ {.ebnf .gram}
mod_item : "mod" ident ( ';' | '{' mod '}' );
mod : [ view_item | item ] * ;
~~~~~~~~

A module is a container for zero or more [view items](#view-items) and zero or
more [items](#items). The view items manage the visibility of the items
defined within the module, as well as the visibility of names from outside the
module when referenced from inside the module.

A _module item_ is a module, surrounded in braces, named, and prefixed with
the keyword `mod`. A module item introduces a new, named module into the tree
of modules making up a crate. Modules can nest arbitrarily.

An example of a module:

~~~~~~~~
mod math {
    type complex = (f64, f64);
    fn sin(f: f64) -> f64 {
        ...
# die!();
    }
    fn cos(f: f64) -> f64 {
        ...
# die!();
    }
    fn tan(f: f64) -> f64 {
        ...
# die!();
    }
}
~~~~~~~~

Modules and types share the same namespace.
Declaring a named type that has the same name as a module in scope is forbidden:
that is, a type definition, trait, struct, enumeration, or type parameter
can't shadow the name of a module in scope, or vice versa.

A module without a body is loaded from an external file, by default with the same
name as the module, plus the `.rs` extension.
When a nested submodule is loaded from an external file,
it is loaded from a subdirectory path that mirrors the module hierarchy.

~~~ {.xfail-test}
// Load the `vec` module from `vec.rs`
mod vec;

mod task {
    // Load the `local_data` module from `task/local_data.rs`
    mod local_data;
}
~~~

The directories and files used for loading external file modules can be influenced
with the `path` attribute.

~~~ {.xfail-test}
#[path = "task_files"]
mod task {
    // Load the `local_data` module from `task_files/tls.rs`
    #[path = "tls.rs"]
    mod local_data;
}
~~~

#### View items

~~~~~~~~ {.ebnf .gram}
view_item : extern_mod_decl | use_decl ;
~~~~~~~~

A view item manages the namespace of a module.
View items do not define new items, but rather, simply change other items' visibility.
There are several kinds of view item:

 * [`extern mod` declarations](#extern-mod-declarations)
 * [`use` declarations](#use-declarations)

##### Extern mod declarations

~~~~~~~~ {.ebnf .gram}
extern_mod_decl : "extern" "mod" ident [ '(' link_attrs ')' ] ? ;
link_attrs : link_attr [ ',' link_attrs ] + ;
link_attr : ident '=' literal ;
~~~~~~~~

An _`extern mod` declaration_ specifies a dependency on an external crate.
The external crate is then bound into the declaring scope as the `ident` provided in the `extern_mod_decl`.

The external crate is resolved to a specific `soname` at compile time, and a
runtime linkage requirement to that `soname` is passed to the linker for
loading at runtime. The `soname` is resolved at compile time by scanning the
compiler's library path and matching the `link_attrs` provided in the
`use_decl` against any `#link` attributes that were declared on the external
crate when it was compiled. If no `link_attrs` are provided, a default `name`
attribute is assumed, equal to the `ident` given in the `use_decl`.

Three examples of `extern mod` declarations:

~~~~~~~~{.xfail-test}
extern mod pcre (uuid = "54aba0f8-a7b1-4beb-92f1-4cf625264841");

extern mod std; // equivalent to: extern mod std ( name = "std" );

extern mod ruststd (name = "std"); // linking to 'std' under another name
~~~~~~~~

##### Use declarations

~~~~~~~~ {.ebnf .gram}
use_decl : "pub"? "use" ident [ '=' path
                          | "::" path_glob ] ;

path_glob : ident [ "::" path_glob ] ?
          | '*'
          | '{' ident [ ',' ident ] * '}'
~~~~~~~~

A _use declaration_ creates one or more local name bindings synonymous
with some other [path](#paths). Usually a `use` declaration is used to
shorten the path required to refer to a module item.

*Note*: unlike many languages, Rust's `use` declarations do *not* declare
linkage-dependency with external crates. Linkage dependencies are
independently declared with
[`extern mod` declarations](#extern-mod-declarations).

Use declarations support a number of "convenience" notations:

  * Rebinding the target name as a new local name, using the
    syntax `use x = p::q::r;`.
  * Simultaneously binding a list of paths differing only in final element,
    using the glob-like brace syntax `use a::b::{c,d,e,f};`
  * Binding all paths matching a given prefix,
    using the glob-like asterisk syntax `use a::b::*;`

An example of `use` declarations:

~~~~
use foo = core::info;
use core::float::sin;
use core::str::{slice, to_upper};
use core::option::Some;

fn main() {
    // Equivalent to 'log(core::info, core::float::sin(1.0));'
    log(foo, sin(1.0));

    // Equivalent to 'log(core::info, core::option::Some(1.0));'
    log(info, Some(1.0));

    // Equivalent to 'log(core::info,
    //                    core::str::to_upper(core::str::slice("foo", 0, 1)));'
    log(info, to_upper(slice("foo", 0, 1)));
}
~~~~

Like items, `use` declarations are private to the containing module, by default.
Also like items, a `use` declaration can be public, if qualified by the `pub` keyword.
Such a `use` declaration serves to _re-export_ a name.
A public `use` declaration can therefore be used to _redirect_ some public name to a different target definition,
even a definition with a private canonical path, inside a different module.
If a sequence of such redirections form a cycle or cannot be unambiguously resolved, they represent a compile-time error.

An example of re-exporting:
~~~~
# fn main() { }
mod quux {
    pub mod foo {
        pub fn bar() { }
        pub fn baz() { }
    }

    pub use quux::foo::*;
}
~~~~

In this example, the module `quux` re-exports all of the public names defined in `foo`.

Also note that the paths contained in `use` items are relative to the crate root; so, in the previous
example, the use refers to `quux::foo::*`, and not simply to `foo::*`.

### Functions

A _function item_ defines a sequence of [statements](#statements) and an optional final [expression](#expressions), along with a name and a set of parameters.
Functions are declared with the keyword `fn`.
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.

A function may also be copied into a first class *value*, in which case the
value has the corresponding [*function type*](#function-types), and can be
used otherwise exactly as a function item (with a minor additional cost of
calling the function indirectly).

Every control path in a function logically ends with a `return` expression or a
diverging expression. If the outermost block of a function has a
value-producing expression in its final-expression position, that expression
is interpreted as an implicit `return` expression applied to the
final-expression.

An example of a function:

~~~~
fn add(x: int, y: int) -> int {
    return x + y;
}
~~~~

As with `let` bindings, function arguments are irrefutable patterns,
so any pattern that is valid in a let binding is also valid as an argument.

~~~
fn first((value, _): (int, int)) -> int { value }
~~~


#### Generic functions

A _generic function_ allows one or more _parameterized types_ to
appear in its signature. Each type parameter must be explicitly
declared, in an angle-bracket-enclosed, comma-separated list following
the function name.

~~~~ {.xfail-test}
fn iter<T>(seq: &[T], f: fn(T)) {
    for seq.each |elt| { f(elt); }
}
fn map<T, U>(seq: &[T], f: fn(T) -> U) -> ~[U] {
    let mut acc = ~[];
    for seq.each |elt| { acc.push(f(elt)); }
    acc
}
~~~~

Inside the function signature and body, the name of the type parameter
can be used as a type name.

When a generic function is referenced, its type is instantiated based
on the context of the reference. For example, calling the `iter`
function defined above on `[1, 2]` will instantiate type parameter `T`
with `int`, and require the closure parameter to have type
`fn(int)`.

Since a parameter type is opaque to the generic function, the set of
operations that can be performed on it is limited. Values of parameter
type can always be moved, but they can only be copied when the
parameter is given a [`Copy` bound](#type-kinds).

~~~~
fn id<T: Copy>(x: T) -> T { x }
~~~~

Similarly, [trait](#traits) bounds can be specified for type
parameters to allow methods with that trait to be called on values
of that type.


#### Unsafe functions

Unsafe functions are those containing unsafe operations that are not contained in an [`unsafe` block](#unsafe-blocks).
Such a function must be prefixed with the keyword `unsafe`.

Unsafe operations are those that potentially violate the memory-safety guarantees of Rust's static semantics.
Specifically, the following operations are considered unsafe:

  - Dereferencing a [raw pointer](#pointer-types).
  - Casting a [raw pointer](#pointer-types) to a safe pointer type.
  - Breaking the [purity-checking rules](#pure-functions) in a `pure` function.
  - Calling an unsafe function.

##### Unsafe blocks

A block of code can also be prefixed with the `unsafe` keyword, to permit a sequence of unsafe operations in an otherwise-safe function.
This facility exists because the static semantics of Rust are a necessary approximation of the dynamic semantics.
When a programmer has sufficient conviction that a sequence of unsafe operations is actually safe, they can encapsulate that sequence (taken as a whole) within an `unsafe` block. The compiler will consider uses of such code "safe", to the surrounding context.


#### Pure functions

A pure function declaration is identical to a function declaration, except that
it is declared with the additional keyword `pure`. In addition, the typechecker
checks the body of a pure function with a restricted set of typechecking rules.
A pure function may only modify data owned by its own stack frame.
So, a pure function may modify a local variable allocated on the stack, but not a mutable reference that it takes as an argument.
A pure function may only call other pure functions, not general functions.

An example of a pure function:

~~~~
pure fn lt_42(x: int) -> bool {
    return (x < 42);
}
~~~~

Pure functions may call other pure functions:

~~~~{.xfail-test}
pure fn pure_length<T>(ls: List<T>) -> uint { ... }

pure fn nonempty_list<T>(ls: List<T>) -> bool { pure_length(ls) > 0u }
~~~~

These purity-checking rules approximate the concept of referential transparency:
that a call-expression could be rewritten with the literal-expression of its return value, without changing the meaning of the program.
Since they are an approximation, sometimes these rules are *too* restrictive.
Rust allows programmers to violate these rules using [`unsafe` blocks](#unsafe-blocks), which we already saw.
As with any `unsafe` block, those that violate static purity carry transfer the burden of safety-proof from the compiler to the programmer.
Programmers should exercise caution when breaking such rules.

For more details on purity, see [the borrowed pointer tutorial][borrow].

[borrow]: tutorial-borrowed-ptr.html

#### Diverging functions

A special kind of function can be declared with a `!` character where the
output slot type would normally be. For example:

~~~~
fn my_err(s: &str) -> ! {
    log(info, s);
    die!();
}
~~~~

We call such functions "diverging" because they never return a value to the
caller. Every control path in a diverging function must end with a
`fail!()` or a call to another diverging function on every
control path. The `!` annotation does *not* denote a type. Rather, the result
type of a diverging function is a special type called $\bot$ ("bottom") that
unifies with any type. Rust has no syntax for $\bot$.

It might be necessary to declare a diverging function because as mentioned
previously, the typechecker checks that every control path in a function ends
with a [`return`](#return-expressions) or diverging expression. So, if `my_err`
were declared without the `!` annotation, the following code would not
typecheck:

~~~~
# fn my_err(s: &str) -> ! { die!() }

fn f(i: int) -> int {
   if i == 42 {
     return 42;
   }
   else {
     my_err("Bad number!");
   }
}
~~~~

This will not compile without the `!` annotation on `my_err`,
since the `else` branch of the conditional in `f` does not return an `int`,
as required by the signature of `f`.
Adding the `!` annotation to `my_err` informs the typechecker that,
should control ever enter `my_err`, no further type judgments about `f` need to hold,
since control will never resume in any context that relies on those judgments.
Thus the return type on `f` only needs to reflect the `if` branch of the conditional.


#### Extern functions

Extern functions are part of Rust's foreign function interface, providing
the opposite functionality to [foreign modules](#foreign-modules). Whereas
foreign modules allow Rust code to call foreign code, extern functions with
bodies defined in Rust code _can be called by foreign code_. They are defined the
same as any other Rust function, except that they are prepended with the
`extern` keyword.

~~~
extern fn new_vec() -> ~[int] { ~[] }
~~~

Extern functions may not be called from Rust code, but their value
may be taken as a raw `u8` pointer.

~~~
# extern fn new_vec() -> ~[int] { ~[] }
let fptr: *u8 = new_vec;
~~~

The primary motivation of extern functions is to create callbacks
for foreign functions that expect to receive function pointers.

### Type definitions

A _type definition_ defines a new name for an existing [type](#types). Type
definitions are declared with the keyword `type`. Every value has a single,
specific type; the type-specified aspects of a value include:

* Whether the value is composed of sub-values or is indivisible.
* Whether the value represents textual or numerical information.
* Whether the value represents integral or floating-point information.
* The sequence of memory operations required to access the value.
* The [kind](#type-kinds) of the type.

For example, the type `(u8, u8)` defines the set of immutable values that are composite pairs,
each containing two unsigned 8-bit integers accessed by pattern-matching and laid out in memory with the `x` component preceding the `y` component.

### Structures

A _structure_ is a nominal [structure type](#structure-types) defined with the keyword `struct`.

An example of a `struct` item and its use:

~~~~
struct Point {x: int, y: int}
let p = Point {x: 10, y: 11};
let px: int = p.x;
~~~~

A _tuple structure_ is a nominal [tuple type](#tuple-types), also defined with the keyword `struct`.
For example:

~~~~
struct Point(int, int);
let p = Point(10, 11);
let px: int = match p { Point(x, _) => x };
~~~~

### Enumerations

An _enumeration_ is a simultaneous definition of a nominal [enumerated type](#enumerated-types) as well as a set of *constructors*,
that can be used to create or pattern-match values of the corresponding enumerated type.

Enumerations are declared with the keyword `enum`.

An example of an `enum` item and its use:

~~~~
enum Animal {
  Dog,
  Cat
}

let mut a: Animal = Dog;
a = Cat;
~~~~

Enumeration constructors can have either named or unnamed fields:
~~~~
enum Animal {
    Dog (~str, float),
    Cat { name: ~str, weight: float }
}

let mut a: Animal = Dog(~"Cocoa", 37.2);
a = Cat{ name: ~"Spotty", weight: 2.7 };
~~~~

In this example, `Cat` is a _struct-like enum variant_,
whereas `Dog` is simply called an enum variant.
### Constants

~~~~~~~~ {.ebnf .gram}
const_item : "const" ident ':' type '=' expr ';' ;
~~~~~~~~

A *constant* is a named value stored in read-only memory in a crate.
The value bound to a constant is evaluated at compile time.
Constants are declared with the `const` keyword.
A constant item must have an expression giving its definition.
The definition expression of a constant is limited to expression forms that can be evaluated at compile time.

Constants must be explicitly typed. The type may be ```bool```, ```char```, a number, or a type derived from
those primitive types. The derived types are borrowed pointers, static arrays, tuples, and structs.

~~~~
const bit1: uint = 1 << 0;
const bit2: uint = 1 << 1;

const bits: [uint * 2] = [bit1, bit2];
const string: &str = "bitstring";

struct BitsNStrings {
    mybits: [uint *2],
    mystring: &str
}

const bits_n_strings: BitsNStrings = BitsNStrings {
    mybits: bits,
    mystring: string
};
~~~~

### Traits

A _trait_ describes a set of method types.

Traits can include default implementations of methods,
written in terms of some unknown [`self` type](#self-types);
the `self` type may either be completely unspecified,
or constrained by some other trait.

Traits are implemented for specific types through separate [implementations](#implementations).

~~~~
# type Surface = int;
# type BoundingBox = int;

trait Shape {
    fn draw(Surface);
    fn bounding_box() -> BoundingBox;
}
~~~~

This defines a trait with two methods.
All values that have [implementations](#implementations) of this trait in scope can have their `draw` and `bounding_box` methods called,
using `value.bounding_box()` [syntax](#method-call-expressions).

Type parameters can be specified for a trait to make it generic.
These appear after the trait name, using the same syntax used in [generic functions](#generic-functions).

~~~~
trait Seq<T> {
   fn len() -> uint;
   fn elt_at(n: uint) -> T;
   fn iter(fn(T));
}
~~~~

Generic functions may use traits as _bounds_ on their type parameters.
This will have two effects: only types that have the trait may instantiate the parameter,
and within the generic function,
the methods of the trait can be called on values that have the parameter's type.
For example:

~~~~
# type Surface = int;
# trait Shape { fn draw(Surface); }

fn draw_twice<T: Shape>(surface: Surface, sh: T) {
    sh.draw(surface);
    sh.draw(surface);
}
~~~~

Traits also define an [object type](#object-types) with the same name as the trait.
Values of this type are created by [casting](#type-cast-expressions) pointer values
(pointing to a type for which an implementation of the given trait is in scope)
to pointers to the trait name, used as a type.

~~~~
# trait Shape { }
# impl int: Shape { }
# let mycircle = 0;

let myshape: Shape = @mycircle as @Shape;
~~~~

The resulting value is a managed box containing the value that was cast,
along with information that identifies the methods of the implementation that was used.
Values with a trait type can have [methods called](#method-call-expressions) on them,
for any method in the trait,
and can be used to instantiate type parameters that are bounded by the trait.

Trait methods may be static,
which means that they lack a `self` argument.
This means that they can only be called with function call syntax (`f(x)`)
and not method call syntax (`obj.f()`).
The way to refer to the name of a static method is to qualify it with the trait name,
treating the trait name like a module.
For example:

~~~~
trait Num {
    static pure fn from_int(n: int) -> Self;
}
impl float: Num {
    static pure fn from_int(n: int) -> float { n as float }
}
let x: float = Num::from_int(42);
~~~~

Traits may inherit from other traits. For example, in

~~~~
trait Shape { fn area() -> float; }
trait Circle : Shape { fn radius() -> float; }
~~~~

the syntax `Circle : Shape` means that types that implement `Circle` must also have an implementation for `Shape`.
Multiple supertraits are separated by spaces, `trait Circle : Shape Eq { }`.
In an implementation of `Circle` for a given type `T`, methods can refer to `Shape` methods,
since the typechecker checks that any type with an implementation of `Circle` also has an implementation of `Shape`.

In type-parameterized functions,
methods of the supertrait may be called on values of subtrait-bound type parameters.
Refering to the previous example of `trait Circle : Shape`:

~~~
# trait Shape { fn area() -> float; }
# trait Circle : Shape { fn radius() -> float; }
fn radius_times_area<T: Circle>(c: T) -> float {
    // `c` is both a Circle and a Shape
    c.radius() * c.area()
}
~~~

Likewise, supertrait methods may also be called on trait objects.

~~~ {.xfail-test}
# trait Shape { fn area() -> float; }
# trait Circle : Shape { fn radius() -> float; }
# impl int: Shape { fn area() -> float { 0.0 } }
# impl int: Circle { fn radius() -> float { 0.0 } }
# let mycircle = 0;

let mycircle: Circle = @mycircle as @Circle;
let nonsense = mycircle.radius() * mycircle.area();
~~~

### Implementations

An _implementation_ is an item that implements a [trait](#traits) for a specific type.

Implementations are defined with the keyword `impl`.

~~~~
# type Point = {x: float, y: float};
# type Surface = int;
# type BoundingBox = {x: float, y: float, width: float, height: float};
# trait Shape { fn draw(Surface); fn bounding_box() -> BoundingBox; }
# fn do_draw_circle(s: Surface, c: Circle) { }

type Circle = {radius: float, center: Point};

impl Circle: Shape {
    fn draw(s: Surface) { do_draw_circle(s, self); }
    fn bounding_box() -> BoundingBox {
        let r = self.radius;
        {x: self.center.x - r, y: self.center.y - r,
         width: 2.0 * r, height: 2.0 * r}
    }
}
~~~~

It is possible to define an implementation without referring to a trait.
The methods in such an implementation can only be used statically
(as direct calls on the values of the type that the implementation targets).
In such an implementation, the type after the colon is omitted.
Such implementations are limited to nominal types (enums, structs),
and the implementation must appear in the same module or a sub-module as the `self` type.

When a trait _is_ specified in an `impl`,
all methods declared as part of the trait must be implemented,
with matching types and type parameter counts.

An implementation can take type parameters,
which can be different from the type parameters taken by the trait it implements.
Implementation parameters are written after after the `impl` keyword.

~~~~
# trait Seq<T> { }

impl<T> ~[T]: Seq<T> {
   ...
}
impl u32: Seq<bool> {
   /* Treat the integer as a sequence of bits */
}
~~~~

### Foreign modules

~~~ {.ebnf .gram}
foreign_mod_item : "extern mod" ident '{' foreign_mod '} ;
foreign_mod : [ foreign_fn ] * ;
~~~

Foreign modules form the basis for Rust's foreign function interface. A
foreign module describes functions in external, non-Rust
libraries.
Functions within foreign modules are declared in the same way as other Rust functions,
with the exception that they may not have a body and are instead terminated by a semicolon.

~~~
# use libc::{c_char, FILE};
# #[nolink]

extern mod c {
    fn fopen(filename: *c_char, mode: *c_char) -> *FILE;
}
~~~

Functions within foreign modules may be called by Rust code, just like functions defined in Rust.
The Rust compiler automatically translates between the Rust ABI and the foreign ABI.

The name of the foreign module has special meaning to the Rust compiler in
that it will treat the module name as the name of a library to link to,
performing the linking as appropriate for the target platform. The name
given for the foreign module will be transformed in a platform-specific way
to determine the name of the library. For example, on Linux the name of the
foreign module is prefixed with 'lib' and suffixed with '.so', so the
foreign mod 'rustrt' would be linked to a library named 'librustrt.so'.

A number of [attributes](#attributes) control the behavior of foreign
modules.

By default foreign modules assume that the library they are calling use the
standard C "cdecl" ABI. Other ABIs may be specified using the `abi`
attribute as in

~~~{.xfail-test}
// Interface to the Windows API
#[abi = "stdcall"]
extern mod kernel32 { }
~~~

The `link_name` attribute allows the default library naming behavior to
be overridden by explicitly specifying the name of the library.

~~~{.xfail-test}
#[link_name = "crypto"]
extern mod mycrypto { }
~~~

The `nolink` attribute tells the Rust compiler not to do any linking for the foreign module.
This is particularly useful for creating foreign
modules for libc, which tends to not follow standard library naming
conventions and is linked to all Rust programs anyway.

## Attributes

~~~~~~~~{.ebnf .gram}
attribute : '#' '[' attr_list ']' ;
attr_list : attr [ ',' attr_list ]*
attr : ident [ '=' literal
             | '(' attr_list ')' ] ? ;
~~~~~~~~

Static entities in Rust -- crates, modules and items -- may have _attributes_
applied to them. ^[Attributes in Rust are modeled on Attributes in ECMA-335,
C#]
An attribute is a general, free-form metadatum that is interpreted according to name, convention, and language and compiler version.
Attributes may appear as any of

* A single identifier, the attribute name
* An identifier followed by the equals sign '=' and a literal, providing a key/value pair
* An identifier followed by a parenthesized list of sub-attribute arguments

Attributes terminated by a semi-colon apply to the entity that the attribute is declared
within. Attributes that are not terminated by a semi-colon apply to the next entity.

An example of attributes:

~~~~~~~~{.xfail-test}
// General metadata applied to the enclosing module or crate.
#[license = "BSD"];

// A function marked as a unit test
#[test]
fn test_foo() {
  ...
}

// A conditionally-compiled module
#[cfg(target_os="linux")]
mod bar {
  ...
}

// A lint attribute used to suppress a warning/error
#[allow(non_camel_case_types)]
pub type int8_t = i8;
~~~~~~~~

> **Note:** In future versions of Rust, user-provided extensions to the compiler will be able to interpret attributes.
> When this facility is provided, the compiler will distinguish between language-reserved and user-available attributes.

At present, only the Rust compiler interprets attributes, so all attribute
names are effectively reserved. Some significant attributes include:

* The `doc` attribute, for documenting code in-place.
* The `cfg` attribute, for conditional-compilation by build-configuration.
* The `link` attribute, for describing linkage metadata for a crate.
* The `test` attribute, for marking functions as unit tests.
* The `allow`, `warn`, `forbid`, and `deny` attributes, for controling lint checks. Lint checks supported
by the compiler can be found via `rustc -W help`.

Other attributes may be added or removed during development of the language.


# Statements and expressions

Rust is _primarily_ an expression language. This means that most forms of
value-producing or effect-causing evaluation are directed by the uniform
syntax category of _expressions_. Each kind of expression can typically _nest_
within each other kind of expression, and rules for evaluation of expressions
involve specifying both the value produced by the expression and the order in
which its sub-expressions are themselves evaluated.

In contrast, statements in Rust serve _mostly_ to contain and explicitly
sequence expression evaluation.

## Statements

A _statement_ is a component of a block, which is in turn a component of an
outer [expression](#expressions) or [function](#functions).

Rust has two kinds of statement:
[declaration statements](#declaration-statements) and
[expression statements](#expression-statements).

### Declaration statements

A _declaration statement_ is one that introduces one or more *names* into the enclosing statement block.
The declared names may denote new slots or new items.

#### Item declarations

An _item declaration statement_ has a syntactic form identical to an
[item](#items) declaration within a module. Declaring an item -- a function,
enumeration, type, constant, trait, implementation or module -- locally
within a statement block is simply a way of restricting its scope to a narrow
region containing all of its uses; it is otherwise identical in meaning to
declaring the item outside the statement block.

Note: there is no implicit capture of the function's dynamic environment when
declaring a function-local item.


#### Slot declarations

~~~~~~~~{.ebnf .gram}
let_decl : "let" pat [':' type ] ? [ init ] ? ';' ;
init : [ '=' ] expr ;
~~~~~~~~

A _slot declaration_ introduces a new set of slots, given by a pattern.
The pattern may be followed by a type annotation, and/or an initializer expression.
When no type annotation is given, the compiler will infer the type,
or signal an error if insufficient type information is available for definite inference.
Any slots introduced by a slot declaration are visible from the point of declaration until the end of the enclosing block scope.

### Expression statements

An _expression statement_ is one that evaluates an [expression](#expressions)
and drops its result. The purpose of an expression statement is often to cause
the side effects of the expression's evaluation.

## Expressions

An expression plays the dual roles of causing side effects and producing a
*value*. Expressions are said to *evaluate to* a value, and the side effects
are caused during *evaluation*. Many expressions contain sub-expressions as
operands; the definition of each kind of expression dictates whether or not,
and in which order, it will evaluate its sub-expressions, and how the
expression's value derives from the value of its sub-expressions.

In this way, the structure of execution -- both the overall sequence of
observable side effects and the final produced value -- is dictated by the
structure of expressions. Blocks themselves are expressions, so the nesting
sequence of block, statement, expression, and block can repeatedly nest to an
arbitrary depth.

#### Lvalues, rvalues and temporaries

Expressions are divided into two main categories: _lvalues_ and _rvalues_.
Likewise within each expression, sub-expressions may occur in _lvalue context_ or _rvalue context_.
The evaluation of an expression depends both on its own category and the context it occurs within.

[Path](#path-expressions), [field](#field-expressions) and [index](#index-expressions) expressions are lvalues.
All other expressions are rvalues.

The left operand of an [assignment](#assignment-expressions),
[binary move](#binary-move-expressions) or
[compound-assignment](#compound-assignment-expressions) expression is an lvalue context,
as is the single operand of a unary [borrow](#unary-operator-expressions),
or [move](#unary-move-expressions) expression,
and _both_ operands of a [swap](#swap-expressions) expression.
All other expression contexts are rvalue contexts.

When an lvalue is evaluated in an _lvalue context_, it denotes a memory location;
when evaluated in an _rvalue context_, it denotes the value held _in_ that memory location.

When an rvalue is used in lvalue context, a temporary un-named lvalue is created and used instead.
A temporary's lifetime equals the largest lifetime of any borrowed pointer that points to it.

#### Moved and copied types

When a [local variable](#memory-slots) is used as an [rvalue](#lvalues-rvalues-and-temporaries)
the variable will either be [moved](#move-expressions) or [copied](#copy-expressions),
depending on its type.
For types that contain mutable fields or [owning pointers](#owning-pointers), the variable is moved.
All other types are copied.


### Literal expressions

A _literal expression_ consists of one of the [literal](#literals)
forms described earlier. It directly describes a number, character,
string, boolean value, or the unit value.

~~~~~~~~ {.literals}
();        // unit type
"hello";   // string type
'5';       // character type
5;         // integer type
~~~~~~~~

### Path expressions

A [path](#paths) used as an expression context denotes either a local variable or an item.
Path expressions are [lvalues](#lvalues-rvalues-and-temporaries).

### Tuple expressions

Tuples are written by enclosing two or more comma-separated
expressions in parentheses. They are used to create [tuple-typed](#tuple-types)
values.

~~~~~~~~ {.tuple}
(0f, 4.5f);
("a", 4u, true);
~~~~~~~~

### Structure expressions

~~~~~~~~{.ebnf .gram}
struct_expr : expr_path '{' ident ':' expr
                      [ ',' ident ':' expr ] *
                      [ ".." expr ] '}' |
              expr_path '(' expr
                      [ ',' expr ] * ')'
~~~~~~~~

There are several forms of structure expressions.
A _structure expression_ consists of the [path](#paths) of a [structure item](#structures),
followed by a brace-enclosed list of one or more comma-separated name-value pairs,
providing the field values of a new instance of the structure.
A field name can be any identifier, and is separated from its value expression by a colon.
To indicate that a field is mutable, the `mut` keyword is written before its name.

A _tuple structure expression_ constists of the [path](#paths) of a [structure item](#structures),
followed by a parenthesized list of one or more comma-separated expressions
(in other words, the path of a structured item followed by a tuple expression).
The structure item must be a tuple structure item.

The following are examples of structure expressions:

~~~~
# struct Point { x: float, y: float }
# struct TuplePoint(float, float);
# mod game { pub struct User { name: &str, age: uint, mut score: uint } }
# use game;
Point {x: 10f, y: 20f};
TuplePoint(10f, 20f);
let u = game::User {name: "Joe", age: 35u, mut score: 100_000};
~~~~

A structure expression forms a new value of the named structure type.

A structure expression can terminate with the syntax `..` followed by an expression to denote a functional update.
The expression following `..` (the base) must be of the same structure type as the new structure type being formed.
A new structure will be created, of the same type as the base expression, with the given values for the fields that were explicitly specified,
and the values in the base record for all other fields.

~~~~
# struct Point3d { x: int, y: int, z: int }
let base = Point3d {x: 1, y: 2, z: 3};
Point3d {y: 0, z: 10, .. base};
~~~~

### Record expressions

~~~~~~~~{.ebnf .gram}
rec_expr : '{' ident ':' expr
               [ ',' ident ':' expr ] *
               [ ".." expr ] '}'
~~~~~~~~

> **Note:** In future versions of Rust, record expressions and [record types](#record-types) will be removed.

A [_record_](#record-types) _expression_ is one or more comma-separated
name-value pairs enclosed by braces. A fieldname can be any identifier,
and is separated from its value expression by a
colon. To indicate that a field is mutable, the `mut` keyword is
written before its name.

~~~~
{x: 10f, y: 20f};
{name: "Joe", age: 35u, score: 100_000};
{ident: "X", mut count: 0u};
~~~~

The order of the fields in a record expression is significant, and
determines the type of the resulting value. `{a: u8, b: u8}` and `{b:
u8, a: u8}` are two different fields.

A record expression can terminate with the syntax `..` followed by an
expression to denote a functional update. The expression following
`..` (the base) must be of a record type that includes at least all the
fields mentioned in the record expression. A new record will be
created, of the same type as the base expression, with the given
values for the fields that were explicitly specified, and the values
in the base record for all other fields. The ordering of the fields in
such a record expression is not significant.

~~~~
let base = {x: 1, y: 2, z: 3};
{y: 0, z: 10, .. base};
~~~~

### Method-call expressions

~~~~~~~~{.ebnf .gram}
method_call_expr : expr '.' ident paren_expr_list ;
~~~~~~~~

A _method call_ consists of an expression followed by a single dot, an identifier, and a parenthesized expression-list.
Method calls are resolved to methods on specific traits,
either statically dispatching to a method if the exact `self`-type of the left-hand-side is known,
or dynamically dispatching if the left-hand-side expression is an indirect [object type](#object-types).


### Field expressions

~~~~~~~~{.ebnf .gram}
field_expr : expr '.' ident
~~~~~~~~

A _field expression_ consists of an expression followed by a single dot and an identifier,
when not immediately followed by a parenthesized expression-list (the latter is a [method call expression](#method-call-expressions)).
A field expression denotes a field of a [structure](#structure-types) or [record](#record-types).

~~~~~~~~ {.field}
myrecord.myfield;
{a: 10, b: 20}.a;
~~~~~~~~

A field access on a record is an [lvalue](#lvalues-rvalues-and-temporaries) referring to the value of that field.
When the field is mutable, it can be [assigned](#assignment-expressions) to.

When the type of the expression to the left of the dot is a pointer to a record or structure,
it is automatically derferenced to make the field access possible.


### Vector expressions

~~~~~~~~{.ebnf .gram}
vec_expr : '[' "mut"? vec_elems? ']'

vec_elems : [expr [',' expr]*] | [expr ',' ".." expr]
~~~~~~~~

A [_vector_](#vector-types) _expression_ is written by enclosing zero or
more comma-separated expressions of uniform type in square brackets.

~~~~
[1, 2, 3, 4];
["a", "b", "c", "d"];
[0, ..128];             // vector with 128 zeros
[0u8, 0u8, 0u8, 0u8];
~~~~

### Index expressions

~~~~~~~~{.ebnf .gram}
idx_expr : expr '[' expr ']'
~~~~~~~~


[Vector](#vector-types)-typed expressions can be indexed by writing a
square-bracket-enclosed expression (the index) after them. When the
vector is mutable, the resulting [lvalue](#lvalues-rvalues-and-temporaries) can be assigned to.

Indices are zero-based, and may be of any integral type. Vector access
is bounds-checked at run-time. When the check fails, it will put the
task in a _failing state_.

~~~~
# do task::spawn_unlinked {

([1, 2, 3, 4])[0];
(["a", "b"])[10]; // fails

# }
~~~~

### Unary operator expressions

Rust defines six symbolic unary operators,
in addition to the unary [copy](#unary-copy-expressions) and [move](#unary-move-expressions) operators.
They are all written as prefix operators, before the expression they apply to.

`-`
  : Negation. May only be applied to numeric types.
`*`
  : Dereference. When applied to a [pointer](#pointer-types) it denotes the pointed-to location.
    For pointers to mutable locations, the resulting [lvalue](#lvalues-rvalues-and-temporaries) can be assigned to.
    For [enums](#enumerated-types) that have only a single variant, containing a single parameter,
    the dereference operator accesses this parameter.
`!`
  : Logical negation. On the boolean type, this flips between `true` and
    `false`. On integer types, this inverts the individual bits in the
    two's complement representation of the value.
`@` and `~`
  :  [Boxing](#pointer-types) operators. Allocate a box to hold the value they are applied to,
     and store the value in it. `@` creates a managed box, whereas `~` creates an owned box.
`&`
  : Borrow operator. Returns a borrowed pointer, pointing to its operand.
    The operand of a borrowed pointer is statically proven to outlive the resulting pointer.
    If the borrow-checker cannot prove this, it is a compilation error.

### Binary operator expressions

~~~~~~~~{.ebnf .gram}
binop_expr : expr binop expr ;
~~~~~~~~

Binary operators expressions are given in terms of
[operator precedence](#operator-precedence).

#### Arithmetic operators

Binary arithmetic expressions are syntactic sugar for calls to built-in traits,
defined in the `core::ops` module of the `core` library.
This means that arithmetic operators can be overridden for user-defined types.
The default meaning of the operators on standard types is given here.

`+`
  : Addition and vector/string concatenation.
    Calls the `add` method on the `core::ops::Add` trait.
`-`
  : Subtraction.
    Calls the `sub` method on the `core::ops::Sub` trait.
`*`
  : Multiplication.
    Calls the `mul` method on the `core::ops::Mul` trait.
`/`
  : Division.
    Calls the `div` method on the `core::ops::Div` trait.
`%`
  : Modulo (a.k.a. "remainder").
    Calls the `modulo` method on the `core::ops::Modulo` trait.

#### Bitwise operators

Bitwise operators are, like the [arithmetic operators](#arithmetic-operators),
syntactic sugar for calls to built-in traits.
This means that bitwise operators can be overridden for user-defined types.
The default meaning of the operators on standard types is given here.

`&`
  : And.
    Calls the `bitand` method on the `core::ops::BitAnd` trait.
`|`
  : Inclusive or.
    Calls the `bitor` method on the `core::ops::BitOr` trait.
`^`
  : Exclusive or.
    Calls the `bitxor` method on the `core::ops::BitXor` trait.
`<<`
  : Logical left shift.
    Calls the `shl` method on the `core::ops::Shl` trait.
`>>`
  : Logical right shift.
    Calls the `shr` method on the `core::ops::Shr` trait.

#### Lazy boolean operators

The operators `||` and `&&` may be applied to operands of boolean
type. The first performs the 'or' operation, and the second the 'and'
operation. They differ from `|` and `&` in that the right-hand operand
is only evaluated when the left-hand operand does not already
determine the outcome of the expression. That is, `||` only evaluates
its right-hand operand when the left-hand operand evaluates to `false`,
and `&&` only when it evaluates to `true`.

#### Comparison operators

Comparison operators are, like the [arithmetic operators](#arithmetic-operators),
and [bitwise operators](#bitwise-operators),
syntactic sugar for calls to built-in traits.
This means that comparison operators can be overridden for user-defined types.
The default meaning of the operators on standard types is given here.

`==`
  : Equal to.
    Calls the `eq` method on the `core::cmp::Eq` trait.
`!=`
  : Unequal to.
    Calls the `ne` method on the `core::cmp::Eq` trait.
`<`
  : Less than.
    Calls the `lt` method on the `core::cmp::Ord` trait.
`>`
  : Greater than.
    Calls the `gt` method on the `core::cmp::Ord` trait.
`<=`
  : Less than or equal.
    Calls the `le` method on the `core::cmp::Ord` trait.
`>=`
  : Greater than or equal.
    Calls the `ge` method on the `core::cmp::Ord` trait.


#### Type cast expressions

A type cast expression is denoted with the binary operator `as`.

Executing an `as` expression casts the value on the left-hand side to the type
on the right-hand side.

A numeric value can be cast to any numeric type.
A raw pointer value can be cast to or from any integral type or raw pointer type.
Any other cast is unsupported and will fail to compile.

An example of an `as` expression:

~~~~
# fn sum(v: &[float]) -> float { 0.0 }
# fn len(v: &[float]) -> int { 0 }

fn avg(v: &[float]) -> float {
  let sum: float = sum(v);
  let sz: float = len(v) as float;
  return sum / sz;
}
~~~~

#### Swap expressions

A _swap expression_ consists of an [lvalue](#lvalues-rvalues-and-temporaries) followed by a bi-directional arrow (`<->`) and another [lvalue](#lvalues-rvalues-and-temporaries).

Evaluating a swap expression causes, as a side effect, the values held in the left-hand-side and right-hand-side [lvalues](#lvalues-rvalues-and-temporaries) to be exchanged indivisibly.

Evaluating a swap expression neither changes reference counts,
nor deeply copies any owned structure pointed to by the moved [rvalue](#lvalues-rvalues-and-temporaries).
Instead, the swap expression represents an indivisible *exchange of ownership*,
between the right-hand-side and the left-hand-side of the expression.
No allocation or destruction is entailed.

An example of three different swap expressions:

~~~~~~~~
# let mut x = &mut [0];
# let mut a = &mut [0];
# let i = 0;
# let y = {mut z: 0};
# let b = {mut c: 0};

x <-> a;
x[i] <-> a[i];
y.z <-> b.c;
~~~~~~~~


#### Assignment expressions

An _assignment expression_ consists of an [lvalue](#lvalues-rvalues-and-temporaries) expression followed by an
equals sign (`=`) and an [rvalue](#lvalues-rvalues-and-temporaries) expression.

Evaluating an assignment expression [either copies or moves](#moved-and-copied-types) its right-hand operand to its left-hand operand.

~~~~
# let mut x = 0;
# let y = 0;

x = y;
~~~~

#### Compound assignment expressions

The `+`, `-`, `*`, `/`, `%`, `&`, `|`, `^`, `<<`, and `>>`
operators may be composed with the `=` operator. The expression `lval
OP= val` is equivalent to `lval = lval OP val`. For example, `x = x +
1` may be written as `x += 1`.

Any such expression always has the [`unit`](#primitive-types) type.

#### Operator precedence

The precedence of Rust binary operators is ordered as follows, going
from strong to weak:

~~~~ {.precedence}
* / %
as
+ -
<< >>
&
^
|
< > <= >=
== !=
&&
||
= <->
~~~~

Operators at the same precedence level are evaluated left-to-right.

### Grouped expressions

An expression enclosed in parentheses evaluates to the result of the enclosed
expression.  Parentheses can be used to explicitly specify evaluation order
within an expression.

~~~~~~~~{.ebnf .gram}
paren_expr : '(' expr ')' ;
~~~~~~~~

An example of a parenthesized expression:

~~~~
let x = (2 + 3) * 4;
~~~~

### Unary copy expressions

~~~~~~~~{.ebnf .gram}
copy_expr : "copy" expr ;
~~~~~~~~

A _unary copy expression_ consists of the unary `copy` operator applied to
some argument expression.

Evaluating a copy expression first evaluates the argument expression, then
copies the resulting value, allocating any memory necessary to hold the new
copy.

[Managed boxes](#pointer-types) (type `@`) are, as usual, shallow-copied,
as are raw and borrowed pointers.
[Owned boxes](#pointer-types), [owned vectors](#vector-types) and similar owned types are deep-copied.

Since the binary [assignment operator](#assignment-expressions) `=` performs a copy or move implicitly,
the unary copy operator is typically only used to cause an argument to a function to be copied and passed by value.

An example of a copy expression:

~~~~
fn mutate(mut vec: ~[int]) {
   vec[0] = 10;
}

let v = ~[1,2,3];

mutate(copy v);   // Pass a copy

assert v[0] == 1; // Original was not modified
~~~~

### Unary move expressions

~~~~~~~~{.ebnf .gram}
move_expr : "move" expr ;
~~~~~~~~

A _unary move expression_ is similar to a [unary copy](#unary-copy-expressions) expression,
except that it can only be applied to a [local variable](#memory-slots),
and it performs a _move_ on its operand, rather than a copy.
That is, the memory location denoted by its operand is de-initialized after evaluation,
and the resulting value is a shallow copy of the operand,
even if the operand is an [owning type](#type-kinds).


> **Note:** In future versions of Rust, `move` may be removed as a separate operator;
> moves are now [automatically performed](#moved-and-copied-types) for most cases `move` would be appropriate.


### Call expressions

~~~~~~~~ {.abnf .gram}
expr_list : [ expr [ ',' expr ]* ] ? ;
paren_expr_list : '(' expr_list ')' ;
call_expr : expr paren_expr_list ;
~~~~~~~~

A _call expression_ invokes a function, providing zero or more input slots and
an optional reference slot to serve as the function's output, bound to the
`lval` on the right hand side of the call. If the function eventually returns,
then the expression completes.

An example of a call expression:

~~~~
# fn add(x: int, y: int) -> int { 0 }

let x: int = add(1, 2);
~~~~

### Lambda expressions

~~~~~~~~ {.abnf .gram}
ident_list : [ ident [ ',' ident ]* ] ? ;
lambda_expr : '|' ident_list '|' expr ;
~~~~~~~~

A _lambda expression_ (a.k.a. "anonymous function expression") defines a function and denotes it as a value,
in a single expression.
Lambda expressions are written by prepending a list of identifiers, surrounded by pipe symbols (`|`),
to an expression.

A lambda expression denotes a function mapping parameters to the expression to the right of the `ident_list`.
The identifiers in the `ident_list` are the parameters to the function, with types inferred from context.

Lambda expressions are most useful when passing functions as arguments to other functions,
as an abbreviation for defining and capturing a separate fucntion.

Significantly, lambda expressions _capture their environment_,
which regular [function definitions](#functions) do not.

The exact type of capture depends on the [function type](#function-types) inferred for the lambda expression;
in the simplest and least-expensive form, the environment is captured by reference,
effectively borrowing pointers to all outer variables referenced inside the function.
Other forms of capture include making copies of captured variables,
and moving values from the environment into the lambda expression's captured environment.

An example of a lambda expression:

~~~~
fn ten_times(f: fn(int)) {
    let mut i = 0;
    while i < 10 {
        f(i);
        i += 1;
    }
}

ten_times(|j| io::println(fmt!("hello, %d", j)));

~~~~

### While loops

~~~~~~~~{.ebnf .gram}
while_expr : "while" expr '{' block '}' ;
~~~~~~~~

A `while` loop begins by evaluating the boolean loop conditional expression.
If the loop conditional expression evaluates to `true`, the loop body block
executes and control returns to the loop conditional expression. If the loop
conditional expression evaluates to `false`, the `while` expression completes.

An example:

~~~~
let mut i = 0;

while i < 10 {
    io::println("hello\n");
    i = i + 1;
}
~~~~

### Infinite loops

The keyword `loop` in Rust appears both in _loop expressions_ and in _continue expressions_.
A loop expression denotes an infinite loop;
see [Continue expressions](#continue-expressions) for continue expressions.

~~~~~~~~{.ebnf .gram}
loop_expr : "loop" [ ident ':' ] '{' block '}';
~~~~~~~~

A `loop` expression may optionally have a _label_.
If a label is present,
then labeled `break` and `loop` expressions nested within this loop may exit out of this loop or return control to its head.
See [Break expressions](#break-expressions).

### Break expressions

~~~~~~~~{.ebnf .gram}
break_expr : "break" [ ident ];
~~~~~~~~

A `break` expression has an optional `label`.
If the label is absent, then executing a `break` expression immediately terminates the innermost loop enclosing it.
It is only permitted in the body of a loop.
If the label is present, then `break foo` terminates the loop with label `foo`,
which need not be the innermost label enclosing the `break` expression,
but must enclose it.

### Continue expressions

~~~~~~~~{.ebnf .gram}
continue_expr : "loop" [ ident ];
~~~~~~~~

A continue expression, written `loop`, also has an optional `label`.
If the label is absent,
then executing a `loop` expression immediately terminates the current iteration of the innermost loop enclosing it,
returning control to the loop *head*.
In the case of a `while` loop,
the head is the conditional expression controlling the loop.
In the case of a `for` loop, the head is the call-expression controlling the loop.
If the label is present, then `loop foo` returns control to the head of the loop with label `foo`,
which need not be the innermost label enclosing the `break` expression,
but must enclose it.

A `loop` expression is only permitted in the body of a loop.


### Do expressions

~~~~~~~~{.ebnf .gram}
do_expr : "do" expr [ '|' ident_list '|' ] ? '{' block '}' ;
~~~~~~~~

A _do expression_ provides a more-familiar block-syntax for a [lambda expression](#lambda-expressions),
including a special translation of [return expressions](#return-expressions) inside the supplied block.

The optional `ident_list` and `block` provided in a `do` expression are parsed as though they constitute a lambda expression;
if the `ident_list` is missing, an empty `ident_list` is implied.

The lambda expression is then provided as a _trailing argument_
to the outermost [call](#call-expressions) or [method call](#method-call-expressions) expression
in the `expr` following `do`.
If the `expr` is a [path expression](#path-expressions), it is parsed as though it is a call expression.
If the `expr` is a [field expression](#field-expressions), it is parsed as though it is a method call expression.

In this example, both calls to `f` are equivalent:

~~~~
# fn f(f: fn(int)) { }
# fn g(i: int) { }

f(|j| g(j));

do f |j| {
    g(j);
}
~~~~


### For expressions

~~~~~~~~{.ebnf .gram}
for_expr : "for" expr [ '|' ident_list '|' ] ? '{' block '}' ;
~~~~~~~~

A _for expression_ is similar to a [`do` expression](#do-expressions),
in that it provides a special block-form of lambda expression,
suited to passing the `block` function to a higher-order function implementing a loop.

Like a `do` expression, a `return` expression inside a `for` expresison is rewritten,
to access a local flag that causes an early return in the caller.

Additionally, any occurrence of a [return expression](#return-expressions)
inside the `block` of a `for` expression is rewritten
as a reference to an (anonymous) flag set in the caller's environment,
which is checked on return from the `expr` and, if set,
causes a corresponding return from the caller.
In this way, the meaning of `return` statements in language built-in control blocks is preserved,
if they are rewritten using lambda functions and `do` expressions as abstractions.

Like `return` expressions, any [`break`](#break-expressions) and [`loop`](#loop-expressions) expressions
are rewritten inside `for` expressions, with a combination of local flag variables,
and early boolean-valued returns from the `block` function,
such that the meaning of `break` and `loop` is preserved in a primitive loop
when rewritten as a `for` loop controlled by a higher order function.

An example a for loop:

~~~~
# type foo = int;
# fn bar(f: foo) { }
# let a = 0, b = 0, c = 0;

let v: &[foo] = &[a, b, c];

for v.each |e| {
    bar(*e);
}
~~~~


### If expressions

~~~~~~~~{.ebnf .gram}
if_expr : "if" expr '{' block '}'
          else_tail ? ;

else_tail : "else" [ if_expr
                   | '{' block '}' ] ;
~~~~~~~~

An `if` expression is a conditional branch in program control. The form of
an `if` expression is a condition expression, followed by a consequent
block, any number of `else if` conditions and blocks, and an optional
trailing `else` block. The condition expressions must have type
`bool`. If a condition expression evaluates to `true`, the
consequent block is executed and any subsequent `else if` or `else`
block is skipped. If a condition expression evaluates to `false`, the
consequent block is skipped and any subsequent `else if` condition is
evaluated. If all `if` and `else if` conditions evaluate to `false`
then any `else` block is executed.


### Match expressions

~~~~~~~~{.ebnf .gram}
match_expr : "match" expr '{' match_arm [ '|' match_arm ] * '}' ;

match_arm : match_pat '=>' [ expr "," | '{' block '}' ] ;

match_pat : pat [ ".." pat ] ? [ "if" expr ] ;
~~~~~~~~


A `match` expression branches on a *pattern*. The exact form of matching that
occurs depends on the pattern. Patterns consist of some combination of
literals, destructured enum constructors, structures, records and tuples, variable binding
specifications, wildcards (`*`), and placeholders (`_`). A `match` expression has a *head
expression*, which is the value to compare to the patterns. The type of the
patterns must equal the type of the head expression.

In a pattern whose head expression has an `enum` type, a placeholder (`_`) stands for a
*single* data field, whereas a wildcard `*` stands for *all* the fields of a particular
variant. For example:

~~~~
enum List<X> { Nil, Cons(X, @List<X>) }

let x: List<int> = Cons(10, @Cons(11, @Nil));

match x {
    Cons(_, @Nil) => die!(~"singleton list"),
    Cons(*)       => return,
    Nil           => die!(~"empty list")
}
~~~~

The first pattern matches lists constructed by applying `Cons` to any head value, and a
tail value of `@Nil`. The second pattern matches _any_ list constructed with `Cons`,
ignoring the values of its arguments. The difference between `_` and `*` is that the pattern `C(_)` is only type-correct if
`C` has exactly one argument, while the pattern `C(*)` is type-correct for any enum variant `C`, regardless of how many arguments `C` has.

To execute an `match` expression, first the head expression is evaluated, then
its value is sequentially compared to the patterns in the arms until a match
is found. The first arm with a matching pattern is chosen as the branch target
of the `match`, any variables bound by the pattern are assigned to local
variables in the arm's block, and control enters the block.

An example of an `match` expression:


~~~~
# fn process_pair(a: int, b: int) { }
# fn process_ten() { }

enum List<X> { Nil, Cons(X, @List<X>) }

let x: List<int> = Cons(10, @Cons(11, @Nil));

match x {
    Cons(a, @Cons(b, _)) => {
        process_pair(a,b);
    }
    Cons(10, _) => {
        process_ten();
    }
    Nil => {
        return;
    }
    _ => {
        die!();
    }
}
~~~~

Records and structures can also be pattern-matched and their fields bound to variables.
When matching fields of a record,
the fields being matched are specified first,
then a placeholder (`_`) represents the remaining fields.

~~~~
# type options = {choose: bool, size: ~str};
# type player = {player: ~str, stats: (), options: options};
# fn load_stats() { }
# fn choose_player(r: &player) { }
# fn next_player() { }

fn main() {
    let r = {
        player: ~"ralph",
        stats: load_stats(),
        options: {
            choose: true,
            size: ~"small"
        }
    };

    match r {
      {options: {choose: true, _}, _} => {
        choose_player(&r)
      }
      {player: ref p, options: {size: ~"small", _}, _} => {
        log(info, (copy *p) + ~" is small");
      }
      _ => {
        next_player();
      }
    }
}
~~~~

Patterns that bind variables default to binding to a copy of the matched value. This can be made
explicit using the ```copy``` keyword, changed to bind to a borrowed pointer by using the ```ref```
keyword, or to a mutable borrowed pointer using ```ref mut```, or the value can be moved into
the new binding using ```move```.

A pattern that's just an identifier,
like `Nil` in the previous answer,
could either refer to an enum variant that's in scope,
or bind a new variable.
The compiler resolves this ambiguity by forbidding variable bindings that occur in ```match``` patterns from shadowing names of variants that are in scope.
For example, wherever ```List``` is in scope,
a ```match``` pattern would not be able to bind ```Nil``` as a new name.
The compiler interprets a variable pattern `x` as a binding _only_ if there is no variant named `x` in scope.
A convention you can use to avoid conflicts is simply to name variants with upper-case letters,
and local variables with lower-case letters.

Multiple match patterns may be joined with the `|` operator.
A range of values may be specified with `..`.
For example:

~~~~
# let x = 2;

let message = match x {
  0 | 1  => "not many",
  2 .. 9 => "a few",
  _      => "lots"
};
~~~~

Range patterns only work on scalar types
(like integers and characters; not like vectors and structs, which have sub-components).
A range pattern may not be a sub-range of another range pattern inside the same `match`.

Finally, match patterns can accept *pattern guards* to further refine the
criteria for matching a case. Pattern guards appear after the pattern and
consist of a bool-typed expression following the `if` keyword. A pattern
guard may refer to the variables bound within the pattern they follow.

~~~~
# let maybe_digit = Some(0);
# fn process_digit(i: int) { }
# fn process_other(i: int) { }

let message = match maybe_digit {
  Some(x) if x < 10 => process_digit(x),
  Some(x) => process_other(x),
  None => die!()
};
~~~~

### Return expressions

~~~~~~~~{.ebnf .gram}
return_expr : "return" expr ? ;
~~~~~~~~

Return expressions are denoted with the keyword `return`. Evaluating a `return`
expression moves its argument into the output slot of the current
function, destroys the current function activation frame, and transfers
control to the caller frame.

An example of a `return` expression:

~~~~
fn max(a: int, b: int) -> int {
   if a > b {
      return a;
   }
   return b;
}
~~~~

### Log expressions

~~~~~~~~{.ebnf .gram}
log_expr : "log" '(' level ',' expr ')' ;
~~~~~~~~

Evaluating a `log` expression may, depending on runtime configuration, cause a
value to be appended to an internal diagnostic logging buffer provided by the
runtime or emitted to a system console. Log expressions are enabled or
disabled dynamically at run-time on a per-task and per-item basis. See
[logging system](#logging-system).

Each `log` expression must be provided with a *level* argument in
addition to the value to log. The logging level is a `u32` value, where
lower levels indicate more-urgent levels of logging. By default, the lowest
four logging levels (`1_u32 ... 4_u32`) are predefined as the constants
`error`, `warn`, `info` and `debug` in the `core` library.

Additionally, the macros `error!`, `warn!`, `info!` and `debug!` are defined
in the default syntax-extension namespace. These expand into calls to the
logging facility composed with calls to the `fmt!` string formatting
syntax-extension.

The following examples all produce the same output, logged at the `error`
logging level:

~~~~
# let filename = "bulbasaur";

// Full version, logging a value.
log(core::error, ~"file not found: " + filename);

// Log-level abbreviated, since core::* is used by default.
log(error, ~"file not found: " + filename);

// Formatting the message using a format-string and fmt!
log(error, fmt!("file not found: %s", filename));

// Using the error! macro, that expands to the previous call.
error!("file not found: %s", filename);
~~~~

A `log` expression is *not evaluated* when logging at the specified logging-level, module or task is disabled at runtime.
This makes inactive `log` expressions very cheap;
they should be used extensively in Rust code, as diagnostic aids,
as they add little overhead beyond a single integer-compare and branch at runtime.

Logging is presently implemented as a language built-in feature,
as it makes use of compiler-provided, per-module data tables and flags.
In the future, logging will move into a library, and will no longer be a core expression type.
It is therefore recommended to use the macro forms of logging (`error!`, `debug!`, etc.) to minimize disruption in code that uses logging.


### Assert expressions

~~~~~~~~{.ebnf .gram}
assert_expr : "assert" expr ;
~~~~~~~~

> **Note:** In future versions of Rust, `assert` will be changed from a full expression to a macro.

An `assert` expression causes the program to fail if its `expr` argument evaluates to `false`.
The failure carries string representation of the false expression.

# Type system

## Types

Every slot, item and value in a Rust program has a type. The _type_ of a *value*
defines the interpretation of the memory holding it.

Built-in types and type-constructors are tightly integrated into the language,
in nontrivial ways that are not possible to emulate in user-defined
types. User-defined types have limited capabilities.

### Primitive types

The primitive types are the following:

* The "unit" type `()`, having the single "unit" value `()` (occasionally called "nil").
  ^[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.]
* The boolean type `bool` with values `true` and `false`.
* The machine types.
* The machine-dependent integer and floating-point types.

#### Machine types

The machine types are the following:


* The unsigned word types `u8`, `u16`, `u32` and `u64`, with values drawn from
  the integer intervals $[0, 2^8 - 1]$, $[0, 2^{16} - 1]$, $[0, 2^{32} - 1]$ and
  $[0, 2^{64} - 1]$ respectively.

* The signed two's complement word types `i8`, `i16`, `i32` and `i64`, with
  values drawn from the integer intervals $[-(2^7), 2^7 - 1]$,
  $[-(2^{15}), 2^{15} - 1]$, $[-(2^{31}), 2^{31} - 1]$, $[-(2^{63}), 2^{63} - 1]$
  respectively.

* The IEEE 754-2008 `binary32` and `binary64` floating-point types: `f32` and
  `f64`, respectively.

#### Machine-dependent integer types

The Rust type `uint`^[A Rust `uint` is analogous to a C99 `uintptr_t`.] is an
unsigned integer type with target-machine-dependent size. Its size, in
bits, is equal to the number of bits required to hold any memory address on
the target machine.

The Rust type `int`^[A Rust `int` is analogous to a C99 `intptr_t`.] is a
two's complement signed integer type with target-machine-dependent size. Its
size, in bits, is equal to the size of the rust type `uint` on the same target
machine.


#### Machine-dependent floating point type

The Rust type `float` is a machine-specific type equal to one of the supported
Rust floating-point machine types (`f32` or `f64`). It is the largest
floating-point type that is directly supported by hardware on the target
machine, or if the target machine has no floating-point hardware support, the
largest floating-point type supported by the software floating-point library
used to support the other floating-point machine types.

Note that due to the preference for hardware-supported floating-point, the
type `float` may not be equal to the largest *supported* floating-point type.


### Textual types

The types `char` and `str` hold textual data.

A value of type `char` is a Unicode character, represented as a 32-bit
unsigned word holding a UCS-4 codepoint.

A value of type `str` is a Unicode string, represented as a vector of 8-bit
unsigned bytes holding a sequence of UTF-8 codepoints.
Since `str` is of indefinite size, it is not a _first class_ type,
but can only be instantiated through a pointer type,
such as `&str`, `@str` or `~str`.


### Tuple types

The tuple type-constructor forms a new heterogeneous product of values similar
to the record type-constructor. The differences are as follows:

* tuple elements cannot be mutable, unlike record fields
* tuple elements are not named and can be accessed only by pattern-matching

Tuple types and values are denoted by listing the types or values of their
elements, respectively, in a parenthesized, comma-separated
list. Single-element tuples are not legal; all tuples have two or more values.

The members of a tuple are laid out in memory contiguously, like a record, in
order specified by the tuple type.

An example of a tuple type and its use:

~~~~
type Pair = (int,&str);
let p: Pair = (10,"hello");
let (a, b) = p;
assert b != "world";
~~~~


### Vector types

The vector type-constructor represents a homogeneous array of values of a given type.
A vector has a fixed size.
A vector type can be accompanied by _definite_ size, written with a trailing asterisk and integer literal, such as `[int * 10]`.
Such a definite-sized vector can be treated as a first class type since its size is known statically.
A vector without such a size is said to be of _indefinite_ size,
and is therefore not a _first class_ type,
can only be instantiated through a pointer type,
such as `&[T]`, `@[T]` or `~[T]`.
The kind of a vector type depends on the kind of its member type, as with other simple structural types.

An example of a vector type and its use:

~~~~
let v: &[int] = &[7, 5, 3];
let i: int = v[2];
assert (i == 3);
~~~~

All accessible elements of a vector are always initialized, and access to a vector is always bounds-checked.


### Structure types

A `struct` *type* is a heterogeneous product of other types, called the *fields* of the type.
^[`struct` types are analogous `struct` types in C,
the *record* types of the ML family,
or the *structure* types of the Lisp family.]

New instances of a `struct` can be constructed with a [struct expression](#struct-expressions).

The memory order of fields in a `struct` is given by the item defining it.
Fields may be given in any order in a corresponding struct *expression*;
the resulting `struct` value will always be laid out in memory in the order specified by the corresponding *item*.

The fields of a `struct` may be qualified by [visibility modifiers](#visibility-modifiers),
to restrict access to implementation-private data in a structure.

A `tuple struct` type is just like a structure type, except that the fields are anonymous.

### Enumerated types

An *enumerated type* is a nominal, heterogeneous disjoint union type,
denoted by the name of an [`enum` item](#enumerations).
^[The `enum` type is analogous to a `data` constructor declaration in ML,
or a *pick ADT* in Limbo.]

An [`enum` item](#enumerations) declares both the type and a number of *variant constructors*,
each of which is independently named and takes an optional tuple of arguments.

New instances of an `enum` can be constructed by calling one of the variant constructors,
in a [call expression](#call-expressions).

Any `enum` value consumes as much memory as the largest variant constructor for its corresponding `enum` type.

Enum types cannot be denoted *structurally* as types,
but must be denoted by named reference to an [`enum` item](#enumerations).


### Recursive types

Nominal types -- [enumerations](#enumerated-types) and [structures](#structure-types) -- may be recursive.
That is, each `enum` constructor or `struct` field may refer, directly or indirectly, to the enclosing `enum` or `struct` type itself.
Such recursion has restrictions:

* Recursive types must include a nominal type in the recursion
  (not mere [type definitions](#type-definitions),
   or other structural types such as [vectors](#vector-types) or [tuples](#tuple-types)).
* A recursive `enum` item must have at least one non-recursive constructor
  (in order to give the recursion a basis case).
* The size of a recursive type must be finite;
  in other words the recursive fields of the type must be [pointer types](#pointer-types).
* Recursive type definitions can cross module boundaries, but not module *visibility* boundaries,
  or crate boundaries (in order to simplify the module system and type checker).

An example of a *recursive* type and its use:

~~~~
enum List<T> {
  Nil,
  Cons(T, @List<T>)
}

let a: List<int> = Cons(7, @Cons(13, @Nil));
~~~~


### Record types

> **Note:** Records are not nominal types, thus do not directly support recursion, visibility control,
> out-of-order field initialization, or coherent trait implementation.
> Records are therefore deprecated and will be removed in future versions of Rust.
> [Structure types](#structure-types) should be used instead.

The record type-constructor forms a new heterogeneous product of values.
Fields of a record type are accessed by name and are arranged in memory in the order specified by the record type.

An example of a record type and its use:

~~~~
type Point = {x: int, y: int};
let p: Point = {x: 10, y: 11};
let px: int = p.x;
~~~~


### Pointer types

All pointers in Rust are explicit first-class values.
They can be copied, stored into data structures, and returned from functions.
There are four varieties of pointer in Rust:

Managed pointers (`@`)
  : These point to managed heap allocations (or "boxes") in the task-local, managed heap.
    Managed pointers are written `@content`,
    for example `@int` means a managed pointer to a managed box containing an integer.
    Copying a managed pointer is a "shallow" operation:
    it involves only copying the pointer itself
    (as well as any reference-count or GC-barriers required by the managed heap).
    Dropping a managed pointer does not necessarily release the box it points to;
    the lifecycles of managed boxes are subject to an unspecified garbage collection algorithm.

Owning pointers (`~`)
  : These point to owned heap allocations (or "boxes") in the shared, inter-task heap.
    Each owned box has a single owning pointer; pointer and pointee retain a 1:1 relationship at all times.
    Owning pointers are written `~content`,
    for example `~int` means an owning pointer to an owned box containing an integer.
    Copying an owned box is a "deep" operation:
    it involves allocating a new owned box and copying the contents of the old box into the new box.
    Releasing an owning pointer immediately releases its corresponding owned box.

Borrowed pointers (`&`)
  : These point to memory _owned by some other value_.
    Borrowed pointers arise by (automatic) conversion from owning pointers, managed pointers,
    or by applying the borrowing operator `&` to some other value,
    including [lvalues, rvalues or temporaries](#lvalues-rvalues-and-temporaries).
    Borrowed pointers are written `&content`, or in some cases `&f/content` for some lifetime-variable `f`,
    for example `&int` means a borrowed pointer to an integer.
    Copying a borrowed pointer is a "shallow" operation:
    it involves only copying the pointer itself.
    Releasing a borrowed pointer typically has no effect on the value it points to,
    with the exception of temporary values,
    which are released when the last borrowed pointer to them is released.

Raw pointers (`*`)
  : Raw pointers are pointers without safety or liveness guarantees.
    Raw pointers are written `*content`,
    for example `*int` means a raw pointer to an integer.
    Copying or dropping a raw pointer is has no effect on the lifecycle of any other value.
    Dereferencing a raw pointer or converting it to any other pointer type is an [`unsafe` operation](#unsafe-functions).
    Raw pointers are generally discouraged in Rust code;
    they exist to support interoperability with foreign code,
    and writing performance-critical or low-level functions.


### Function types

The function type-constructor `fn` forms new function types. A function type
consists of a set of function-type modifiers (`pure`, `unsafe`, `extern`, etc.),
a sequence of input slots and an output slot.

An example of a `fn` type:

~~~~~~~~
fn add(x: int, y: int) -> int {
  return x + y;
}

let mut x = add(5,7);

type Binop = fn(int,int) -> int;
let bo: Binop = add;
x = bo(5,7);
~~~~~~~~

### Object types

Every trait item (see [traits](#traits)) defines a type with the same name as the trait.
This type is called the _object type_ of the trait.
Object types permit "late binding" of methods, dispatched using _virtual method tables_ ("vtables").
Whereas most calls to trait methods are "early bound" (statically resolved) to specific implementations at compile time,
a call to a method on an object type is only resolved to a vtable entry at compile time.
The actual implementation for each vtable entry can vary on an object-by-object basis.

Given a pointer-typed expression `E` of type `&T`, `~T` or `@T`, where `T` implements trait `R`,
casting `E` to the corresponding pointer type `&R`, `~R` or `@R` results in a value of the _object type_ `R`.
This result is represented as a pair of pointers:
the vtable pointer for the `T` implementation of `R`, and the pointer value of `E`.

An example of an object type:

~~~~~~~~
trait Printable {
  fn to_str() -> ~str;
}

impl int: Printable {
  fn to_str() -> ~str { int::to_str(self) }
}

fn print(a: @Printable) {
   io::println(a.to_str());
}

fn main() {
   print(@10 as @Printable);
}
~~~~~~~~

In this example, the trait `Printable` occurs as an object type in both the type signature of `print`,
and the cast expression in `main`.

### Type parameters

Within the body of an item that has type parameter declarations, the names of its type parameters are types:

~~~~~~~
fn map<A: Copy, B: Copy>(f: fn(A) -> B, xs: &[A]) -> ~[B] {
   if xs.len() == 0 { return ~[]; }
   let first: B = f(xs[0]);
   let rest: ~[B] = map(f, xs.slice(1, xs.len()));
   return ~[first] + rest;
}
~~~~~~~

Here, `first` has type `B`, referring to `map`'s `B` type parameter; and `rest` has
type `~[B]`, a vector type with element type `B`.

### Self types

The special type `self` has a meaning within methods inside an
impl item. It refers to the type of the implicit `self` argument. For
example, in:

~~~~~~~~
trait Printable {
  fn make_string() -> ~str;
}

impl ~str: Printable {
  fn make_string() -> ~str { copy self }
}
~~~~~~~~

`self` refers to the value of type `~str` that is the receiver for a
call to the method `make_string`.

## Type kinds

Types in Rust are categorized into kinds, based on various properties of the components of the type.
The kinds are:

`Const`
  : Types of this kind are deeply immutable;
    they contain no mutable memory locations directly or indirectly via pointers.
`Owned`
  : Types of this kind can be safely sent between tasks.
    This kind includes scalars, owning pointers, owned closures, and
    structural types containing only other owned types. All `Owned` types are `Static`.
`Static`
  : Types of this kind do not contain any borrowed pointers;
    this can be a useful guarantee for code that breaks borrowing assumptions using [`unsafe` operations](#unsafe-functions).
`Copy`
  : This kind includes all types that can be copied. All types with
    sendable kind are copyable, as are managed boxes, managed closures,
    trait types, and structural types built out of these.
    Types with destructors (types that implement `Drop`) can not implement `Copy`.
`Drop`
  : This is not strictly a kind, but its presence interacts with kinds: the `Drop`
    trait provides a single method `finalize` that takes no parameters, and is run
    when values of the type are dropped. Such a method is called a "destructor",
    and are always executed in "top-down" order: a value is completely destroyed
    before any of the values it owns run their destructors. Only `Owned` types
    that do not implement `Copy` can implement `Drop`.

> **Note:** The `finalize` method may be renamed in future versions of Rust.

_Default_
  : Types with destructors, closure environments,
    and various other _non-first-class_ types,
    are not copyable at all.
    Such types can usually only be accessed through pointers,
    or in some cases, moved between mutable locations.

Kinds can be supplied as _bounds_ on type parameters, like traits,
in which case the parameter is constrained to types satisfying that kind.

By default, type parameters do not carry any assumed kind-bounds at all.

Any operation that causes a value to be copied requires the type of that value to be of copyable kind,
so the `Copy` bound is frequently required on function type parameters.
For example, this is not a valid program:

~~~~{.xfail-test}
fn box<T>(x: T) -> @T { @x }
~~~~

Putting `x` into a managed box involves copying, and the `T` parameter has the default (non-copyable) kind.
To change that, a bound is declared:

~~~~
fn box<T: Copy>(x: T) -> @T { @x }
~~~~

Calling this second version of `box` on a noncopyable type is not
allowed. When instantiating a type parameter, the kind bounds on the
parameter are checked to be the same or narrower than the kind of the
type that it is instantiated with.

Sending operations are not part of the Rust language, but are
implemented in the library. Generic functions that send values bound
the kind of these values to sendable.

# Memory and concurrency models

Rust has a memory model centered around concurrently-executing _tasks_. Thus
its memory model and its concurrency model are best discussed simultaneously,
as parts of each only make sense when considered from the perspective of the
other.

When reading about the memory model, keep in mind that it is partitioned in
order to support tasks; and when reading about tasks, keep in mind that their
isolation and communication mechanisms are only possible due to the ownership
and lifetime semantics of the memory model.

## Memory model

A Rust program's memory consists of a static set of *items*, a set of
[tasks](#tasks) each with its own *stack*, and a *heap*. Immutable portions of
the heap may be shared between tasks, mutable portions may not.

Allocations in the stack consist of *slots*, and allocations in the heap
consist of *boxes*.


### Memory allocation and lifetime

The _items_ of a program are those functions, modules and types
that have their value calculated at compile-time and stored uniquely in the
memory image of the rust process. Items are neither dynamically allocated nor
freed.

A task's _stack_ consists of activation frames automatically allocated on
entry to each function as the task executes. A stack allocation is reclaimed
when control leaves the frame containing it.

The _heap_ is a general term that describes two separate sets of boxes:
managed boxes -- which may be subject to garbage collection -- and owned
boxes.  The lifetime of an allocation in the heap depends on the lifetime of
the box values pointing to it. Since box values may themselves be passed in
and out of frames, or stored in the heap, heap allocations may outlive the
frame they are allocated within.

### Memory ownership

A task owns all memory it can *safely* reach through local variables,
as well as managed, owning and borrowed pointers.

When a task sends a value that has the `Owned` trait to another task,
it loses ownership of the value sent and can no longer refer to it.
This is statically guaranteed by the combined use of "move semantics",
and the compiler-checked _meaning_ of the `Owned` trait:
it is only instantiated for (transitively) sendable kinds of data constructor and pointers,
never including managed or borrowed pointers.

When a stack frame is exited, its local allocations are all released, and its
references to boxes (both managed and owned) are dropped.

A managed box may (in the case of a recursive, mutable managed type) be cyclic;
in this case the release of memory inside the managed structure may be deferred
until task-local garbage collection can reclaim it. Code can ensure no such
delayed deallocation occurs by restricting itself to owned boxes and similar
unmanaged kinds of data.

When a task finishes, its stack is necessarily empty and it therefore has no
references to any boxes; the remainder of its heap is immediately freed.


### Memory slots

A task's stack contains slots.

A _slot_ is a component of a stack frame, either a function parameter,
a [temporary](#lvalues-rvalues-and-temporaries), or a local variable.

A _local variable_ (or *stack-local* allocation) holds a value directly,
allocated within the stack's memory. The value is a part of the stack frame.

Local variables are immutable unless declared with `let mut`.  The
`mut` keyword applies to all local variables declared within that
declaration (so `let mut x, y` declares two mutable variables, `x` and
`y`).

Function parameters are immutable unless declared with `mut`. The
`mut` keyword applies only to the following parameter (so `|mut x, y|`
and `fn f(mut x: ~int, y: ~int)` declare one mutable variable `x` and
one immutable variable `y`).

Local variables are not initialized when allocated; the entire frame worth of
local variables are allocated at once, on frame-entry, in an uninitialized
state. Subsequent statements within a function may or may not initialize the
local variables. Local variables can be used only after they have been
initialized; this is enforced by the compiler.


### Memory boxes

A _box_ is a reference to a heap allocation holding another value. There
are two kinds of boxes: *managed boxes* and *owned boxes*.

A _managed box_ type or value is constructed by the prefix *at* sigil `@`.

An _owned box_ type or value is constructed by the prefix *tilde* sigil `~`.

Multiple managed box values can point to the same heap allocation; copying a
managed box value makes a shallow copy of the pointer (optionally incrementing
a reference count, if the managed box is implemented through
reference-counting).

Owned box values exist in 1:1 correspondence with their heap allocation;
copying an owned box value makes a deep copy of the heap allocation and
produces a pointer to the new allocation.

An example of constructing one managed box type and value, and one owned box
type and value:

~~~~~~~~
let x: @int = @10;
let x: ~int = ~10;
~~~~~~~~

Some operations (such as field selection) implicitly dereference boxes. An
example of an _implicit dereference_ operation performed on box values:

~~~~~~~~
let x = @{y: 10};
assert x.y == 10;
~~~~~~~~

Other operations act on box values as single-word-sized address values. For
these operations, to access the value held in the box requires an explicit
dereference of the box value. Explicitly dereferencing a box is indicated with
the unary *star* operator `*`. Examples of such _explicit dereference_
operations are:

* copying box values (`x = y`)
* passing box values to functions (`f(x,y)`)


An example of an explicit-dereference operation performed on box values:

~~~~~~~~
fn takes_boxed(b: @int) {
}

fn takes_unboxed(b: int) {
}

fn main() {
    let x: @int = @10;
    takes_boxed(x);
    takes_unboxed(*x);
}
~~~~~~~~

## Tasks

An executing Rust program consists of a tree of tasks. A Rust _task_
consists of an entry function, a stack, a set of outgoing communication
channels and incoming communication ports, and ownership of some portion of
the heap of a single operating-system process.

Multiple Rust tasks may coexist in a single operating-system process. The
runtime scheduler maps tasks to a certain number of operating-system threads;
by default a number of threads is used based on the number of concurrent
physical CPUs detected at startup, but this can be changed dynamically at
runtime. When the number of tasks exceeds the number of threads -- which is
quite possible -- the tasks are multiplexed onto the threads ^[This is an M:N
scheduler, which is known to give suboptimal results for CPU-bound concurrency
problems. In such cases, running with the same number of threads as tasks can
give better results. The M:N scheduling in Rust exists to support very large
numbers of tasks in contexts where threads are too resource-intensive to use
in a similar volume. The cost of threads varies substantially per operating
system, and is sometimes quite low, so this flexibility is not always worth
exploiting.]


### Communication between tasks

Rust tasks are isolated and generally unable to interfere with one another's memory directly,
except through [`unsafe` code](#unsafe-functions).
All contact between tasks is mediated by safe forms of ownership transfer,
and data races on memory are prohibited by the type system.

Inter-task communication and co-ordination facilities are provided in the standard library.
These include:

  - synchronous and asynchronous communication channels with various communication topologies
  - read-only and read-write shared variables with various safe mutual exclusion patterns
  - simple locks and semaphores

When such facilities carry values, the values are restricted to the [`Owned` type-kind](#type-kinds).
Restricting communication interfaces to this kind ensures that no borrowed or managed pointers move between tasks.
Thus access to an entire data structure can be mediated through its owning "root" value;
no further locking or copying is required to avoid data races within the substructure of such a value.


### Task lifecycle

The _lifecycle_ of a task consists of a finite set of states and events
that cause transitions between the states. The lifecycle states of a task are:

* running
* blocked
* failing
* dead

A task begins its lifecycle -- once it has been spawned -- in the *running*
state. In this state it executes the statements of its entry function, and any
functions called by the entry function.

A task may transition from the *running* state to the *blocked*
state any time it makes a blocking communication call. When the
call can be completed -- when a message arrives at a sender, or a
buffer opens to receive a message -- then the blocked task will
unblock and transition back to *running*.

A task may transition to the *failing* state at any time, due being
killed by some external event or internally, from the evaluation of a
`fail!()` macro. Once *failing*, a task unwinds its stack and
transitions to the *dead* state. Unwinding the stack of a task is done by
the task itself, on its own control stack. If a value with a destructor is
freed during unwinding, the code for the destructor is run, also on the task's
control stack. Running the destructor code causes a temporary transition to a
*running* state, and allows the destructor code to cause any subsequent
state transitions.  The original task of unwinding and failing thereby may
suspend temporarily, and may involve (recursive) unwinding of the stack of a
failed destructor. Nonetheless, the outermost unwinding activity will continue
until the stack is unwound and the task transitions to the *dead*
state. There is no way to "recover" from task failure.  Once a task has
temporarily suspended its unwinding in the *failing* state, failure
occurring from within this destructor results in *hard* failure.  The
unwinding procedure of hard failure frees resources but does not execute
destructors.  The original (soft) failure is still resumed at the point where
it was temporarily suspended.

A task in the *dead* state cannot transition to other states; it exists
only to have its termination status inspected by other tasks, and/or to await
reclamation when the last reference to it drops.


### Task scheduling

The currently scheduled task is given a finite *time slice* in which to
execute, after which it is *descheduled* at a loop-edge or similar
preemption point, and another task within is scheduled, pseudo-randomly.

An executing task can yield control at any time, by making a library call to
`core::task::yield`, which deschedules it immediately. Entering any other
non-executing state (blocked, dead) similarly deschedules the task.


# Runtime services, linkage and debugging


The Rust _runtime_ is a relatively compact collection of C++ and Rust code
that provides fundamental services and datatypes to all Rust tasks at
run-time. It is smaller and simpler than many modern language runtimes. It is
tightly integrated into the language's execution model of memory, tasks,
communication and logging.

> **Note:** The runtime library will merge with the `core` library in future versions of Rust.

### Memory allocation

The runtime memory-management system is based on a _service-provider
interface_, through which the runtime requests blocks of memory from its
environment and releases them back to its environment when they are no longer
in use. The default implementation of the service-provider interface consists
of the C runtime functions `malloc` and `free`.

The runtime memory-management system in turn supplies Rust tasks with
facilities for allocating, extending and releasing stacks, as well as
allocating and freeing boxed values.


### Built in types

The runtime provides C and Rust code to assist with various built-in types,
such as vectors, strings, and the low level communication system (ports,
channels, tasks).

Support for other built-in types such as simple types, tuples, records, and
enums is open-coded by the Rust compiler.



### Task scheduling and communication

The runtime provides code to manage inter-task communication.  This includes
the system of task-lifecycle state transitions depending on the contents of
queues, as well as code to copy values between queues and their recipients and
to serialize values for transmission over operating-system inter-process
communication facilities.


### Logging system

The runtime contains a system for directing [logging
expressions](#log-expressions) to a logging console and/or internal logging
buffers. Logging expressions can be enabled per module.

Logging output is enabled by setting the `RUST_LOG` environment
variable.  `RUST_LOG` accepts a logging specification made up of a
comma-separated list of paths, with optional log levels. For each
module containing log expressions, if `RUST_LOG` contains the path to
that module or a parent of that module, then logs of the appropriate
level will be output to the console.

The path to a module consists of the crate name, any parent modules,
then the module itself, all separated by double colons (`::`).  The
optional log level can be appended to the module path with an equals
sign (`=`) followed by the log level, from 1 to 4, inclusive. Level 1
is the error level, 2 is warning, 3 info, and 4 debug. Any logs
less than or equal to the specified level will be output. If not
specified then log level 4 is assumed.

As an example, to see all the logs generated by the compiler, you would set
`RUST_LOG` to `rustc`, which is the crate name (as specified in its `link`
[attribute](#attributes)). To narrow down the logs to just crate resolution,
you would set it to `rustc::metadata::creader`. To see just error logging
use `rustc=0`.

Note that when compiling either `.rs` or `.rc` files that don't specify a
crate name the crate is given a default name that matches the source file,
with the extension removed. In that case, to turn on logging for a program
compiled from, e.g. `helloworld.rs`, `RUST_LOG` should be set to `helloworld`.

As a convenience, the logging spec can also be set to a special pseudo-crate,
`::help`. In this case, when the application starts, the runtime will
simply output a list of loaded modules containing log expressions, then exit.

The Rust runtime itself generates logging information. The runtime's logs are
generated for a number of artificial modules in the `::rt` pseudo-crate,
and can be enabled just like the logs for any standard module. The full list
of runtime logging modules follows.

* `::rt::mem` Memory management
* `::rt::comm` Messaging and task communication
* `::rt::task` Task management
* `::rt::dom` Task scheduling
* `::rt::trace` Unused
* `::rt::cache` Type descriptor cache
* `::rt::upcall` Compiler-generated runtime calls
* `::rt::timer` The scheduler timer
* `::rt::gc` Garbage collection
* `::rt::stdlib` Functions used directly by the standard library
* `::rt::kern` The runtime kernel
* `::rt::backtrace` Log a backtrace on task failure
* `::rt::callback` Unused


# Appendix: Rationales and design tradeoffs

*TODO*.

# Appendix: Influences and further references

## Influences


>  The essential problem that must be solved in making a fault-tolerant
>  software system is therefore that of fault-isolation. Different programmers
>  will write different modules, some modules will be correct, others will have
>  errors. We do not want the errors in one module to adversely affect the
>  behaviour of a module which does not have any errors.
>
>  &mdash; Joe Armstrong


>  In our approach, all data is private to some process, and processes can
>  only communicate through communications channels. *Security*, as used
>  in this paper, is the property which guarantees that processes in a system
>  cannot affect each other except by explicit communication.
>
>  When security is absent, nothing which can be proven about a single module
>  in isolation can be guaranteed to hold when that module is embedded in a
>  system [...]
>
>  &mdash;  Robert Strom and Shaula Yemini


>  Concurrent and applicative programming complement each other. The
>  ability to send messages on channels provides I/O without side effects,
>  while the avoidance of shared data helps keep concurrent processes from
>  colliding.
>
>  &mdash; Rob Pike


Rust is not a particularly original language. It may however appear unusual
by contemporary standards, as its design elements are drawn from a number of
"historical" languages that have, with a few exceptions, fallen out of
favour. Five prominent lineages contribute the most, though their influences
have come and gone during the course of Rust's development:

* The NIL (1981) and Hermes (1990) family. These languages were developed by
  Robert Strom, Shaula Yemini, David Bacon and others in their group at IBM
  Watson Research Center (Yorktown Heights, NY, USA).

* The Erlang (1987) language, developed by Joe Armstrong, Robert Virding, Claes
  Wikstr&ouml;m, Mike Williams and others in their group at the Ericsson Computer
  Science Laboratory (&Auml;lvsj&ouml;, Stockholm, Sweden) .

* The Sather (1990) language, developed by Stephen Omohundro, Chu-Cheow Lim,
  Heinz Schmidt and others in their group at The International Computer
  Science Institute of the University of California, Berkeley (Berkeley, CA,
  USA).

* The Newsqueak (1988), Alef (1995), and Limbo (1996) family. These
  languages were developed by Rob Pike, Phil Winterbottom, Sean Dorward and
  others in their group at Bell Labs Computing Sciences Research Center
  (Murray Hill, NJ, USA).

* The Napier (1985) and Napier88 (1988) family. These languages were
  developed by Malcolm Atkinson, Ron Morrison and others in their group at
  the University of St. Andrews (St. Andrews, Fife, UK).

Additional specific influences can be seen from the following languages:

* The stack-growth implementation of Go.
* The structural algebraic types and compilation manager of SML.
* The attribute and assembly systems of C#.
* The references and deterministic destructor system of C++.
* The memory region systems of the ML Kit and Cyclone.
* The typeclass system of Haskell.
* The lexical identifier rule of Python.
* The block syntax of Ruby.