rust/doc/rust.md

% Rust Reference Manual % January 2012

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 at http://www.rust-lang.org/doc/tutorial 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. Formatted documentation can be found at the following locations:

• Core library: http://doc.rust-lang.org/doc/core
• Standard library: http://doc.rust-lang.org/doc/std

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, 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.

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:

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.

The grammar for Rust given in this document is extracted and verified as LL(1) by an automated grammar-analysis tool, and further tested against the Rust sources. The generated parser is currently not the one used by the Rust compiler itself, but in the future we hope to relate the two together more precisely. As of this writing they are only related by testing against existing source code.

Unicode productions

A small number of productions in Rust's grammar permit Unicode codepoints ouside 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.

String table productions

Some rules in the grammar -- notably operators, keywords and reserved words -- 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 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 for more information.

Lexical structure

Input format

Rust input is interpreted in as a sequence of Unicode codepoints encoded in UTF-8. No normalization is performed during input processing. 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. ^[Surrogate 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.

Identifier

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 or reserved words.

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

comment : block_comment | line_comment ;
block_comment : "/*" block_comment_body * "*/" ;
block_comment_body : block_comment | non_star * | '*' non_slash ;
line_comment : "//" non_eol * ;

Comments in Rust code follow the general C++ style of line and block-comment forms, with proper nesting of block-comment delimiters. Comments are interpreted as a form of whitespace.

Whitespace

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 meaning.

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

simple_token : keyword | reserved | unop | binop ; 
token : simple_token | ident | immediate | symbol | whitespace token ;

Tokens are primitive productions in the grammar defined by regular (non-recursive) languages. "Simple" tokens are given in string table production 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 are the following strings:

import export use mod dir

The keywords in source files are the following strings:

alt any as assert
be bind block bool break
char check claim const cont
do
else export
f32 f64 fail false float fn for
i16 i32 i64 i8 if import in int
let log
mod mutable
native note
obj  
prove pure
resource ret
self str syntax
tag true type
u16 u32 u64 u8 uint unchecked unsafe use
vec
while with

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

Reserved words

The reserved words are the following strings:

m32 m64 m128
f80 f16 f128
class trait

Any of these may have special meaning in future versions of the language, do are excluded from the ident rule.

Immediates

Immediates are a subset of all possible literals: those that are defined as single tokens, rather than sequences of tokens.

An immediate is a form of constant expression, so is evaluated (primarily) at compile time.

immediate : string_lit | char_lit | num_lit ;

Character and string literals

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

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.

By default, an integer literal is of type int. 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 u suffix gives the literal type uint.
• Each of the signed and unsigned machine types u8, i8, u16, i16, u32, i32, u64 and i64 give the literal the corresponding machine type.

Examples of integer literals of various forms:

123;                               // type int
123u;                              // type uint
123_u;                             // type uint
0xff00;                            // type int
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 only two floating-point suffixes: f32 and f64. Each of these gives the floating point literal the associated type, rather than float.

A set of suffixes are also reserved to accommodate literal support for types corresponding to reserved tokens. The reserved suffixes are f16, f80, f128, m, m32, m64 and m128.

Examples of floating-point literals of various forms:

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

Symbols

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

Symbols are a general class of printable token that play structural roles in a variety of grammar productions. They are catalogued here for completeness as the set of remaining miscellaneous printable token that do not otherwise appear as operators, keywords or reserved words.

Paths

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 or a (variable)[#variables) 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:

x;
x::y::z;

Path components are usually identifiers, but the trailing component of a path may be an angle-bracket enclosed list of type arguments. In expression 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:

type t = map::hashtbl<int,str>;  // Type arguments used in a type expression
let x = id::<int>(10);           // Type arguments used in a call expression

Crates and source files

Rust is a compiled language. Its semantics are divided along a phase distinction between compile-time and run-time. Those semantic rules that have a static interpretation govern the success or failure of compilation. A program that fails to compile due to violation of a compile-time rule has no defined semantics at run-time; the compiler should halt with an error report, and produce no executable artifact.

The compilation model centres on artifacts called crates. Each compilation is directed towards 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 a unit of compilation and linking, as well as versioning, distribution and runtime loading. A crate contains a tree of nested module 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 denoting its location within the crate's module tree.

Crates are provided to the Rust compiler through two kinds of file:

• crate files, that end in .rc and each define a crate.
• source files, that end in .rs and each define a module.

The Rust compiler is always invoked with a single input file, and always produces a single output crate.

When the Rust compiler is invoked with a crate file, it reads the explicit definition of the crate it's compiling from that file, and populates the crate with modules derived from all the source files referenced by the crate, reading and processing all the referenced modules at once.

When the Rust compiler is invoked with a source file, it creates an implicit crate and treats the source file and though it was referenced as the sole module populating this implicit crate. The module name is derived from the source file name, with the .rs extension removed.

Crate files

crate : attribute [ ';' | attribute* directive ] 
      | directive ;
directive : view_item | dir_directive | source_directive ;

A crate file contains a crate definition, for which the production above defines the grammar. It is a declarative grammar that guides the compiler in assembling a crate from component source files.^[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 file describes:

• Attributes about the crate, such as author, name, version, and copyright. These are used for linking, versioning and distributing crates.
• The source-file and directory modules that make up the crate.
• Any use, import or export view items that apply to the anonymous module at the top-level of the crate's module tree.

An example of a crate file:

// 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" ];

// Import a module.
use std (ver = "1.0");

// Define some modules.
#[path = "foo.rs"]
mod foo;
mod bar {
    #[path =  "quux.rs"]
    mod quux;
}

Dir directives

A dir_directive forms a module in the module tree making up the crate, as well as implicitly relating that module to a directory in the filesystem containing source files and/or further subdirectories. The filesystem directory associated with a dir_directive module can either be explicit, or if omitted, is implicitly the same name as the module.

A source_directive references a source file, either explicitly or implicitly by combining the module name with the file extension .rs. The module contained in that source file is bound to the module path formed by the dir_directive modules containing the source_directive.

Source file

A source file contains a module, that is, a sequence of zero-or-more item definitions. Each source file is an implicit module, the name and location of which -- in the module tree of the current crate -- is defined from outside the source file: either by an explicit source_directive in a referencing crate file, or by the filename of the source file itself.

Items and attributes

Attributes

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 piece of metadata 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 are applied to an entity by placing them within a hash-list (#[...]) as either a prefix to the entity or as a semicolon-delimited declaration within the entity body.

An example of attributes:

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

// General metadata applied to the enclosing module or crate.
#[license = "BSD"];

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

// A documentation attribute
#[doc = "Add two numbers together."
fn add(x: int, y: int) { x + y }

In future versions of Rust, user-provided extensions to the compiler will be able to interpret attributes. When this facility is provided, a distinction will be made 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 where it's written.
• 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.

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

Statements and expressions

Call expressions

expr_list : [ expr [ ',' expr ]* ] ? ;
paren_expr_list : '(' expr_list ')' ;
call_expr : expr paren_expr_list ;

Operators

Unary operators

+ - * ! @ ~

Binary operators

.
+ - * / %
& | ^
|| &&
< <= == >= >
<< >> >>>
<- <-> = += -= *= /= %= &= |= ^= <<= >>= >>>=

Syntax extensions

syntax_ext_expr : '#' ident paren_expr_list ? brace_match ? ;

Rust provides a notation for syntax extension. The notation for invoking a syntax extension is a marked syntactic form that can appear as an expression in the body of a Rust program.

After parsing, a syntax-extension invocation is expanded into a Rust expression. The name of the extension determines the translation performed. In future versions of Rust, user-provided syntax extensions aside from macros will be provided via external crates.

At present, only a set of built-in syntax extensions, as well as macros introduced inline in source code using the macro extension, may be used. The current built-in syntax extensions are:

• fmt expands into code to produce a formatted string, similar to printf from C.
• env expands into a string literal containing the value of that environment variable at compile-time.
• concat_idents expands into an identifier which is the concatenation of its arguments.
• ident_to_str expands into a string literal containing the name of its argument (which must be a literal).
• log_syntax causes the compiler to pretty-print its arguments.

Finally, macro is used to define a new macro. A macro can abstract over second-class Rust concepts that are present in syntax. The arguments to macro are pairs (two-element vectors). The pairs consist of an invocation and the syntax to expand into. An example:

#macro([#apply[fn, [args, ...]], fn(args, ...)]);

In this case, the invocation #apply[sum, 5, 8, 6] expands to sum(5,8,6). If ... follows an expression (which need not be as simple as a single identifier) in the input syntax, the matcher will expect an arbitrary number of occurrences of the thing preceding it, and bind syntax to the identifiers it contains. If it follows an expression in the output syntax, it will transcribe that expression repeatedly, according to the identifiers (bound to syntax) that it contains.

The behaviour of ... is known as Macro By Example. It allows you to write a macro with arbitrary repetition by specifying only one case of that repetition, and following it by ..., both where the repeated input is matched, and where the repeated output must be transcribed. A more sophisticated example:

#macro([#zip_literals[[x, ...], [y, ...]), [[x, y], ...]]);
#macro([#unzip_literals[[x, y], ...], [[x, ...], [y, ...]]]);

In this case, #zip_literals[[1,2,3], [1,2,3]] expands to [[1,1],[2,2],[3,3]], and #unzip_literals[[1,1], [2,2], [3,3]] expands to [[1,2,3],[1,2,3]].

Macro expansion takes place outside-in: that is, #unzip_literals[#zip_literals[[1,2,3],[1,2,3]]] will fail because unzip_literals expects a list, not a macro invocation, as an argument.

The macro system currently has some limitations. It's not possible to destructure anything other than vector literals (therefore, the arguments to complicated macros will tend to be an ocean of square brackets). Macro invocations and ... can only appear in expression positions. Finally, macro expansion is currently unhygienic. That is, name collisions between macro-generated and user-written code can cause unintentional capture.

Future versions of Rust will address these issues.

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 task's memory consists of a static set of items, a set of 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, objects, 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: shared boxes -- which may be subject to garbage collection -- and unique 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, shared or unique boxes, and/or references. Sharing memory between tasks can only be accomplished using unsafe constructs, such as raw pointer operations or calling C code.

When a task sends a value satisfying the send interface over a channel, 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 send interface: it is only instantiated for (transitively) unique kinds of data constructor and pointers, never shared pointers.

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

A shared box may (in the case of a recursive, mutable shared type) be cyclic; in this case the release of memory inside the shared structure may be deferred until task-local garbage collection can reclaim it. Code can ensure no such delayed deallocation occurs by restricting itself to unique boxes and similar unshared 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. A slot is either local or a reference.

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

A reference references a value outside the frame. It may refer to a value allocated in another frame or a boxed value in the heap. The reference-formation rules ensure that the referent will outlive the reference.

Local slots are always implicitly mutable.

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

References are created for function arguments. If the compiler can not prove that the referred-to value will outlive the reference, it will try to set aside a copy of that value to refer to. If this is not semantically safe (for example, if the referred-to value contains mutable fields), it will reject the program. If the compiler deems copying the value expensive, it will warn.

A function can be declared to take an argument by mutable reference. This allows the function to write to the slot that the reference refers to.

An example function that accepts an value by mutable reference:

fn incr(&i: int) {
    i = i + 1;
}

Memory boxes

A box is a reference to a heap allocation holding another value. There are two kinds of boxes: shared boxes and unique boxes.

A shared box type or value is constructed by the prefix at sigil @.

A unique box type or value is constructed by the prefix tilde sigil ~.

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

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

An example of constructing one shared box type and value, and one unique box type and value:

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

Some operations implicitly dereference boxes. Examples of such @dfn{implicit dereference} operations are:

• arithmetic operators (x + y - z)
• field selection (x.y.z)

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

let x: @int = @10;
let y: @int = @12;
assert (x + y == 22);

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 @dfn{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

With the exception of unsafe blocks, Rust tasks are isolated from interfering with one another's memory directly. Instead of manipulating shared storage, Rust tasks communicate with one another using a typed, asynchronous, simplex message-passing system.

A port is a communication endpoint that can receive messages. Ports receive messages from channels.

A channel is a communication endpoint that can send messages. Channels send messages to ports.

Each port is implicitly boxed and mutable; as such a port has a unique per-task identity and cannot be replicated or transmitted. If a port value is copied, both copies refer to the same port. New ports can be constructed dynamically and stored in data structures.

Each channel is bound to a port when the channel is constructed, so the destination port for a channel must exist before the channel itself. A channel cannot be rebound to a different port from the one it was constructed with.

Channels are weak: a channel does not keep the port it is bound to alive. Ports are owned by their allocating task and cannot be sent over channels; if a task dies its ports die with it, and all channels bound to those ports no longer function. Messages sent to a channel connected to a dead port will be dropped.

Channels are immutable types with meaning known to the runtime; channels can be sent over channels.

Many channels can be bound to the same port, but each channel is bound to a single port. In other words, channels and ports exist in an N:1 relationship, N channels to 1 port. ^[It may help to remember nautical terminology when differentiating channels from ports. Many different waterways -- channels -- may lead to the same port.}

Each port and channel can carry only one type of message. The message type is encoded as a parameter of the channel or port type. The message type of a channel is equal to the message type of the port it is bound to. The types of messages must satisfy the send built-in interface.

Messages are generally sent asynchronously, with optional rate-limiting on the transmit side. A channel contains a message queue and asynchronously sending a message merely inserts it into the sending channel's queue; message receipt is the responsibility of the receiving task.

Messages are sent on channels and received on ports using standard library functions.

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 recieve call on a port, or attempts a rate-limited blocking send on a channel. When the communication expression can be completed -- when a message arrives at a sender, or a queue drains sufficiently to complete a rate-limited send -- 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 expression. 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 std::task::yield, which deschedules it immediately. Entering any other non-executing state (blocked, dead) similarly deschedules the task.

Spawning tasks

A call to std::task::spawn, passing a 0-argument function as its single argument, causes the runtime to construct a new task executing the passed function. The passed function is referred to as the entry function for the spawned task, and any captured environment is carries is moved from the spawning task to the spawned task before the spawned task begins execution.

The result of a spawn call is a std::task::task value.

An example of a spawn call:

import std::task::*;
import std::comm::*;

fn helper(c: chan<u8>) {
    // do some work.
    let result = ...;
    send(c, result);
}

let p: port<u8>;

spawn(bind helper(chan(p)));
// let task run, do other things.
// ...
let result = recv(p);

Sending values into channels

Sending a value into a channel is done by a library call to std::comm::send, which takes a channel and a value to send, and moves the value into the channel's outgoing buffer.

An example of a send:

import std::comm::*;
let c: chan<str> = ...;
send(c, "hello, world");

Receiving values from ports

Receiving a value is done by a call to the recv method, on a value of type std::comm::port. This call causes the receiving task to enter the blocked reading state until a task is sending a value to the port, at which point the runtime pseudo-randomly selects a sending task and moves a value from the head of one of the task queues to the call's return value, and un-blocks the receiving task. See communication system.

An example of a receive:

import std::comm::*;
let p: port<str> = ...;
let s: str = recv(p);

Runtime services, linkage and debugging

Appendix: Rationales and design tradeoffs

TBD.

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.

— 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 [...]

— 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.

— 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 Wikström, Mike Williams and others in their group at the Ericsson Computer Science Laboratory (Älvsjö, 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 Reserch 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 deterministic destructor system of C++.
• The typeclass system of Haskell.
• The lexical identifier rule of Python.
• The block syntax of Ruby.

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