Tokens - The Rust Reference

# Tokens

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.

## 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.

### Examples

#### Characters and strings

| Example | `#` sets | Characters | Escapes
--- | --- | --- | --- | ---
Character | `'H'` | `N/A` | All Unicode | Quote & Byte & Unicode
String | `"hello"` | `N/A` | All Unicode | Quote & Byte & Unicode
Raw | `r#"hello"#` | `0...` | All Unicode | `N/A`
Byte | `b'H'` | `N/A` | All ASCII | Quote & Byte
Byte string | `b"hello"` | `N/A` | All ASCII | Quote & Byte
Raw byte string | `br#"hello"#` | `0...` | All ASCII | `N/A`

#### Byte escapes

| Name
--- | ---
`\x7F` | 8-bit character code (exactly 2 digits)
`\n` | Newline
`\r` | Carriage return
`\t` | Tab
`\\` | Backslash
`\0` | Null

#### Unicode escapes

| Name
--- | ---
`\u{7FFF}` | 24-bit Unicode character code (up to 6 digits)

#### Quote escapes

| Name
--- | ---
`\'` | Single quote
`\"` | Double quote

#### Numbers

Number literals`*` | Example | Exponentiation | Suffixes
--- | --- | --- | ---
Decimal integer | `98_222` | `N/A` | Integer suffixes
Hex integer | `0xff` | `N/A` | Integer suffixes
Octal integer | `0o77` | `N/A` | Integer suffixes
Binary integer | `0b1111_0000` | `N/A` | Integer suffixes
Floating-point | `123.0E+77` | `Optional` | Floating-point suffixes

`*` All number literals allow `_` as a visual separator: `1_234.0E+18f64`

#### Suffixes

Integer | Floating-point
--- | ---
`u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`, `i64`, `isize`, `usize` | `f32`, `f64`

### Character and string literals

#### Character literals

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 (`\`).

#### String literals

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 (`\`).

Line-break characters are allowed in string literals. Normally they represent themselves (i.e. no translation), but as a special exception, when an unescaped `U+005C` character (`\`) occurs immediately before the newline (`U+000A`), the `U+005C` character, the newline, and all whitespace at the beginning of the next line are ignored. Thus `a` and `b` are equal:

    # #![allow(unused_variables)]
    #fn main() {
    let a = "foobar";
    let b = "foo\
             bar";

    assert_eq!(a,b);

    #}

#### Character escapes

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

- An 8-bit code point escape starts with `U+0078` (`x`) and is followed by exactly two hex digits. It denotes the Unicode code point equal to the provided hex value.
- A 24-bit code point escape starts with `U+0075` (`u`) and is followed by up to six hex digits surrounded by braces `U+007B` (`{`) and `U+007D` (`}`). It denotes the Unicode code point 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 null escape is the character `U+0030` (`0`) and denotes the Unicode value `U+0000` (NUL).
- The backslash escape is the character `U+005C` (`\`) which must be escaped in order to denote itself.

#### Raw string literals

Raw string literals do not process any escapes. They start with the character `U+0072` (`r`), followed by zero or more of the character `U+0023` (`#`) and a `U+0022` (double-quote) character. The raw string body can contain any sequence of Unicode characters and is terminated only by another `U+0022` (double-quote) character, followed by the same number of `U+0023` (`#`) characters that preceded the opening `U+0022` (double-quote) character.

All Unicode characters contained in the raw string body represent themselves, the characters `U+0022` (double-quote) (except when followed by at least as many `U+0023` (`#`) characters as were used to start the raw string literal) or `U+005C` (`\`) do not have any special meaning.

Examples for string literals:

    # #![allow(unused_variables)]
    #fn main() {
    "foo"; r"foo";                     // foo
    "\"foo\""; r#""foo""#;             // "foo"

    "foo #\"# bar";
    r##"foo #"# bar"##;                // foo #"# bar

    "\x52"; "R"; r"R";                 // R
    "\\x52"; r"\x52";                  // \x52

    #}

### Byte and byte string literals

#### Byte literals

A byte literal is a single ASCII character (in the `U+0000` to `U+007F` range) or a single escape preceded by the characters `U+0062` (`b`) and `U+0027` (single-quote), and followed by the character `U+0027`. If the character `U+0027` is present within the literal, it must be escaped by a preceding `U+005C` (`\`) character. It is equivalent to a `u8` unsigned 8-bit integer number literal.

#### Byte string literals

A non-raw byte string literal is a sequence of ASCII characters and escapes, preceded by the characters `U+0062` (`b`) and `U+0022` (double-quote), and followed by the character `U+0022`. If the character `U+0022` is present within the literal, it must be escaped by a preceding `U+005C` (`\`) character. Alternatively, a byte string literal can be a raw byte string literal, defined below. A byte string literal of length `n` is equivalent to a `&'static [u8; n]` borrowed fixed-sized array of unsigned 8-bit integers.

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

- A byte escape escape starts with `U+0078` (`x`) and is followed by exactly two hex digits. It denotes the byte 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 bytes values `0x0A` (ASCII LF), `0x0D` (ASCII CR) or `0x09` (ASCII HT) respectively.
- The null escape is the character `U+0030` (`0`) and denotes the byte value `0x00` (ASCII NUL).
- The backslash escape is the character `U+005C` (`\`) which must be escaped in order to denote its ASCII encoding `0x5C`.

#### Raw byte string literals

Raw byte string literals do not process any escapes. They start with the character `U+0062` (`b`), followed by `U+0072` (`r`), followed by zero or more of the character `U+0023` (`#`), and a `U+0022` (double-quote) character. The raw string body can contain any sequence of ASCII characters and is terminated only by another `U+0022` (double-quote) character, followed by the same number of `U+0023` (`#`) characters that preceded the opening `U+0022` (double-quote) character. A raw byte string literal can not contain any non-ASCII byte.

All characters contained in the raw string body represent their ASCII encoding, the characters `U+0022` (double-quote) (except when followed by at least as many `U+0023` (`#`) characters as were used to start the raw string literal) or `U+005C` (`\`) do not have any special meaning.

Examples for byte string literals:

    # #![allow(unused_variables)]
    #fn main() {
    b"foo"; br"foo";                     // foo
    b"\"foo\""; br#""foo""#;             // "foo"

    b"foo #\"# bar";
    br##"foo #"# bar"##;                 // foo #"# bar

    b"\x52"; b"R"; br"R";                // R
    b"\\x52"; br"\x52";                  // \x52

    #}

### Number literals

A number literal is either an integer literal or a floating-point literal. The grammar for recognizing the two kinds of literals is mixed.

#### Integer literals

An integer literal has one of four 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 of hex digits and underscores.
- An octal literal starts with the character sequence `U+0030` `U+006F` (`0o`) and continues as any mixture of octal digits and underscores.
- A binary literal starts with the character sequence `U+0030` `U+0062` (`0b`) and continues as any mixture of binary digits and underscores.

Like any literal, an integer literal may be followed (immediately, without any spaces) by an integer suffix, which forcibly sets the type of the literal. The integer suffix must be the name of one of the integral types: `u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`, `i64`, `isize`, or `usize`.

The type of an unsuffixed integer literal is determined by type inference:

- If an integer type can be uniquely determined from the surrounding program context, the unsuffixed integer literal has that type.

- If the program context under-constrains the type, it defaults to the signed 32-bit integer `i32`.

- If the program context over-constrains the type, it is considered a static type error.

Examples of integer literals of various forms:

    # #![allow(unused_variables)]
    #fn main() {
    123i32;                            // type i32
    123u32;                            // type u32
    123_u32;                           // type u32
    0xff_u8;                           // type u8
    0o70_i16;                          // type i16
    0b1111_1111_1001_0000_i32;         // type i32
    0usize;                            // type usize

    #}

Note that the Rust syntax considers `-1i8` as an application of the unary minus operator to an integer literal `1i8`, rather than a single integer literal.

#### Floating-point literals

A floating-point literal has one of two forms:

- A decimal literal followed by a period character `U+002E` (`.`). This is optionally followed by another decimal literal, with an optional exponent.
- A single decimal literal followed by an exponent.

Like integer literals, a floating-point literal may be followed by a suffix, so long as the pre-suffix part does not end with `U+002E` (`.`). The suffix forcibly sets the type of the literal. There are two valid floating-point suffixes, `f32` and `f64` (the 32-bit and 64-bit floating point types), which explicitly determine the type of the literal.

The type of an unsuffixed floating-point literal is determined by type inference:

- If a floating-point type can be uniquely determined from the surrounding program context, the unsuffixed floating-point literal has that type.

- If the program context under-constrains the type, it defaults to `f64`.

- If the program context over-constrains the type, it is considered a static type error.

Examples of floating-point literals of various forms:

    # #![allow(unused_variables)]
    #fn main() {
    123.0f64;        // type f64
    0.1f64;          // type f64
    0.1f32;          // type f32
    12E+99_f64;      // type f64
    let x: f64 = 2.; // type f64

    #}

This last example is different because it is not possible to use the suffix syntax with a floating point literal ending in a period. `2.f64` would attempt to call a method named `f64` on `2`.

The representation semantics of floating-point numbers are described in "Machine Types".

### Boolean literals

The two values of the boolean type are written `true` and `false`.

## Symbols

Symbols are a general class of printable tokens that play structural roles in a variety of grammar productions. They are a set of remaining miscellaneous printable tokens that do not otherwise appear as unary operators, binary operators, or keywords. They are catalogued in the Symbols section of the Grammar document.