Tokens

Tokens are primitive productions in the grammar defined by regular (non-recursive) languages. Rust source input can be broken down into the following kinds of tokens:

• Keywords
• Identifiers
• Literals
• Lifetimes
• Punctuation
• Delimiters

Within this documentation's grammar, "simple" tokens are given in string table production form, and appear in monospace font.

Literals

Literals are tokens used in literal expressions.

Examples

Characters and strings

* The number of #s on each side of the same literal must be equivalent.

ASCII escapes

Byte escapes

Unicode escapes

Quote escapes

Numbers

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

Suffixes

A suffix is a sequence of characters following the primary part of a literal (without intervening whitespace), of the same form as a non-raw identifier or keyword.

^Lexer
SUFFIX : IDENTIFIER_OR_KEYWORD
SUFFIX_NO_E : SUFFIX _{not beginning with e or E}

Any kind of literal (string, integer, etc) with any suffix is valid as a token.

A literal token with any suffix can be passed to a macro without producing an error. The macro itself will decide how to interpret such a token and whether to produce an error or not. In particular, the literal fragment specifier for by-example macros matches literal tokens with arbitrary suffixes.

#![allow(unused)]
fn main() {
macro_rules! blackhole { ($tt:tt) => () }
macro_rules! blackhole_lit { ($l:literal) => () }

blackhole!("string"suffix); // OK
blackhole_lit!(1suffix); // OK
}

However, suffixes on literal tokens which are interpreted as literal expressions or patterns are restricted. Any suffixes are rejected on non-numeric literal tokens, and numeric literal tokens are accepted only with suffixes from the list below.

Character and string literals

Character literals

^Lexer
CHAR_LITERAL :
   ' ( ~[' \ \n \r \t] | QUOTE_ESCAPE | ASCII_ESCAPE | UNICODE_ESCAPE ) ' SUFFIX^?

QUOTE_ESCAPE :
   \' | \"

ASCII_ESCAPE :
      \x OCT_DIGIT HEX_DIGIT
   | \n | \r | \t | \\ | \0

UNICODE_ESCAPE :
   \u{ ( HEX_DIGIT _^* )^1..6 }

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

^Lexer
STRING_LITERAL :
   " (
      ~[" \ IsolatedCR]
      | QUOTE_ESCAPE
      | ASCII_ESCAPE
      | UNICODE_ESCAPE
      | STRING_CONTINUE
   )^* " SUFFIX^?

STRING_CONTINUE :
   \ followed by \n

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-breaks are allowed in string literals. A line-break is either a newline (U+000A) or a pair of carriage return and newline (U+000D, U+000A). Both byte sequences are normally translated to U+000A, but as a special exception, when an unescaped U+005C character (\) occurs immediately before a line break, then the line break character(s), and all immediately following (U+0020), \t (U+0009), \n (U+000A) and \r (U+0000D) characters are ignored. Thus a, b and c are equal:

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

     bar";

assert_eq!(a, b);
assert_eq!(b, c);
}

Note: Rust skipping additional newlines (like in example c) is potentially confusing and unexpected. This behavior may be adjusted in the future. Until a decision is made, it is recommended to avoid relying on this, i.e. skipping multiple newlines with line continuations. See this issue for more information.

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:

• A 7-bit code point escape starts with U+0078 (x) and is followed by exactly two hex digits with value up to 0x7F. It denotes the ASCII character with value equal to the provided hex value. Higher values are not permitted because it is ambiguous whether they mean Unicode code points or byte values.
• 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

^Lexer
RAW_STRING_LITERAL :
   r RAW_STRING_CONTENT SUFFIX^?

RAW_STRING_CONTENT :
      " ( ~ IsolatedCR )^{* (non-greedy)} "
   | # RAW_STRING_CONTENT #

Raw string literals do not process any escapes. They start with the character U+0072 (r), followed by fewer than 256 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)]
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

^Lexer
BYTE_LITERAL :
   b' ( ASCII_FOR_CHAR | BYTE_ESCAPE ) ' SUFFIX^?

ASCII_FOR_CHAR :
   any ASCII (i.e. 0x00 to 0x7F), except ', \, \n, \r or \t

BYTE_ESCAPE :
      \x HEX_DIGIT HEX_DIGIT
   | \n | \r | \t | \\ | \0 | \' | \"

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

^Lexer
BYTE_STRING_LITERAL :
   b" ( ASCII_FOR_STRING | BYTE_ESCAPE | STRING_CONTINUE )^* " SUFFIX^?

ASCII_FOR_STRING :
   any ASCII (i.e 0x00 to 0x7F), except ", \ and IsolatedCR

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. The type of a byte string literal of length n is &'static [u8; n].

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

^Lexer
RAW_BYTE_STRING_LITERAL :
   br RAW_BYTE_STRING_CONTENT SUFFIX^?

RAW_BYTE_STRING_CONTENT :
      " ASCII^{* (non-greedy)} "
   | # RAW_BYTE_STRING_CONTENT #

ASCII :
   any ASCII (i.e. 0x00 to 0x7F)

Raw byte string literals do not process any escapes. They start with the character U+0062 (b), followed by U+0072 (r), followed by fewer than 256 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)]
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

^Lexer
INTEGER_LITERAL :
   ( DEC_LITERAL | BIN_LITERAL | OCT_LITERAL | HEX_LITERAL ) SUFFIX_NO_E^?

DEC_LITERAL :
   DEC_DIGIT (DEC_DIGIT|_)^*

BIN_LITERAL :
   0b (BIN_DIGIT|_)^* BIN_DIGIT (BIN_DIGIT|_)^*

OCT_LITERAL :
   0o (OCT_DIGIT|_)^* OCT_DIGIT (OCT_DIGIT|_)^*

HEX_LITERAL :
   0x (HEX_DIGIT|_)^* HEX_DIGIT (HEX_DIGIT|_)^*

BIN_DIGIT : [0-1]

OCT_DIGIT : [0-7]

DEC_DIGIT : [0-9]

HEX_DIGIT : [0-9 a-f A-F]

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 (with at least one digit) of hex digits and underscores.
• An octal literal starts with the character sequence U+0030 U+006F (0o) and continues as any mixture (with at least one digit) of octal digits and underscores.
• A binary literal starts with the character sequence U+0030 U+0062 (0b) and continues as any mixture (with at least one digit) of binary digits and underscores.

Like any literal, an integer literal may be followed (immediately, without any spaces) by a suffix as described above. The suffix may not begin with e or E, as that would be interpreted as the exponent of a floating-point literal. See literal expressions for the effect of these suffixes.

Examples of integer literals which are accepted as literal expressions:

#![allow(unused)]
fn main() {
#![allow(overflowing_literals)]
123;
123i32;
123u32;
123_u32;

0xff;
0xff_u8;
0x01_f32; // integer 7986, not floating-point 1.0
0x01_e3;  // integer 483, not floating-point 1000.0

0o70;
0o70_i16;

0b1111_1111_1001_0000;
0b1111_1111_1001_0000i64;
0b________1;

0usize;

// These are too big for their type, but are accepted as literal expressions.
128_i8;
256_u8;

// This is an integer literal, accepted as a floating-point literal expression.
5f32;
}

Note that -1i8, for example, is analyzed as two tokens: - followed by 1i8.

Examples of integer literals which are not accepted as literal expressions:

#![allow(unused)]
fn main() {
#[cfg(FALSE)] {
0invalidSuffix;
123AFB43;
0b010a;
0xAB_CD_EF_GH;
0b1111_f32;
}
}

Tuple index

^Lexer
TUPLE_INDEX:
   INTEGER_LITERAL

A tuple index is used to refer to the fields of tuples, tuple structs, and tuple variants.

Tuple indices are compared with the literal token directly. Tuple indices start with 0 and each successive index increments the value by 1 as a decimal value. Thus, only decimal values will match, and the value must not have any extra 0 prefix characters.

#![allow(unused)]
fn main() {
let example = ("dog", "cat", "horse");
let dog = example.0;
let cat = example.1;
// The following examples are invalid.
let cat = example.01;  // ERROR no field named `01`
let horse = example.0b10;  // ERROR no field named `0b10`
}

Note: Tuple indices may include certain suffixes, but this is not intended to be valid, and may be removed in a future version. See https://github.com/rust-lang/rust/issues/60210 for more information.

Floating-point literals

^Lexer
FLOAT_LITERAL :
      DEC_LITERAL . (not immediately followed by ., _ or an XID_Start character)
   | DEC_LITERAL . DEC_LITERAL SUFFIX_NO_E^?
   | DEC_LITERAL (. DEC_LITERAL)^? FLOAT_EXPONENT SUFFIX^?

FLOAT_EXPONENT :
   (e|E) (+|-)^? (DEC_DIGIT|_)^* DEC_DIGIT (DEC_DIGIT|_)^*

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 may not begin with e or E if the literal does not include an exponent. See literal expressions for the effect of these suffixes.

Examples of floating-point literals which are accepted as literal expressions:

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

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.

Note that -1.0, for example, is analyzed as two tokens: - followed by 1.0.

Examples of floating-point literals which are not accepted as literal expressions:

#![allow(unused)]
fn main() {
#[cfg(FALSE)] {
2.0f80;
2e5f80;
2e5e6;
2.0e5e6;
1.3e10u64;
}
}

Reserved forms similar to number literals

^Lexer
RESERVED_NUMBER :
      BIN_LITERAL [2-9​]
   | OCT_LITERAL [8-9​]
   | ( BIN_LITERAL | OCT_LITERAL | HEX_LITERAL ) .
         (not immediately followed by ., _ or an XID_Start character)
   | ( BIN_LITERAL | OCT_LITERAL ) (e|E)
   | 0b _^* end of input or not BIN_DIGIT
   | 0o _^* end of input or not OCT_DIGIT
   | 0x _^* end of input or not HEX_DIGIT
   | DEC_LITERAL ( . DEC_LITERAL)^? (e|E) (+|-)^? end of input or not DEC_DIGIT

The following lexical forms similar to number literals are reserved forms. Due to the possible ambiguity these raise, they are rejected by the tokenizer instead of being interpreted as separate tokens.

• An unsuffixed binary or octal literal followed, without intervening whitespace, by a decimal digit out of the range for its radix.

• An unsuffixed binary, octal, or hexadecimal literal followed, without intervening whitespace, by a period character (with the same restrictions on what follows the period as for floating-point literals).

• An unsuffixed binary or octal literal followed, without intervening whitespace, by the character e or E.

• Input which begins with one of the radix prefixes but is not a valid binary, octal, or hexadecimal literal (because it contains no digits).

• Input which has the form of a floating-point literal with no digits in the exponent.

Examples of reserved forms:

#![allow(unused)]
fn main() {
0b0102;  // this is not `0b010` followed by `2`
0o1279;  // this is not `0o127` followed by `9`
0x80.0;  // this is not `0x80` followed by `.` and `0`
0b101e;  // this is not a suffixed literal, or `0b101` followed by `e`
0b;      // this is not an integer literal, or `0` followed by `b`
0b_;     // this is not an integer literal, or `0` followed by `b_`
2e;      // this is not a floating-point literal, or `2` followed by `e`
2.0e;    // this is not a floating-point literal, or `2.0` followed by `e`
2em;     // this is not a suffixed literal, or `2` followed by `em`
2.0em;   // this is not a suffixed literal, or `2.0` followed by `em`
}

Lifetimes and loop labels

^Lexer
LIFETIME_TOKEN :
      ' IDENTIFIER_OR_KEYWORD
   | '_

LIFETIME_OR_LABEL :
      ' NON_KEYWORD_IDENTIFIER

Lifetime parameters and loop labels use LIFETIME_OR_LABEL tokens. Any LIFETIME_TOKEN will be accepted by the lexer, and for example, can be used in macros.

Punctuation

Punctuation symbol tokens are listed here for completeness. Their individual usages and meanings are defined in the linked pages.

Delimiters

Bracket punctuation is used in various parts of the grammar. An open bracket must always be paired with a close bracket. Brackets and the tokens within them are referred to as "token trees" in macros. The three types of brackets are:

Reserved prefixes

^{Lexer 2021+}
RESERVED_TOKEN_DOUBLE_QUOTE : ( IDENTIFIER_OR_KEYWORD _{Except b or r or br} | _ ) "
RESERVED_TOKEN_SINGLE_QUOTE : ( IDENTIFIER_OR_KEYWORD _{Except b} | _ ) '
RESERVED_TOKEN_POUND : ( IDENTIFIER_OR_KEYWORD _{Except r or br} | _ ) #

Some lexical forms known as reserved prefixes are reserved for future use.

Source input which would otherwise be lexically interpreted as a non-raw identifier (or a keyword or _) which is immediately followed by a #, ', or " character (without intervening whitespace) is identified as a reserved prefix.

Note that raw identifiers, raw string literals, and raw byte string literals may contain a # character but are not interpreted as containing a reserved prefix.

Similarly the r, b, and br prefixes used in raw string literals, byte literals, byte string literals, and raw byte string literals are not interpreted as reserved prefixes.

Edition Differences: Starting with the 2021 edition, reserved prefixes are reported as an error by the lexer (in particular, they cannot be passed to macros).

Before the 2021 edition, reserved prefixes are accepted by the lexer and interpreted as multiple tokens (for example, one token for the identifier or keyword, followed by a # token).

Examples accepted in all editions:

#![allow(unused)]
fn main() {
macro_rules! lexes {($($_:tt)*) => {}}
lexes!{a #foo}
lexes!{continue 'foo}
lexes!{match "..." {}}
lexes!{r#let#foo}         // three tokens: r#let # foo
}

Examples accepted before the 2021 edition but rejected later:

#![allow(unused)]
fn main() {
macro_rules! lexes {($($_:tt)*) => {}}
lexes!{a#foo}
lexes!{continue'foo}
lexes!{match"..." {}}
}