Literal expressions

^Syntax
LiteralExpression :
      CHAR_LITERAL
   | STRING_LITERAL
   | RAW_STRING_LITERAL
   | BYTE_LITERAL
   | BYTE_STRING_LITERAL
   | RAW_BYTE_STRING_LITERAL
   | INTEGER_LITERAL¹
   | FLOAT_LITERAL
   | true | false

¹
A value ≥ 2¹²⁸ is not allowed.

A literal expression 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.

Each of the lexical literal forms described earlier can make up a literal expression, as can the keywords true and false.

#![allow(unused)]
fn main() {
"hello";   // string type
'5';       // character type
5;         // integer type
}

Character literal expressions

A character literal expression consists of a single CHAR_LITERAL token.

Note: This section is incomplete.

String literal expressions

A string literal expression consists of a single STRING_LITERAL or RAW_STRING_LITERAL token.

Note: This section is incomplete.

Byte literal expressions

A byte literal expression consists of a single BYTE_LITERAL token.

Note: This section is incomplete.

Byte string literal expressions

A string literal expression consists of a single BYTE_STRING_LITERAL or RAW_BYTE_STRING_LITERAL token.

Note: This section is incomplete.

Integer literal expressions

An integer literal expression consists of a single INTEGER_LITERAL token.

If the token has a suffix, the suffix will be the name of one of the primitive integer types: u8, i8, u16, i16, u32, i32, u64, i64, u128, i128, usize, or isize, and the expression has that type.

If the token has no suffix, the expression's type is determined by type inference:

If an integer type can be uniquely determined from the surrounding program context, the expression 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 literal expressions:

#![allow(unused)]
fn main() {
123;                               // type i32
123i32;                            // type i32
123u32;                            // type u32
123_u32;                           // type u32
let a: u64 = 123;                  // type u64

0xff;                              // type i32
0xff_u8;                           // type u8

0o70;                              // type i32
0o70_i16;                          // type i16

0b1111_1111_1001_0000;             // type i32
0b1111_1111_1001_0000i64;          // type i64

0usize;                            // type usize
}

The value of the expression is determined from the string representation of the token as follows:

An integer radix is chosen by inspecting the first two characters of the string, as follows:
- 0b indicates radix 2
- 0o indicates radix 8
- 0x indicates radix 16
- otherwise the radix is 10.
If the radix is not 10, the first two characters are removed from the string.
Any underscores are removed from the string.
The string is converted to a u128 value as if by u128::from_str_radix with the chosen radix. If the value does not fit in u128, the expression is rejected by the parser.
The u128 value is converted to the expression's type via a numeric cast.

Note: The final cast will truncate the value of the literal if it does not fit in the expression's type. rustc includes a lint check named overflowing_literals, defaulting to deny, which rejects expressions where this occurs.

Note: -1i8, for example, is an application of the negation operator to the literal expression 1i8, not a single integer literal expression. See Overflow for notes on representing the most negative value for a signed type.

Floating-point literal expressions

A floating-point literal expression consists of a single FLOAT_LITERAL token.

If the token has a suffix, the suffix will be the name of one of the primitive floating-point types: f32 or f64, and the expression has that type.

If the token has no suffix, the expression's type is determined by type inference:

If a floating-point type can be uniquely determined from the surrounding program context, the expression 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 literal expressions:

#![allow(unused)]
fn main() {
123.0f64;        // type f64
0.1f64;          // type f64
0.1f32;          // type f32
12E+99_f64;      // type f64
5f32;            // type f32
let x: f64 = 2.; // type f64
}

The value of the expression is determined from the string representation of the token as follows:

Any underscores are removed from the string.
The string is converted to the expression's type as if by f32::from_str or f64::from_str.

Note: -1.0, for example, is an application of the negation operator to the literal expression 1.0, not a single floating-point literal expression.

Note: inf and NaN are not literal tokens. The f32::INFINITY, f64::INFINITY, f32::NAN, and f64::NAN constants can be used instead of literal expressions. In rustc, a literal large enough to be evaluated as infinite will trigger the overflowing_literals lint check.

Boolean literal expressions

A boolean literal expression consists of one of the keywords true or false.

The expression's type is the primitive boolean type, and its value is:

true if the keyword is true
false if the keyword is false

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