Numeric literal expressions and literal suffixes

src/expressions/literal-expr.md

@@ -8,19 +8,129 @@
 >    | [BYTE_LITERAL]\
 >    | [BYTE_STRING_LITERAL]\
 >    | [RAW_BYTE_STRING_LITERAL]\
->    | [INTEGER_LITERAL]\
+>    | [INTEGER_LITERAL][^out-of-range]\
 >    | [FLOAT_LITERAL]\
 >    | [BOOLEAN_LITERAL]
+>
+> [^out-of-range]: A value ≥ 2<sup>128</sup> is not allowed.
 
-A _literal expression_ consists of one of the [literal](../tokens.md#literals) forms described earlier.
-It directly describes a number, character, string, or boolean value.
+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][literal tokens] forms described earlier can make up a literal expression.
 
 ```rust
 "hello";   // string type
 '5';       // character type
 5;         // integer type
 ```
 
+## 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][numeric 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:
+
+```rust
+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: `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.
+
+## 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][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:
+
+```rust
+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.
+
+[constant expression]: ../const_eval.md#constant-expressions
+[floating-point types]: ../types/numeric.md#floating-point-types
+[lint check]: ../attributes/diagnostics.md#lint-check-attributes
+[literal tokens]: ../tokens.md#literals
+[numeric cast]: operator-expr.md#numeric-cast
+[numeric types]: ../types/numeric.md
+[suffix]: ../tokens.md#suffixes
+[negation operator]: operator-expr.md#negation-operators
+[`f32::from_str`]: ../../core/primitive.f32.md#method.from_str
+[`f32::INFINITY`]: ../../core/primitive.f32.md#associatedconstant.INFINITY
+[`f32::NAN`]: ../../core/primitive.f32.md#associatedconstant.NAN
+[`f64::from_str`]: ../../core/primitive.f64.md#method.from_str
+[`f64::INFINITY`]: ../../core/primitive.f64.md#associatedconstant.INFINITY
+[`f64::NAN`]: ../../core/primitive.f64.md#associatedconstant.NAN
+[`u128::from_str_radix`]: ../../core/primitive.u128.md#method.from_str_radix
 [CHAR_LITERAL]: ../tokens.md#character-literals
 [STRING_LITERAL]: ../tokens.md#string-literals
 [RAW_STRING_LITERAL]: ../tokens.md#raw-string-literals