diff --git a/src/expressions/operator-expr.md b/src/expressions/operator-expr.md index f35d0328f980b..3635cc03285dc 100644 --- a/src/expressions/operator-expr.md +++ b/src/expressions/operator-expr.md @@ -367,16 +367,15 @@ same trait object. * **[NOTE: currently this will cause Undefined Behavior if the rounded value cannot be represented by the target integer type][float-int]**. This includes Inf and NaN. This is a bug and will be fixed. - * Casting from an integer to float will produce the floating point - representation of the integer, rounded if necessary (rounding strategy - unspecified) + * Casting from an integer to float will produce the closest possible float \* + * if necessary, rounding is according to `roundTiesToEven` mode \*\*\* + * on overflow, infinity (of the same sign as the input) is produced + * note: with the current set of numeric types, overflow can only happen + on `u128 as f32` for values greater or equal to `f32::MAX + (0.5 ULP)` * Casting from an f32 to an f64 is perfect and lossless - * Casting from an f64 to an f32 will produce the closest possible value - (rounding strategy unspecified) - * **[NOTE: currently this will cause Undefined Behavior if the value - is finite but larger or smaller than the largest or smallest finite - value representable by f32][float-float]**. This is a bug and will - be fixed. + * Casting from an f64 to an f32 will produce the closest possible f32 \*\* + * if necessary, rounding is according to `roundTiesToEven` mode \*\*\* + * on overflow, infinity (of the same sign as the input) is produced * Enum cast * Casts an enum to its discriminant, then uses a numeric cast if needed. * Primitive to integer cast @@ -385,8 +384,19 @@ same trait object. * `u8` to `char` cast * Casts to the `char` with the corresponding code point. +\* if integer-to-float casts with this rounding mode and overflow behavior are +not supported natively by the hardware, these casts will likely be slower than +expected. + +\*\* if f64-to-f32 casts with this rounding mode and overflow behavior are not +supported natively by the hardware, these casts will likely be slower than +expected. + +\*\*\* as defined in IEEE 754-2008 §4.3.1: pick the nearest floating point +number, preferring the one with an even least significant digit if exactly +halfway between two floating point numbers. + [float-int]: /~https://github.com/rust-lang/rust/issues/10184 -[float-float]: /~https://github.com/rust-lang/rust/issues/15536 ## Assignment expressions