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Specify behavior of int->float and f64->f32 casts #607

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Sep 8, 2019
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Specify behavior of int->float and f64->f32 casts #607

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9774995
Specify behavior of int->float and f64->f32 casts
Aug 15, 2019
1254ddb
Fix typo: ode -> mode
Aug 16, 2019
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30 changes: 20 additions & 10 deletions src/expressions/operator-expr.md
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Expand Up @@ -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
Expand All @@ -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
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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

Expand Down