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Add information about numeric casts, from the nomicon

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Martin Pool 2015-11-29 11:54:29 -08:00
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26
src/doc/book/casting-between-types.md
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@ -67,6 +67,32 @@ For example
let one = true as u8;
let at_sign = 64 as char;
```
For numeric casts, there are quite a few cases to consider:
* Casting between two integers of the same size (e.g. i32 -> u32) is a no-op
* Casting from a larger integer to a smaller integer (e.g. u32 -> u8) will
truncate
* Casting from a smaller integer to a larger integer (e.g. u8 -> u32) will
* zero-extend if the source is unsigned
* sign-extend if the source is signed
* Casting from a float to an integer will round the float towards zero
* **[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 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.
[float-int]: https://github.com/rust-lang/rust/issues/10184
[float-float]: https://github.com/rust-lang/rust/issues/15536
## Pointer casts