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59 lines
2.5 KiB
59 lines
2.5 KiB
% Casts
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Casts are a superset of coercions: every coercion can be explicitly invoked via
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a cast, but some conversions *require* a cast. These "true casts" are generally
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regarded as dangerous or problematic actions. True casts revolve around raw
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pointers and the primitive numeric types. True casts aren't checked.
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Here's an exhaustive list of all the true casts. For brevity, we will use `*`
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to denote either a `*const` or `*mut`, and `integer` to denote any integral
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primitive:
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* `*T as *U` where `T, U: Sized`
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* `*T as *U` TODO: explain unsized situation
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* `*T as integer`
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* `integer as *T`
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* `number as number`
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* `C-like-enum as integer`
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* `bool as integer`
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* `char as integer`
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* `u8 as char`
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* `&[T; n] as *const T`
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* `fn as *T` where `T: Sized`
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* `fn as integer`
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where `&.T` and `*T` are references of either mutability,
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and where unsize_kind(`T`) is the kind of the unsize info
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in `T` - the vtable for a trait definition (e.g. `fmt::Display` or
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`Iterator`, not `Iterator<Item=u8>`) or a length (or `()` if `T: Sized`).
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Note that lengths are not adjusted when casting raw slices -
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`T: *const [u16] as *const [u8]` creates a slice that only includes
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half of the original memory.
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Casting is not transitive, that is, even if `e as U1 as U2` is a valid
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expression, `e as U2` is not necessarily so (in fact it will only be valid if
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`U1` coerces to `U2`).
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For numeric casts, there are quite a few cases to consider:
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* casting between two integers of the same size (e.g. i32 -> u32) is a no-op
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* casting from a larger integer to a smaller integer (e.g. u32 -> u8) will
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truncate
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* casting from a smaller integer to a larger integer (e.g. u8 -> u32) will
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* zero-extend if the source is unsigned
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* sign-extend if the source is signed
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* casting from a float to an integer will round the float towards zero
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* **NOTE: currently this will cause Undefined Behaviour if the rounded
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value cannot be represented by the target integer type**. This includes
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Inf and NaN. This is a bug and will be fixed.
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* casting from an integer to float will produce the floating point
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representation of the integer, rounded if necessary (rounding strategy
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unspecified)
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* casting from an f32 to an f64 is perfect and lossless
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* casting from an f64 to an f32 will produce the closest possible value
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(rounding strategy unspecified)
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* **NOTE: currently this will cause Undefined Behaviour if the value
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is finite but larger or smaller than the largest or smallest finite
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value representable by f32**. This is a bug and will be fixed.
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