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Document semantics of various pointer casts
Co-authored-by: Ralf Jung <post@ralfj.de>
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@ -478,15 +478,23 @@ unsafe {
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assert_eq!(values[1], 3);
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```
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#### Slice DST pointer to pointer cast
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#### Pointer-to-pointer cast
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For slice types like `[T]` and `[U]`, the raw pointer types `*const [T]`, `*mut [T]`,
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`*const [U]`, and `*mut [U]` encode the number of elements in this slice. Casts between
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these raw pointer types preserve the number of elements. Note that, as a consequence,
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such casts do *not* necessarily preserve the size of the pointer's referent (e.g.,
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casting `*const [u16]` to `*const [u8]` will result in a raw pointer which refers to an
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object of half the size of the original). The same holds for `str` and any compound type
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whose unsized tail is a slice type, such as struct `Foo(i32, [u8])` or `(u64, Foo)`.
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`*const T` / `*mut T` can be cast to `*const U` / `*mut U` with the following behavior:
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- If `T` and `U` are both sized, the pointer is returned unchanged.
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- If `T` and `U` are both unsized, the pointer is also returned unchanged. In particular,
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the metadata is preserved exactly.
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For instance, a cast from `*const [T]` to `*const [U]` preserves the number of elements.
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Note that, as a consequence, such casts do not necessarily preserve the size of the
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pointer's referent (e.g., casting `*const [u16]` to `*const [u8]` will result in a raw
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pointer which refers to an object of half the size of the original). The same
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holds for `str` and any compound type whose unsized tail is a slice type, such
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as `struct Foo(i32, [u8])` or `(u64, Foo)`.
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- If `T` is unsized and `U` is sized, the cast discards all metadata that
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completes the wide pointer `T` and produces a thin pointer `U` consisting
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of the data part of the unsized pointer.
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## Assignment expressions
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