diff --git a/src/expressions/operator-expr.md b/src/expressions/operator-expr.md index bd4998a..d3fa33b 100644 --- a/src/expressions/operator-expr.md +++ b/src/expressions/operator-expr.md @@ -478,15 +478,20 @@ unsafe { assert_eq!(values[1], 3); ``` -#### Slice DST pointer to pointer cast +#### Pointer-to-pointer cast -For slice types like `[T]` and `[U]`, the raw pointer types `*const [T]`, `*mut [T]`, -`*const [U]`, and `*mut [U]` encode the number of elements in this slice. Casts between -these raw pointer types preserve the number of elements. Note that, as a consequence, -such casts do *not* necessarily preserve the size of the pointer's referent (e.g., -casting `*const [u16]` to `*const [u8]` will result in a raw pointer which refers to an -object of half the size of the original). The same holds for `str` and any compound type -whose unsized tail is a slice type, such as struct `Foo(i32, [u8])` or `(u64, Foo)`. +`*const T` / `*mut T` can be cast to `*const U` / `*mut U` with the following behavior: + +- If `T` and `U` are both sized, the pointer is returned unchanged. +- If `T` and `U` are both unsized, the pointer is also returned unchanged. + In particular, the metadata is preserved exactly. + + For instance, a cast from `*const [T]` to `*const [U]` preserves the number of elements. + Note that, as a consequence, such casts do not necessarily preserve the size of the pointer's referent + (e.g., casting `*const [u16]` to `*const [u8]` will result in a raw pointer which refers to an object of half the size of the original). + The same holds for `str` and any compound type whose unsized tail is a slice type, + such as `struct Foo(i32, [u8])` or `(u64, Foo)`. +- If `T` is unsized and `U` is sized, the cast discards all metadata that completes the wide pointer `T` and produces a thin pointer `U` consisting of the data part of the unsized pointer. ## Assignment expressions @@ -687,3 +692,17 @@ See [this test] for an example of using this dependency. [_TypeNoBounds_]: ../types.md#type-expressions [_RangeExpression_]: ./range-expr.md [_UnderscoreExpression_]: ./underscore-expr.md + +