Document semantics of various pointer casts

Co-authored-by: Ralf Jung <[email protected]>

Author: udoprog

src/expressions/operator-expr.md

@@ -478,15 +478,23 @@ unsafe {
 assert_eq!(values[1], 3);
 ```
 
-#### Slice DST 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)`.
+#### Pointer-to-pointer cast
+
+`*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