Update src/expressions/operator-expr.md

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

Author: udoprog

src/expressions/operator-expr.md

@@ -482,6 +482,16 @@ assert_eq!(values[1], 3);
 
 `*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.