Update operator-expr.md

Author: joshlf

src/expressions/operator-expr.md

@@ -480,12 +480,15 @@ 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).
+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)`.
+
+
 
 ## Assignment expressions