Merge pull request #154 from gnzlbg/usize16

Document that isize/usize are at least 16-bit wide on all platforms we currently support

Author: RalfJung

reference/src/layout/scalars.md

@@ -37,6 +37,11 @@ The `isize` and `usize` types are pointer-sized signed and unsigned integers.
 They have the same layout as the [pointer types] for which the pointee is
 `Sized`, and are layout compatible with C's `uintptr_t` and `intptr_t` types.
 
+> **Note**: C99 [7.18.2.4](https://port70.net/~nsz/c/c99/n1256.html#7.18.2.4)
+> requires `uintptr_t` and `intptr_t` to be at least 16-bit wide. All 
+> platforms we currently support have a C platform, and as a consequence,
+> `isize`/`usize` are at least 16-bit wide for all of them.
+
 > **Note**: Rust's `usize` and C's `unsigned` types are **not** equivalent. C's
 > `unsigned` is at least as large as a short, allowed to have padding bits, etc.
 > but it is not necessarily pointer-sized.