The problem is that, in practice, Rust code uses usize mainly as the integer that we use to index a Vec, and this the role of size_t. There's unfortunately a mismatch between what's been defined (usize as uintptr_t) and what's used in practice. This has been discussed before.
I think the best to do is to admit the mistake and fix the definition of usize. The reason this is possible is that this doesn't break any currently running code, since in all platforms Rust is currently supported, size_t = uintptr_t