I know nothing about writing crypto core code. But... Rust supports inline assembly, so after you're done, you could always zeroize every register, right?
Since the compiler can do whatever it wants, you can't be sure it hasn't put any part of the key anywhere on the stack, and there isn't sufficient information for the asm to know which registers contain tainted values.
Unless you can actually prove all parts of the compiler's data transforms going down to assembly I think the safest thing to do is sandbox your key-handling process so nobody else can examine it.
