Or just skip the trait and require there to be nothing live on the stack that conforms to Drop after a `become`. In fact, the main reason to have a keyword lik

Or just skip the trait and require there to be nothing live on the stack that conforms to Drop after a `become`. In fact, the main reason to have a keyword like `become` is to signal to the compiler to do this kind of analysis, because it's so easy to accidentally break the ability to do TCO when you can't lean on a GC to clean things up later.