I'd argue that it's explicit - that's what a function call does and you don't have implicit function calls in rust.

> Seems the latter is a kind of compiler-level optimization, of which there are already many (I think) that change the semantics internally but guarantee the outward behavior stays the same.

What you're asking for here already exists. Tail calls might be optimized into not allocating extra stack frames, the rust compiler just doesn't guarantee that it will perform that optimization (and almost certainly won't when code is compiled without optimizations... for instance).

What people want is the semantic guarantee that the stack frame won't be allocated. Not just a compiler that often performs the optimization. Otherwise you can't be sure that your code will keep working with new compiler flags/versions/architectures/... You could say "whenever the code is the right shape we'll guarantee the optimization" (C++ famously did this for things like copy elision)... but now the shape of code comes with non-obvious semantic guarantees and that's not rust's style. Hence the proposal for a keyword instead.