Support for guaranteeing TCE is possible and has been proposed, but there remain a few open questions, e.g. what to do about local variables with destructors (personally I'd statically forbid them from being in scope at the end of tail-call recursive functions).
I know, that's why I wrote "when", knowing it's been considered, but it doesn't appear to be missed by enough developers. I'm just spoiled having used those two features, and anytime someone cites the ML family as part of Rust's influence, it reminds me of these two basic, missing features.
Regarding semantics, without having thought about the Rust semantics too much, I'd suggest to check out the most prominent uses for recursive functions in OCaml, SML or Haskell, then try to consider that as the sweet spot to support in Rust.
