Comment by mananaysiempre

It is a feature of the language (with proper tail calls) that a certain class of calls defined in the spec must be TCOd, if you want to put things that way. It’s not just that it’s easier for the compiler to recognize them, it’s that it has to.

(The usual caveats about TCO randomly not working are due to constraints imposed by preexisting ABIs or VMs; if you don’t need to care about those, then the whole thing is quite straightforward.)