Tail call optimisation means no work can be done after the call and before the return. You can't deallocate stack, call destructors, convert types, rethrow exce

Tail call optimisation means no work can be done after the call and before the return. You can't deallocate stack, call destructors, convert types, rethrow exceptions. Therefore _yes_ the compiler can always do it, but it may involve patching the called function to do some work. Splicing code into it and/or changing the calling convention. That's difficult to do for unknown caller/caller pairs, e.g. function pointers or separate compilation to machine code without enough metadata to patch it later. It runs a bit close to "sufficiently smart" compiler which usually means doable in theory but unlikely in practice. If you pick global designs to make them easier - probably most notably having the callee clean up the stack frame from the caller - and that makes other things slower, then you've given the competition an edge. Clang now has tail call annotations for C++, which works mostly because the compiler can reject them in the cases where it hasn't implemented the lowering. Rust can probably have it in the same approximate circumstances as C++.