Moving this to a new thread as TCO discussions tend to quickly explode, and it’s irrelevant to the original discussion of Rust benchmarks (TCO cannot be used for a function like naive fib).
(As Stefan says, TCO is only for “trivial” recursion that can just as easily be written as a loop. Guaranteed TCO is mainly important in functional languages where recursion is favored over loops.)
See this thread and references therein: Does Julia have tail call optimization?