Rust does allow tail call optimization. But that's LLVM's decision to optimize tail calls on a case-by-case basis. An explicit syntax to denote tail calls would be the difference between tail call optimization and guaranteed tall call elimination, which is important because if you're writing a tail-recursive function then it's pretty trivial to blow the stack at any moderate recursion depth unless you can guarantee the elimination.

As for why it's not trivial for Rust to do this by default, consider the question of what should happen in the case of local destructors, which in an ordinary function would be called after `return myfunc()` returns, but in a tail-recursive function would need to be called beforehand. The proposals for `become` tend to handle this by making it a compiler error to have any locals with destructors in scope at the point of the tail-call, further motivating the explicit syntax.