[ufo]

The difficulty with tail recursion optimization is related to calling conventions. Some calling conventions expect the caller to do some cleanup after the callee returns, which effectively means that no function calls actually occur in tail position. For example, in the C calling convention the caller caller is responsible for allocating and freeing the stack memory dedicated for the callee's function parameters. This system makes vararg functions like printf easy to implement but makes it hard to do TCO. Another example is Rust, where having destructors automatically run when a variable got out of scope prevented them from implementing TCO in a clean manner. I'm not familiar with the JVM internals but I think the limitations are going to be similar to the ones I mentioned here.

[steveklabnik]

It's not just that, it's that last time there was serious talk of it, LLVM's musttail wasn't properly supported across important platforms for us. So it got put by the wayside, and there's always so much to do, nobody has worked through the semantics now that support is better.

We did reserve a "become" keyword for this purpose though. Instead of "return", you say "become", and TCO is guaranteed. That's the basic idea anyway, we'll see what happens.