I wonder if this error should be produced on the SSA side instead of panicking when the "too much recursion" is detected. That way it would detect mutual recursion too.

You mean to change the return value into Result? I didn’t investigate the impact, or if it has any other panics that could be covered. There was another PR with a similar case where I changed the panic message and added some CLI side filtering, but didn’t want to cause too much churn by changing the entire call stack.

I’ll defer to @TomAFrench

Ah, you mean that instead of a warning, we could rely purely on the SSA.

I did it this way so it can be a warning during compilation, like the unused variables. Rust has this warning too, so why not? I didn't implement mutual recursion because Rust doesn't seem to have it either, but in theory I think we could: if we detect a cycle where none of the code paths can return without calling any of the FuncId we collected along the way, then it could reject that case too.