What do you mean by saying heap allocation? Do you mean syscall or libc? Zig has a extremely explicit requirement for any allocation. You can have a preallocated region of memory in the allocator for recursion and only move the needle for each “heap allocation”.

Ah, I see, it's a split stack. Well, we experimented with those in Rust for Rust->C calls and they were not fast enough (also, they cause problems with debuggers and tooling). Perhaps if virtual calls are uncommon, though.

At that point I have to wonder what the advantage is over just using explicit checks for overflow on the machine stack though.

You're still forcing a mindset onto the language that isn't present.

It's not 'a split stack' or any other particular solution. You pass in an allocator and it does it whichever way your chose allocator picks.

The goal is to explore sections of design space you're completely ruling out from before you even start.