I would say that in practice 80% of the values go on the stack, 18% in Box and 2% in an Arc/Rc. That's why Rust code tends to be fast: the common cases are real

I would say that in practice 80% of the values go on the stack, 18% in Box and 2% in an Arc/Rc. That's why Rust code tends to be fast: the common cases are really easy to represent with the borrow checker, so a hypothetical GC doesn't have to perform escape analysis to see if it is ok to specialise those allocations, while the more uncommon cases can still be represented, albeit more verbosely than a GCd language would need.