Actually, I got curious and tried it out. Turns out Rust didn't optimize the heap case: it used the caller's stack, and only then copied the value to the heap.

https://play.rust-lang.org/?code=%23!%5Bfeature(core)%5D%0A%...

Because it would change the semantics of the program. Where heap allocations happen is considered a side effect, and forcing a function to be inline(never) also makes it have unconstrained side effects (in some cases). Those side effects cannot be reordered.

That's because of `Box::new`, I'd think. If you use the box keyword, you should get the placement new effect.

Thank you! I thought the box keyword was simply a syntactic sugar -- I'm not that familiar with placement new, even in C++.

Note for the interested: the optimization works right now, even though the keyword is behind the box_syntax feature gate.

Updated test: https://play.rust-lang.org/?code=%23!%5Bfeature(core)%5D%0A%...