There's some ugly hacks that may or may not help here:

https://www.reddit.com/r/rust/comments/xxhp3s/perhaps_not_a_...

    r#box!(make_my_elem())

This crate is marked as deprecated because apparently upstream rust optimises its use-case now, but you never know:

https://github.com/kvark/copyless

    Box::alloc().init(make_my_elem())