Support `FieldRefMut::replace`

[RaitoBezarius]

Extension of #134 for non-enum structures. In non-enum structures, there's no way to move a single non-Copy field out and leave a placeholder (or a default constructed type). The usual Rust pattern is std::mem::replace(&mut foo.field, placeholder) and I don't think zngur supports A C++ equivalent atm.

My proposal would be to generate a Rust extern function per field that does the std::mem::replace and on the C++ side you can call T replace(T placeholder) to FieldRefMut (incl. specializations).
This is always safe because the field is always left initialized.

#133 as a pre-requisite would make sense to put replace in the CRTP perhaps if this idea is retained.

[hkalbasi]

Can't you just use std::mem::replace?

[RaitoBezarius]

Why not (something akin to):

inline T replace(T placeholder) const noexcept {
   auto field_ptr = parent_data_ptr() + this->field_offset();
   auto old = __zngur_internal_move_from_rust<T>(field_ptr);
   __zngur_internal_move_to_rust<T>(field_ptr, placeholder);
   return old;
}

instead?

I am not sure I get your idea of "just" using std::mem::replace, on the C++ side? On the Rust side?

[hkalbasi]

Wrap std::mem::replace in your zng file and then use it on the C++ side.