Transitive R-Value Inference and Move Construction a la Rust

[ag0aep6g via Digitalmars-d]

On 09/26/2016 05:24 PM, Nordlöw wrote:
> I just discovered the following interesting fact about the current state
> of DMD:
[...]
> void top(T)(T a)                // TODO infer that `a` is passed as an
> r-value?
> {
>     assert(a.copyCount == 0);
>     sub(a);                     // so we can do move construction here
> aswell?
> }
[...]
> unittest
> {
[...]
>     top(C());
> }
>
> namely that D will infer move semantics of `top`'s parameter when
> passing an r-value in the call to `top()` but not transitively in
> `top`'s call to `sub` even though this is possible.

The move happens because `C()` is an rvalue. This happens at the call 
site. The code generation for `top` is not affected by this. `top` 
doesn't know or care if the argument it gets is the result of a copy or 
a move.

Inside `top`, `a` is an lvalue. It wouldn't be correct to move `a` just 
because it originated from an rvalue.

There may be cases where `sub(a)` could move `a` into position instead 
of copying it, but `a` having been moved or copied before is not the 
only thing to consider, if it even needs consideration.