Making Drop more magic to make it less magic

[scottmcm]

@CAD97

It’s for this reason that this by-move Drop proposal suggests the the use of the unconditional_recursion lint to prevent unconditional recursion, and confines the drop glue changes to Drop::drop .

unconditional_recursion only catches direct recursion, not mutual recursion. So it wouldn't even catch

impl Drop for Resource {
    fn drop(self) {
        drop(self);
    }
}

@earthengine

is to make the “move everything out of self” being the first step of any drop function body

That's what I mentioned in the thread @CAD97 linked in the OP: we already have syntax for "destructure the type", so we could just require that implementations of the new drop method uses that in the parameter pattern. It's a simple and clear restriction that's entirely local.

On back-compat: we can have drop glue work by calling existing drop with &mut Self, then new drop with Self. People could implement either, both, or neither.