[Pitch] Move Function + "Use After Move" Diagnostic

[taylorswift]

@dabrahams

where you have to scatter move() throughout otherwise-straightforward code if you want it to be efficient. The problem with establishing that practice now in Swift is that even after there's a more complete design, it will probably be cargo-culted into the distant future.

i started using _move(_:) and that’s exactly what ended up happening. once you write _move(_:) once in a function, it’s hard to argue why one variable should get _moved and not another. i often find myself _moveing everything that’s backed by a heap allocation.

a side effect of knowing that _move exists is, psychologically, you start to assume that everything that isn’t explicitly _moved is copyable. this ends up being a real (and not imaginary) problem because of the lack of memory profiling tools for swift on linux.