SE-0366: Move Function + "Use After Move" Diagnostic

[beccadax (Becca Royal-Gordon)]

@ksluder

I’m unclear on how __owned and __consuming relate to move(_:). Do they all deal with the same kind of ownership? Or do __owned and __consuming refer to ARC ownership, while move(_:) refers to a new kind of ownership that’s tracked by flow analysis? If they’re the same thing, then I guess my move parameter modifier suggestion above is just a formalization of __consuming?

To start:

• __consuming just means that self is __owned, in much the same way that mutating means self is inout.
• When I talk about “copying” below, think “retain” for an object; when I talk about “destroying”, think “release”.

__owned means that the callee will destroy the value, so the caller should consider it unusable once the callee returns. The caller can handle this by either copying the value before passing it (the default) or ending its lifetime so it can no longer be used (what happens if you use move).

The alternative is that the callee will not destroy the value, so the caller can keep using it once the callee returns. This means that, if there’s a copy, it will be in the callee, not the caller. The caller can still use move, but it won’t actually eliminate the copy; the callee will still copy the original, but the caller will destroy the original once the callee returns.

So basically, __owned means that if you do move the value in, that will truly eliminate a copy. But using __owned by itself doesn’t eliminate the copy; it just allows the caller to eliminate it.