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

[Andrew_Trick]

@Jumhyn

var y = strawmanDontUseTheseAtTheSameTimeKeyword x
longAlgorithmUsing(&y)
x.append(6) // error: 'x' cannot be modified while 'y' is still alive (used below)
consumeFinalY(y) // OR error: 'y' used after lifetime implicitly ended (by use of 'x' above)

x.append(7) // OK, lifetime of 'y' implicitly ended here

I haven't really thought this through, but it does seem like it would be better if we could properly express the relationship we're worried about rather than relying on ending the lifetime of a single binding.

Looks like you've just reinvented the roadmap's "borrow variables":

A roadmap for improving Swift performance predictability: ARC improvements and ownership control

Swift's high-level semantics try to relieve programmers from thinking about memory management in typical application code. In situations where predictable performance and runtime behavior are needed, though, the variability of ARC and Swift's optimizer have proven difficult for performance-oriented programmers to work with. The Swift Performance team at Apple are working on a series of language changes and features that will make the ARC model easier to understand, while also expanding the bread…