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

[Karl]

I floated this idea in the performance manifesto thread, and I still kind of like it. It's clear for newcomers to understand, and if you could assert that multiple values are no longer used in a single statement, it would be a one-line addition to the function (rather than wrapping every assignment in a call to the magic function move()).

I really believe we should be trying to think outside the box here. One of Swift's biggest strengths is that it is approachable. Let's try to devise a syntax which beginners can understand without having to read articles on temporary variables and object lifetimes. They may not immediately understand why it's there, but they should at least be able to understand what it does.

@Karl

I wonder -- and this is a totally oddball idea that I absolutely have not thought through -- if the goal is just to communicate some performance assertions to the compiler that it could diagnose, why not just write those as some kind of assertion? In as close to real English as possible.

I guess this ultimately becomes a question of syntax and how well concepts are communicated in source code. The ideas floated in the move proposal about adding a drop(x) or assert(no longer using x) to mark the end of a lifetime is a lot more attractive in light of this manifesto.

// Instead of writing:
init(_ values: [String]) {
  // OMG what does 'move' do? This is some low-level performance intrinsic - eek!
  self.values = move(values)
  self.values.sort()
}

// You'd write something more like:
init(_ values: [String]) {
  self.values = values
  // Oh, ok, they want to check that 'values' is no longer being used.
  // It isn't required for correctness. It's just a check.
  #assert(no longer using 'values')
  self.values.sort()
}

This would be a lot more familiar to Swift developers who aren't ready to get in to the weeds about move semantics and ownership. It's just regular Swift, with some additional assertions to the compiler which are very approachable -- the reason for them being there might be a bit opaque, but no more opaque than having 'move' dotted about everywhere.