liveness in stack closures

[nikomatsakis]

The liveness pass should be extended to treat stack closures just like a loop body. This is relatively straightforward. It would allow for a couple of nice things:

• it'd be more efficient in some cases (I believe the last_use code I replaced actually did this right) because we could move values in if there were a loop like

  let mut v = [];
  uint::range(...) { |i|
  use(v);
  v = [some new vec];
  }

• it allows us to right fold for arbitrary iteration with only moves:

  let mut b = b0;
  for as.each { |a|
  b <- op(a, b);
  }
  ret b;

  In this case, the value in op(e, b) is actually moved out temporarily but always restored by the end of the loop. This would work today with a while loop but not with a closure-based loop.

[nikomatsakis]

it is not clear that this would be sound---we'd have to be wary of recursive calls etc. just going to close this.