[NLL] remove "constant" region variables, report free region errors later #45911
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It will be useful later for diagnostics to be able to remember where things were live.
Rather than declaring some region variables to be constant, and reporting errors when they would have to change, we instead populate each free region X with a minimal set of points (the CFG plus end(X)), and then we let inference do its thing. This may add other `end(Y)` points into X; we can then check after the fact that indeed `X: Y` holds. This requires a bit of "blame" detection to find where the bad constraint came from: we are currently using a pretty dumb algorithm. Good place for later expansion.
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I'm going to close this. I'm rebasing it on top of #45825 and will land it later. |
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Rather than declaring some region variables to be constant, and reporting errors when they would have to change, we instead populate each free region X with a minimal set of points (the CFG plus end(X)), and then we let inference do its thing. This may add other
end(Y)points into X; we can then check after the fact that indeedX: Yholds.This will be useful when handling closures properly, since we can then propagate some of those constraints in the form of WF-conditions to our creator. This PR doesn't handle any of that yet.
This requires a bit of "blame" detection to find where the bad constraint came from: we are currently using a pretty dumb algorithm. Good place for later expansion.
r? @arielb1