Location-sensitive polonius prototype: endgame #134980
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it's still partially a skeleton, but works well enough for almost all tests to pass
this is to remove the entire `util` module
it's been simplified over the years, but now it's no longer useful. - document its replacement in `BorrowKind` - use that everywhere instead
in NLLs some locals are marked live at all points if one of their regions escapes the function but that doesn't work in a flow-sensitive setting like polonius
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borrow checking is easy with time travel and spacetime reachability
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18 tasks
we're in in the endgame now set up the location-sensitive analysis end to end: - stop recording inflowing loans and loan liveness in liveness - replace location-insensitive liveness data with live loans computed by reachability - remove equivalence between polonius scopes and NLL scopes, and only run one scope computation
it's a bit mind-bending
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This PR sets up the naive location-sensitive analysis end-to-end, and replaces the location-insensitive analysis. It's roughly all the in-progress work I wanted to land for the prototype, modulo cleanups I still want to do after the holidays, or the polonius debugger, and so on.
Here, we traverse the localized constraint graph, have to deal with kills and time-traveling (👌), and record that as loan liveness for the existing scope and active loans computations.
Then the near future looks like this, especially if the 2025h1 project goal is accepted:
I've tried to gradually introduce this PR's work over 4 commits, because it's kind of subtle/annoying, and Niko/I are not completely convinced yet. That one comment explaining the situation is maybe 30% of the PR 😓. Who knew that spacetime reachability and time-traveling could be mind bending.
I kinda found this late and the impact on this part of the computation was a bit unexpected to us. A bit more care/thought will be needed here. I've described my plan in the comments though. In any case, I believe we have the current implementation is a conservative approximation that shouldn't result in unsoundness but false positives at worst. So it feels fine for now.
r? @jackh726
This PR looks bigger than it is because it's stacked on top of #134920. It contains its 6 commits (rebased) which can be ignored. I've opened this one as draft until review for #134920 is done.
Fixes #127628 -- which was a assertion triggered for a difference in loan computation between NLLs and the location-insensitive analysis. That doesn't exist anymore so I've removed this crash test.