WIP: Trust parameter env when projecting GATs #98742
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If the param-env tells you that, forall 'a, `T::Item<'a> = u32`, we can believe it.
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`call-forall` shows the expected behavior: the where-clause proves too much, so we can't prove it. `backwards-incompat` shows the problem with that: we can construct types that we ought not to be able to construct.
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Thinking on it more, as the backwards-incompat suggests, this behavior isn't more right -- that is, I think we should be trusting the where-clause, but we have the wrong where-clause, so... in a way the current code enforces something closer to the final behavior (but not completely). Anyway, prob shouldn't land, but it was a good exercise for me. |
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☔ The latest upstream changes (presumably #93967) made this pull request unmergeable. Please resolve the merge conflicts. |
jackh726
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Closing this as it was an experiment |
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While investigating the behavior for GAT implied bounds, I realized that we were enforcing stricter requirements when projecting than I believe we should -- in particular, if the where-clause says "this is true for all
'a", we should believe it.This fixes a lot of unnecessary errors from GATs, though not all (see the new test I added, which shows why this is still sound and reasonable).
However, it also may introduce a backwards incompatibility hazard, albeit a very unlikely one, in that we are going to modify the meaning of this where-clause, and hence some functions are accepted now that won't be accepted later, even though they aren't callable today.
r? @jackh726