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Rollup merge of #134553 - lcnr:member-constraints-comment, r=oli-obk

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add member constraints comment

r? `@oli-obk` i guess
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DianQK 2024-12-20 21:47:04 +08:00 committed by GitHub
commit 594e74d56f
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1 changed files with 6 additions and 3 deletions
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9
compiler/rustc_borrowck/src/region_infer/mod.rs
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@ -795,7 +795,6 @@ impl<'tcx> RegionInferenceContext<'tcx> {
// If the member region lives in a higher universe, we currently choose
// the most conservative option by leaving it unchanged.
if !self.constraint_sccs().annotation(scc).min_universe().is_root() {
return;
}
@ -823,12 +822,14 @@ impl<'tcx> RegionInferenceContext<'tcx> {
}
debug!(?choice_regions, "after ub");
// At this point we can pick any member of `choice_regions`, but to avoid potential
// non-determinism we will pick the *unique minimum* choice.
// At this point we can pick any member of `choice_regions` and would like to choose
// it to be a small as possible. To avoid potential non-determinism we will pick the
// smallest such choice.
//
// Because universal regions are only partially ordered (i.e, not every two regions are
// comparable), we will ignore any region that doesn't compare to all others when picking
// the minimum choice.
//
// For example, consider `choice_regions = ['static, 'a, 'b, 'c, 'd, 'e]`, where
// `'static: 'a, 'static: 'b, 'a: 'c, 'b: 'c, 'c: 'd, 'c: 'e`.
// `['d, 'e]` are ignored because they do not compare - the same goes for `['a, 'b]`.
@ -853,6 +854,8 @@ impl<'tcx> RegionInferenceContext<'tcx> {
return;
};
// As we require `'scc: 'min_choice`, we have definitely already computed
// its `scc_values` at this point.
let min_choice_scc = self.constraint_sccs.scc(min_choice);
debug!(?min_choice, ?min_choice_scc);
if self.scc_values.add_region(scc, min_choice_scc) {