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rework min_choice algorithm of member constraints

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See the inline comments for the description of the new algorithm.
This commit is contained in:
Ali MJ Al-Nasrawy 2023-01-16 01:31:58 +03:00
parent 159ba8a92c
commit 837c1aff05
8 changed files with 288 additions and 11 deletions
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35
compiler/rustc_borrowck/src/region_infer/mod.rs
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@ -739,20 +739,33 @@ impl<'tcx> RegionInferenceContext<'tcx> {
}
debug!(?choice_regions, "after ub");
// If we ruled everything out, we're done.
if choice_regions.is_empty() {
return false;
}
// Otherwise, we need to find the minimum remaining choice, if
// any, and take that.
debug!("choice_regions remaining are {:#?}", choice_regions);
let Some(&min_choice) = choice_regions.iter().find(|&r1| {
// At this point we can pick any member of `choice_regions`, but to avoid potential
// non-determinism we will pick the *unique minimum* 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]`.
let totally_ordered_subset = choice_regions.iter().copied().filter(|&r1| {
choice_regions.iter().all(|&r2| {
self.universal_region_relations.outlives(r2, *r1)
self.universal_region_relations.outlives(r1, r2)
|| self.universal_region_relations.outlives(r2, r1)
})
});
// Now we're left with `['static, 'c]`. Pick `'c` as the minimum!
let Some(min_choice) = totally_ordered_subset.reduce(|r1, r2| {
let r1_outlives_r2 = self.universal_region_relations.outlives(r1, r2);
let r2_outlives_r1 = self.universal_region_relations.outlives(r2, r1);
match (r1_outlives_r2, r2_outlives_r1) {
(true, true) => r1.min(r2),
(true, false) => r2,
(false, true) => r1,
(false, false) => bug!("incomparable regions in total order"),
}
}) else {
debug!("no choice region outlived by all others");
debug!("no unique minimum choice");
return false;
};