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Auto merge of #84559 - jackh726:issue-84398, r=nikomatsakis

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Deduplicate ParamCandidates with the same value except for bound vars

Fixes #84398

This is kind of a hack. I wonder if we can get other types of candidates that are the same except for bound vars. This won't be a problem with Chalk, since we don't really need to know that there are two different "candidates" if they both give the same final substitution.

r? `@nikomatsakis`
This commit is contained in:
bors 2021-05-06 17:42:32 +00:00
commit 377d1a984c
2 changed files with 31 additions and 1 deletions
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12
compiler/rustc_trait_selection/src/traits/select/mod.rs
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@ -1361,7 +1361,17 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
) => false,
(ParamCandidate(other), ParamCandidate(victim)) => {
if other.value == victim.value && victim.constness == Constness::NotConst {
let value_same_except_bound_vars = other.value.skip_binder()
== victim.value.skip_binder()
&& !other.value.skip_binder().has_escaping_bound_vars();
if value_same_except_bound_vars {
// See issue #84398. In short, we can generate multiple ParamCandidates which are
// the same except for unused bound vars. Just pick the one with the fewest bound vars
// or the current one if tied (they should both evaluate to the same answer). This is
// probably best characterized as a "hack", since we might prefer to just do our
// best to *not* create essentially duplicate candidates in the first place.
other.value.bound_vars().len() <= victim.value.bound_vars().len()
} else if other.value == victim.value && victim.constness == Constness::NotConst {
// Drop otherwise equivalent non-const candidates in favor of const candidates.
true
} else {