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remove obligation dedup from impl_or_trait_obligations #84944

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Mar 3, 2022
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remove obligation dedup from impl_or_trait_obligations #84944

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22 changes: 0 additions & 22 deletions compiler/rustc_trait_selection/src/traits/select/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2328,28 +2328,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
});
}

// We are performing deduplication here to avoid exponential blowups
// (#38528) from happening, but the real cause of the duplication is
// unknown. What we know is that the deduplication avoids exponential
// amount of predicates being propagated when processing deeply nested
// types.
//
// This code is hot enough that it's worth avoiding the allocation
// required for the FxHashSet when possible. Special-casing lengths 0,
// 1 and 2 covers roughly 75-80% of the cases.
if obligations.len() <= 1 {
// No possibility of duplicates.
} else if obligations.len() == 2 {
// Only two elements. Drop the second if they are equal.
if obligations[0] == obligations[1] {
obligations.truncate(1);
}
} else {
// Three or more elements. Use a general deduplication process.
let mut seen = FxHashSet::default();
obligations.retain(|i| seen.insert(i.clone()));
}

obligations
}
}
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