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[perf] Specialize common (1, 1) case for arg unification. #158865

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rust-bors merged 1 commit into from
Jul 9, 2026
+9 −2
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[perf] Specialize common (1, 1) case for arg unification. #158865

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11 changes: 9 additions & 2 deletions compiler/rustc_type_ir/src/fast_reject.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -233,7 +233,7 @@ impl<I: Interner, const INSTANTIATE_LHS_WITH_INFER: bool, const INSTANTIATE_RHS_
// No need to decrement the depth here as this function is only
// recursively reachable via `types_may_unify_inner` which already
// increments the depth for us.
iter::zip(obligation_args.iter(), impl_args.iter()).all(|(obl, imp)| {
let may_unify = |(obl, imp): (I::GenericArg, I::GenericArg)| {
match (obl.kind(), imp.kind()) {
// We don't fast reject based on regions.
(ty::GenericArgKind::Lifetime(_), ty::GenericArgKind::Lifetime(_)) => true,
Expand All @@ -245,7 +245,14 @@ impl<I: Interner, const INSTANTIATE_LHS_WITH_INFER: bool, const INSTANTIATE_RHS_
}
_ => panic!("kind mismatch: {obl:?} {imp:?}"),
}
})
};

// Specialize the common `(1, 1)` case to avoid iterator machinery.
if let ([obl], [imp]) = (obligation_args.as_slice(), impl_args.as_slice()) {
return may_unify((*obl, *imp));
}

iter::zip(obligation_args.iter(), impl_args.iter()).all(may_unify)
}

fn types_may_unify_inner(self, lhs: I::Ty, rhs: I::Ty, depth: usize) -> bool {
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