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[perf] Fast path for coercions #142701

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[perf] Fast path for coercions #142701

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17 changes: 11 additions & 6 deletions compiler/rustc_hir_typeck/src/coercion.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -597,6 +597,16 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {

// Create an obligation for `Source: CoerceUnsized<Target>`.
let cause = self.cause(self.cause.span, ObligationCauseCode::Coercion { source, target });
let obligation = Obligation::new(
self.tcx,
cause,
self.fcx.param_env,
ty::TraitRef::new(self.tcx, coerce_unsized_did, [coerce_source, coerce_target]),
);
if self.predicate_must_hold_modulo_regions(&obligation) {
coercion.obligations.push(obligation);
return Ok(coercion);
}

// Use a FIFO queue for this custom fulfillment procedure.
//
Expand All @@ -605,12 +615,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
// and almost never more than 3. By using a SmallVec we avoid an
// allocation, at the (very small) cost of (occasionally) having to
// shift subsequent elements down when removing the front element.
let mut queue: SmallVec<[PredicateObligation<'tcx>; 4]> = smallvec![Obligation::new(
self.tcx,
cause,
self.fcx.param_env,
ty::TraitRef::new(self.tcx, coerce_unsized_did, [coerce_source, coerce_target])
)];
let mut queue: SmallVec<[PredicateObligation<'tcx>; 4]> = smallvec![obligation];

// Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
// emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
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