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[Instructions -0.13% vs main; paired PGO estimate] Check MetaSized once per obligation before scanning bounds #129

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[Instructions -0.13% vs main; paired PGO estimate] Check MetaSized once per obligation before scanning bounds #129

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29 changes: 22 additions & 7 deletions compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -218,6 +218,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
// normalization, so try to deduplicate when possible to avoid
// unnecessary ambiguity.
let mut distinct_normalized_bounds = FxHashSet::default();
// Sizedness candidates are only lazily elaborated for `MetaSized`
// obligations, so check the obligation once instead of per bound.
let obligation_is_metasized =
self.tcx().is_lang_item(obligation.predicate.def_id(), LangItem::MetaSized);
let _ = self.for_each_item_bound::<!>(
placeholder_trait_predicate.self_ty(),
|selcx, bound, idx, alias_bound_kind| {
Expand All @@ -229,11 +233,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
}

selcx.infcx.probe(|_| {
let bound = util::lazily_elaborate_sizedness_candidate(
selcx.infcx,
obligation,
bound,
);
let bound = if obligation_is_metasized {
util::lazily_elaborate_sizedness_candidate(
selcx.infcx,
obligation,
bound,
)
} else {
bound
};

// We checked the polarity already
match selcx.match_normalize_trait_ref(
Expand Down Expand Up @@ -287,10 +295,17 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {

let drcx = DeepRejectCtxt::relate_rigid_rigid(self.tcx());
let obligation_args = stack.obligation.predicate.skip_binder().trait_ref.args;
// Sizedness candidates are only lazily elaborated for `MetaSized`
// obligations, so check the obligation once instead of per bound.
let obligation_is_metasized =
self.tcx().is_lang_item(stack.obligation.predicate.def_id(), LangItem::MetaSized);
// Keep only those bounds which may apply, and propagate overflow if it occurs.
for bound in bounds {
let bound =
util::lazily_elaborate_sizedness_candidate(self.infcx, stack.obligation, bound);
let bound = if obligation_is_metasized {
util::lazily_elaborate_sizedness_candidate(self.infcx, stack.obligation, bound)
} else {
bound
};

// Micro-optimization: filter out predicates relating to different traits.
if bound.def_id() != stack.obligation.predicate.def_id() {
Expand Down