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Document why we allow escaping bound vars in LTA norm #141363

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May 22, 2025
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Document why we allow escaping bound vars in LTA norm #141363

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17 changes: 13 additions & 4 deletions compiler/rustc_trait_selection/src/traits/normalize.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -299,12 +299,21 @@ impl<'a, 'b, 'tcx> AssocTypeNormalizer<'a, 'b, 'tcx> {
);
}

// We don't replace bound vars in the generic arguments of the free alias with
// placeholders. This doesn't cause any issues as instantiating parameters with
// bound variables is special-cased to rewrite the debruijn index to be higher
// whenever we fold through a binder.
//
// However, we do replace any escaping bound vars in the resulting goals with
// placeholders as the trait solver does not expect to encounter escaping bound
// vars in obligations.
//
// FIXME(lazy_type_alias): Check how much this actually matters for perf before
// stabilization. This is a bit weird and generally not how we handle binders in
// the compiler so ideally we'd do the same boundvar->placeholder->boundvar dance
// that other kinds of normalization do.
let infcx = self.selcx.infcx;
self.obligations.extend(
// FIXME(BoxyUwU):
// FIXME(lazy_type_alias):
// It seems suspicious to instantiate the predicates with arguments that might be bound vars,
// we might wind up instantiating one of these bound vars underneath a hrtb.
infcx.tcx.predicates_of(free.def_id).instantiate_own(infcx.tcx, free.args).map(
|(mut predicate, span)| {
if free.has_escaping_bound_vars() {
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