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Rollup merge of #141363 - BoxyUwU:doc_lta_norm_binders, r=lcnr

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Document why we allow escaping bound vars in LTA norm

r? lcnr

followup from the const normalization PR. I think I now understand why free alias norm is funny about binders
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Matthias Krüger 2025-05-21 22:15:02 +02:00 • committed by GitHub
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17
compiler/rustc_trait_selection/src/traits/normalize.rs
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@ -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() {