rewrite the comments
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@ -29,20 +29,28 @@ pub(super) fn opt_const_param_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<
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let parent_node = tcx.hir().get(parent_node_id);
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match parent_node {
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// This matches on types who's paths couldn't be resolved without typeck'ing e.g.
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// This match arm is for when the def_id appears in a GAT whose
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// path can't be resolved without typechecking e.g.
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//
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// trait Foo {
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// type Assoc<const N1: usize>;
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// type Assoc<const N: usize>;
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// fn foo() -> Self::Assoc<3>;
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// // note: if the def_id argument is the 3 then in this example
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// // parent_node would be the node for Self::Assoc<_>
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// }
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// We didnt write <Self as Foo>::Assoc so the Self::Assoc<_> is lowered to QPath::TypeRelative.
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//
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// In the above code we would call this query with the def_id of 3 and
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// the parent_node we match on would be the hir node for Self::Assoc<3>
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//
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// `Self::Assoc<3>` cant be resolved without typchecking here as we
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// didnt write <Self as Foo>::Assoc<3>. If we did then another match
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// arm would handle this.
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//
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// I believe this match arm is only needed for GAT but I am not 100% sure - BoxyUwU
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Node::Ty(hir_ty @ Ty { kind: TyKind::Path(QPath::TypeRelative(_, segment)), .. }) => {
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// Walk up from the parent_node to find an item so that
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// we can resolve the relative path to an actual associated type.
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// For the code example above, this item would be the Foo trait.
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// Find the Item containing the associated type so we can create an ItemCtxt.
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// Using the ItemCtxt convert the HIR for the unresolved assoc type into a
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// ty which is a fully resolved projection.
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// For the code example above, this would mean converting Self::Assoc<3>
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// into a ty::Projection(<Self as Foo>::Assoc<3>)
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let item_hir_id = tcx
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.hir()
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.parent_iter(hir_id)
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@ -52,11 +60,12 @@ pub(super) fn opt_const_param_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<
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.unwrap();
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let item_did = tcx.hir().local_def_id(item_hir_id).to_def_id();
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let item_ctxt = &ItemCtxt::new(tcx, item_did) as &dyn crate::astconv::AstConv<'_>;
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// This ty will be the actual associated type so that we can
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// go through its generics to find which param our def_id corresponds to.
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// For the code example above, this ty would be the Assoc<const N1: usize>.
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let ty = item_ctxt.ast_ty_to_ty(hir_ty);
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// Iterate through the generics of the projection to find the one that corresponds to
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// the def_id that this query was called with. We filter to only const args here as a
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// precaution for if it's ever allowed to elide lifetimes in GAT's. It currently isn't
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// but it can't hurt to be safe ^^
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if let ty::Projection(projection) = ty.kind() {
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let generics = tcx.generics_of(projection.item_def_id);
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