Add comment explaining `MaybeConstMaybe` lowering
diff --git a/src/librustc_ast_lowering/lib.rs b/src/librustc_ast_lowering/lib.rs
index bc922fa..7163bf9 100644
--- a/src/librustc_ast_lowering/lib.rs
+++ b/src/librustc_ast_lowering/lib.rs
@@ -1254,6 +1254,8 @@
| GenericBound::Trait(ref ty, TraitBoundModifier::MaybeConst) => {
Some(this.lower_poly_trait_ref(ty, itctx.reborrow()))
}
+ // `?const ?Bound` will cause an error during AST validation
+ // anyways, so treat it like `?Bound` as compilation proceeds.
GenericBound::Trait(_, TraitBoundModifier::Maybe)
| GenericBound::Trait(_, TraitBoundModifier::MaybeConstMaybe) => {
None
@@ -2301,6 +2303,9 @@
match f {
TraitBoundModifier::None => hir::TraitBoundModifier::None,
TraitBoundModifier::MaybeConst => hir::TraitBoundModifier::MaybeConst,
+
+ // `MaybeConstMaybe` will cause an error during AST validation, but we need to pick a
+ // placeholder for compilation to proceed.
TraitBoundModifier::MaybeConstMaybe | TraitBoundModifier::Maybe => {
hir::TraitBoundModifier::Maybe
}
