Fix lowering with supertrait predicates

Previously both valid and invalid Rust code could crash r-a due to a
cyclic query during lowering.

crates/hir-ty/src/lower.rs

@@ -53,7 +53,7 @@ use tracing::debug;
 use triomphe::{Arc, ThinArc};
 
 use crate::{
-    FnAbi, ImplTraitId, TyLoweringDiagnostic, TyLoweringDiagnosticKind,
+    FnAbi, ImplTraitId, TyLoweringDiagnostic, TyLoweringDiagnosticKind, all_super_traits,
     consteval::intern_const_ref,
     db::{HirDatabase, InternedOpaqueTyId},
     generics::{Generics, generics, trait_self_param_idx},
@@ -1624,11 +1624,16 @@ pub(crate) fn field_types_with_diagnostics_query<'db>(
     (res, create_diagnostics(ctx.diagnostics))
 }
 
+/// Predicates for `param_id` of the form `P: SomeTrait`. If
+/// `assoc_name` is provided, only return predicates referencing traits
+/// that have an associated type of that name.
+///
 /// This query exists only to be used when resolving short-hand associated types
 /// like `T::Item`.
 ///
 /// See the analogous query in rustc and its comment:
 /// <https://github.com/rust-lang/rust/blob/9150f844e2624eb013ec78ca08c1d416e6644026/src/librustc_typeck/astconv.rs#L46>
+///
 /// This is a query mostly to handle cycles somewhat gracefully; e.g. the
 /// following bounds are disallowed: `T: Foo<U::Item>, U: Foo<T::Item>`, but
 /// these are fine: `T: Foo<U::Item>, U: Foo<()>`.
@@ -1652,7 +1657,7 @@ pub(crate) fn generic_predicates_for_param<'db>(
     );
 
     // we have to filter out all other predicates *first*, before attempting to lower them
-    let predicate = |pred: &_, ctx: &mut TyLoweringContext<'_, '_>| match pred {
+    let has_relevant_bound = |pred: &_, ctx: &mut TyLoweringContext<'_, '_>| match pred {
         WherePredicate::ForLifetime { target, bound, .. }
         | WherePredicate::TypeBound { target, bound, .. } => {
             let invalid_target = { ctx.lower_ty_only_param(*target) != Some(param_id) };
@@ -1700,11 +1705,7 @@ pub(crate) fn generic_predicates_for_param<'db>(
                         return false;
                     };
 
-                    rustc_type_ir::elaborate::supertrait_def_ids(interner, tr.into()).any(|tr| {
-                        tr.0.trait_items(db).items.iter().any(|(name, item)| {
-                            matches!(item, AssocItemId::TypeAliasId(_)) && name == assoc_name
-                        })
-                    })
+                    trait_or_supertrait_has_assoc_type(db, tr, assoc_name)
                 }
                 TypeBound::Use(_) | TypeBound::Lifetime(_) | TypeBound::Error => false,
             }
@@ -1717,7 +1718,7 @@ pub(crate) fn generic_predicates_for_param<'db>(
     {
         ctx.store = maybe_parent_generics.store();
         for pred in maybe_parent_generics.where_predicates() {
-            if predicate(pred, &mut ctx) {
+            if has_relevant_bound(pred, &mut ctx) {
                 predicates.extend(
                     ctx.lower_where_predicate(
                         pred,
@@ -1757,6 +1758,27 @@ pub(crate) fn generic_predicates_for_param_cycle_result(
     StoredEarlyBinder::bind(Clauses::empty(DbInterner::new_no_crate(db)).store())
 }
 
+/// Check if this trait or any of its supertraits define an associated
+/// type with the given name.
+fn trait_or_supertrait_has_assoc_type(
+    db: &dyn HirDatabase,
+    tr: TraitId,
+    assoc_name: &Name,
+) -> bool {
+    for trait_id in all_super_traits(db, tr) {
+        if trait_id
+            .trait_items(db)
+            .items
+            .iter()
+            .any(|(name, item)| matches!(item, AssocItemId::TypeAliasId(_)) && name == assoc_name)
+        {
+            return true;
+        }
+    }
+
+    false
+}
+
 #[inline]
 pub(crate) fn type_alias_bounds<'db>(
     db: &'db dyn HirDatabase,

crates/hir-ty/src/tests/regression.rs

@@ -2598,3 +2598,51 @@ trait ColumnLike {
     "#,
     );
 }
+
+#[test]
+fn issue_21006_generic_predicates_for_param_supertrait_cycle() {
+    check_no_mismatches(
+        r#"
+trait VCipherSuite {}
+
+trait CipherSuite
+where
+    OprfHash<Self>: Hash,
+{
+}
+
+type Bar<CS: CipherSuite> = <CS::Baz as VCipherSuite>::Hash;
+
+type OprfHash<CS: CipherSuite> = <CS::Baz as VCipherSuite>::Hash;
+
+impl<CS: CipherSuite> Foo<CS> {
+    fn seal() {}
+}
+        "#,
+    );
+}
+
+#[test]
+fn issue_21006_self_assoc_trait() {
+    check_types(
+        r#"
+trait Baz {
+    fn baz(&self);
+}
+
+trait Foo {
+    type Assoc;
+}
+
+trait Bar: Foo
+where
+    Self::Assoc: Baz,
+{
+    fn bar(v: Self::Assoc) {
+        let _ = v.baz();
+        //  ^ ()
+    }
+}
+        "#,
+    );
+}