fix: cyclic dependency panic

crates/hir-ty/src/lower.rs

@@ -1551,7 +1551,7 @@ impl<'db> GenericPredicates<'db> {
     /// Resolve the where clause(s) of an item with generics.
     ///
     /// Diagnostics are computed only for this item's predicates, not for parents.
-    #[salsa::tracked(returns(ref), unsafe(non_update_return_type))]
+    #[salsa::tracked(returns(ref), unsafe(non_update_return_type), cycle_result = generic_predicates_query_with_diagnostics_cycle_result)]
     pub fn query_with_diagnostics(
         db: &'db dyn HirDatabase,
         def: GenericDefId,
@@ -1560,6 +1560,21 @@ impl<'db> GenericPredicates<'db> {
     }
 }
 
+pub(crate) fn generic_predicates_query_with_diagnostics_cycle_result<'db>(
+    _db: &'db dyn HirDatabase,
+    _def: GenericDefId,
+) -> (GenericPredicates<'db>, Diagnostics) {
+    (
+        GenericPredicates {
+            predicates: EarlyBinder::bind(Box::new([])),
+            own_predicates_start: 0,
+            is_trait: false,
+            parent_is_trait: false,
+        },
+        None,
+    )
+}
+
 impl<'db> GenericPredicates<'db> {
     #[inline]
     pub fn query(db: &'db dyn HirDatabase, def: GenericDefId) -> &'db GenericPredicates<'db> {

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

@@ -2562,3 +2562,35 @@ fn main() {
         "#,
     );
 }
+
+#[test]
+fn issue_21006_cyclic_type_alias_in_trait_bounds() {
+    // Test for cyclic dependency in generic predicates when a trait's where clause
+    // references a type alias that depends on the trait itself.
+    // Pattern from opaque-ke: trait CipherSuite where OprfHash<Self>: Trait
+    //                        + type OprfHash<CS: CipherSuite> = <CS::Assoc as Trait>::Type
+    // This should not panic rust-analyzer due to cyclic predicates.
+    check_no_mismatches(
+        r#"
+trait Hash {}
+trait Trait {
+    type Assoc: Trait;
+}
+
+// Type alias that projects an associated type from a trait bound
+type MyHash<T: Trait> = <T::Assoc as Trait>::Assoc;
+
+// Trait with where clause referencing the type alias with Self
+trait CipherSuite
+where
+    MyHash<Self>: Hash,
+{
+    type Assoc: Trait;
+}
+
+fn test<T: CipherSuite>() {
+    // If we reach here without panic, the cycle handler is working
+}
+        "#,
+    );
+}