simplify Interner opaque types API

compiler/rustc_next_trait_solver/src/solve/eval_ctxt/canonical.rs

@@ -425,7 +425,8 @@ where
 
     fn register_new_opaque_types(&mut self, opaque_types: &[(ty::OpaqueTypeKey<I>, I::Ty)]) {
         for &(key, ty) in opaque_types {
-            self.delegate.inject_new_hidden_type_unchecked(key, ty, self.origin_span);
+            let prev = self.delegate.register_hidden_type_in_storage(key, ty, self.origin_span);
+            assert_eq!(prev, None);
         }
     }
 }

compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs

@@ -387,7 +387,8 @@ where
         };
 
         for &(key, ty) in &input.predefined_opaques_in_body.opaque_types {
-            ecx.delegate.inject_new_hidden_type_unchecked(key, ty, ecx.origin_span);
+            let prev = ecx.delegate.register_hidden_type_in_storage(key, ty, ecx.origin_span);
+            assert_eq!(prev, None);
         }
 
         if !ecx.nested_goals.is_empty() {
@@ -1070,16 +1071,12 @@ where
         self.delegate.fetch_eligible_assoc_item(goal_trait_ref, trait_assoc_def_id, impl_def_id)
     }
 
-    pub(super) fn insert_hidden_type(
+    pub(super) fn register_hidden_type_in_storage(
         &mut self,
         opaque_type_key: ty::OpaqueTypeKey<I>,
-        param_env: I::ParamEnv,
         hidden_ty: I::Ty,
-    ) -> Result<(), NoSolution> {
-        let mut goals = Vec::new();
-        self.delegate.insert_hidden_type(opaque_type_key, param_env, hidden_ty, &mut goals)?;
-        self.add_goals(GoalSource::Misc, goals);
-        Ok(())
+    ) -> Option<I::Ty> {
+        self.delegate.register_hidden_type_in_storage(opaque_type_key, hidden_ty, self.origin_span)
     }
 
     pub(super) fn add_item_bounds_for_hidden_type(

compiler/rustc_next_trait_solver/src/solve/normalizes_to/opaque_types.rs

@@ -86,8 +86,8 @@ where
                 }
 
                 // Otherwise, define a new opaque type
-                // FIXME: should we use `inject_hidden_type_unchecked` here?
-                self.insert_hidden_type(opaque_type_key, goal.param_env, expected)?;
+                let prev = self.register_hidden_type_in_storage(opaque_type_key, expected);
+                assert_eq!(prev, None);
                 self.add_item_bounds_for_hidden_type(
                     def_id.into(),
                     opaque_ty.args,