[relay] Show a helpful error if a resolver returns an interface with no implementors

compiler/crates/relay-compiler/tests/compile_relay_artifacts/fixtures/relay-resolver-edge-to-interface-with-no-implementors.expected

@@ -0,0 +1,38 @@
+==================================== INPUT ====================================
+# expected-to-throw
+query relayResolverEdgeToInterfaceWithNoImplementorsQuery {
+  resolver_field {
+    name
+  }
+}
+
+# %extensions%
+
+"""
+An interface with no implementors
+"""
+interface SomeInterface {
+  name: String
+}
+
+extend type Query {
+  resolver_field: SomeInterface
+    @relay_resolver(import_path: "./path/to/Resolver.js")
+}
+==================================== ERROR ====================================
+✖︎ Client Edges that reference client-defined interface types are not currently supported in Relay.
+
+  relay-resolver-edge-to-interface-with-no-implementors.graphql:3:3
+    2 │ query relayResolverEdgeToInterfaceWithNoImplementorsQuery {
+    3 │   resolver_field {
+      │   ^^^^^^^^^^^^^^
+    4 │     name
+
+
+✖︎ No types implement the client interface SomeInterface. For a client interface to be used as a @RelayResolver @outputType, at least one Object type must implement the interface.
+
+  <generated>:2:35
+    1 │ # expected-to-throw
+    2 │ query relayResolverEdgeToInterfaceWithNoImplementorsQuery {
+      │                                   ^^^^^^^^^^^^^
+    3 │   resolver_field {

compiler/crates/relay-compiler/tests/compile_relay_artifacts/fixtures/relay-resolver-edge-to-interface-with-no-implementors.graphql

@@ -0,0 +1,20 @@
+# expected-to-throw
+query relayResolverEdgeToInterfaceWithNoImplementorsQuery {
+  resolver_field {
+    name
+  }
+}
+
+# %extensions%
+
+"""
+An interface with no implementors
+"""
+interface SomeInterface {
+  name: String
+}
+
+extend type Query {
+  resolver_field: SomeInterface
+    @relay_resolver(import_path: "./path/to/Resolver.js")
+}

compiler/crates/relay-compiler/tests/compile_relay_artifacts_test.rs

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<4c83edb5a97709b48ab136e569a1bc3e>>
+ * @generated SignedSource<<27fd5fb3f07346c7af49129d773317a8>>
  */
 
 mod compile_relay_artifacts;
@@ -1104,6 +1104,13 @@ fn relay_resolver_backing_client_edge() {
     test_fixture(transform_fixture, "relay-resolver-backing-client-edge.graphql", "compile_relay_artifacts/fixtures/relay-resolver-backing-client-edge.expected", input, expected);
 }
 
+#[test]
+fn relay_resolver_edge_to_interface_with_no_implementors() {
+    let input = include_str!("compile_relay_artifacts/fixtures/relay-resolver-edge-to-interface-with-no-implementors.graphql");
+    let expected = include_str!("compile_relay_artifacts/fixtures/relay-resolver-edge-to-interface-with-no-implementors.expected");
+    test_fixture(transform_fixture, "relay-resolver-edge-to-interface-with-no-implementors.graphql", "compile_relay_artifacts/fixtures/relay-resolver-edge-to-interface-with-no-implementors.expected", input, expected);
+}
+
 #[test]
 fn relay_resolver_es_modules() {
     let input = include_str!("compile_relay_artifacts/fixtures/relay-resolver-es-modules.graphql");

compiler/crates/relay-transforms/src/client_edges.rs

@@ -363,7 +363,18 @@ impl<'program, 'sc> ClientEdgesTransform<'program, 'sc> {
             }
 
             match edge_to_type {
-                Type::Interface(_) => {
+                Type::Interface(interface_id) => {
+                    let interface = schema.interface(interface_id);
+                    let implementing_objects =
+                        interface.recursively_implementing_objects(Arc::as_ref(schema));
+                    if implementing_objects.is_empty() {
+                        self.errors.push(Diagnostic::error(
+                            ValidationMessage::RelayResolverClientInterfaceMustBeImplemented {
+                                interface_name: interface.name.item,
+                            },
+                            interface.name.location,
+                        ));
+                    }
                     if !has_output_type(resolver_directive) {
                         self.errors.push(Diagnostic::error(
                             ValidationMessage::ClientEdgeToClientInterface,

compiler/crates/relay-transforms/tests/client_edges/fixtures/client-edge-to-client-interface.invalid.expected

@@ -16,6 +16,10 @@ interface ClientOnlyInterface implements Node {
   id: ID!
 }
 
+type BestFriend implements ClientOnlyInterface {
+  id: ID!
+}
+
 extend type User {
   best_friend: ClientOnlyInterface @relay_resolver(fragment_name: "BestFriendResolverFragment_name", import_path: "BestFriendResolver")
 }

compiler/crates/relay-transforms/tests/client_edges/fixtures/client-edge-to-client-interface.invalid.graphql

@@ -15,6 +15,10 @@ interface ClientOnlyInterface implements Node {
   id: ID!
 }
 
+type BestFriend implements ClientOnlyInterface {
+  id: ID!
+}
+
 extend type User {
   best_friend: ClientOnlyInterface @relay_resolver(fragment_name: "BestFriendResolverFragment_name", import_path: "BestFriendResolver")
 }