I'm trying to test a Custom RPC. Let's make it a simple pass through.

p_api::decl_runtime_apis! {
    pub trait SimpleApi<AccountId> where
        AccountId: Codec,
        fn is_true() -> Result<bool, DispatchError>;

Client Server Trait

#[rpc(client, server)]
pub trait SimpleApi<BlockHash, AccountId> {
    #[method(name = "simple_isTrue")]
    fn is_true(
    ) -> RpcResult<bool>>;

pub struct SimpleHandler<C, M> {
    client: Arc<C>,
    _marker: std::marker::PhantomData<M>,

impl<C, M> SimpleHandler<C, M> {
    pub fn new(client: Arc<C>) -> Self {
        Self { client, _marker: Default::default() }

impl<C, Block, AccountId> SimpleServer<<Block as BlockT>::Hash, AccountId> for SimpleHandler<C, Block>
    Block: BlockT,
    C: Send + Sync + 'static,
    C: ProvideRuntimeApi<Block>,
    C: HeaderBackend<Block>,
    C::Api: SimpleRuntimeApi<Block, AccountId>,
    AccountId: Codec,
  fn is_true(&self) -> RpcResult<bool> {
    let api = self.client.runtime_api();
    let at = BlockId::hash(self.client.info().best_hash);
    let runtime_api_result = api.is_true(&at);

So following the test setup from https://github.com/paritytech/substrate/blob/master/client/rpc/src/author/tests.rs

use substrate_test_runtime_client::{
    runtime::Block, Client, Backend, DefaultTestClientBuilderExt,

struct TestSetup {
    pub client: Arc<Client<Backend>>

impl Default for TestSetup {
    fn default() -> Self {
        let client_builder = substrate_test_runtime_client::TestClientBuilder::new();
        let client = Arc::new(client_builder.build());

        TestSetup { client }

impl TestSetup {
    fn simple(&self) -> SimpleHandler<Client<Backend>,Block> {
        SimpleHandler {
            client: self.client.clone(),
            _marker: Default::default(),

    fn into_rpc() -> RpcModule<SimpleHandler<Client<Backend>, Block>> {

async fn testing_simple() {
    let api = TestSetup::default().simple();

    let rpc = api.into_rpc();

    let response = rpc.call("").await.unwrap();

This fails with:

let rpc = api.into_rpc();
         ^^^^^^^^ method cannot be called on `SimpleHandler<...>` due to unsatisfied trait bounds
// ...
C::Api: SimpleRuntimeApi<Block, AccountId>,
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unsatisfied trait bound introduced here

Which makes since, as nothing in the test setup has an implementation for SimpleRuntimeApi. So how do I mock one and tell the client?

My assumption is that I am missing part of the setup for substrate_test_runtime_client (Or perhaps I should actually be using substrate_test_client ?) but the examples around it are few. Always possible I'm barking up the wrong tree entirely.

1 Answer 1


tldr: used the sp_api::mock_impl_runtime_apis! macro to mock the implementation.

Example is in this PR along with a lot of other changes:


See pallets/messages/src/rpc/src/lib.rs for the rpc, pallets/messages/src/rpc/src/tests/mod.rs for the test, and pallets/messages/src/rpc/src/tests/rpc_mock.rs for the general mock setup.

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