support for runtime RPC APIs

[niklasad1]

Currently subxt is great of supporting storage and transactions but as substrate is increasingly moving towards these runtime RPC APIs which is a bit more work to use in context of subxt.

The runtime RPC APIs works as follows:

• make a RPC call to state_call(method, call_data)
• method: is the name of the runtime API such as TransactionPaymentCallApi_query_call_info
• call_data: is the custom types that the certain runtime API takes as input (encoded/decoded as a tuple in the same order as they are defined)

One is example is when I tried to get TransactionPaymentCallApi::query_call_info(call: Call, len: u32) working:

    let tx = runtime::tx().election_provider_multi_phase().submit_unsigned(Default::default(), SolutionOrSnapshotSize { voters, targets });
    
    // call_data is (Call, len)
    let call_data = {
        let mut buffer = Vec::new();

        // this is already encoded so just push it to buffer directly
        let encoded_call = client.tx().call_data(&tx).unwrap();
        // this needs to be encoded....
        let encoded_len = encoded_call.len() as u32;

        buffer.extend(encoded_call);
        encoded_len.encode_to(&mut buffer);

        Bytes(buffer)
    };

    let bytes: Bytes = client
        .rpc()
        .client
        .request(
            "state_call",
            rpc_params!["TransactionPaymentCallApi_query_call_info", call_data],
        )
        .await
        .unwrap();

It's easy to get wrong if you are not really familiar how scale encoding works

Thus, it would be great if subxt could generate these runtime APIs similar to how transactions and storage are supported by subxt but I don't know whether it possible to fetch these via the metadata or something else like inspecting the WASM blob.....

[niklasad1]

Ok, this seems depend on paritytech/substrate#11648

[jsdw]

I'd love this too. I could see an api.call().submit(SomeCall { foo: 2, bar: true }) sort of API that was a similar to the api.tx() one and let you submit dynamic or static call data and decoded the results.

[ascjones]

See use-ink/cargo-contract#701 for an example of manual code it would replace

[jsdw]

This is closed by #918 :)

The node will need to support the (currently unstable) V15 metadata at the right version for it to work, but the logic exists in Subxt master to codegen Runtime API calls now at least (albeit behind an "unstable-metadata" flag.

When V15 metadata stabillises in Substrate, so will this ability in Subxt.