Macros with lifetimes

[ivanovaleksey]

Hello,

I would like to create JSON RPC method with a parameter's type of a struct with a borrowed data.
Something like

#[derive(Deserialize)]
pub struct NewRoom<'a> {
    #[serde(borrow)]
    pub label: Option<Cow<'a, str>>,
    // Or it could be just &str
    // pub label: Option<&'a str>
}

build_rpc_trait! {
    pub trait Rpc {
        type Metadata;

        #[rpc(meta, name = "room.create")]
        fn create<'a>(&self, NewRoom<'a>) -> Result<CreateResponse<'a>>;
    }
}

The problem is that current macro doesn't support functions with lifetimes.
I am not sure if it's possible to create macro with lifetimes at all but I found PR which seems to implemented it rust-lang/rust#46895
https://doc.rust-lang.org/unstable-book/language-features/macro-lifetime-matcher.html
Thought I don't know if it's already available to use and on which Rust version.

My idea is to use borrowed string instead of owned one to avoid extra-allocations.
As far as I understand Serde kindly allows us to do exactly that.

A possible alternative is not to use macro and define RPC server manually. Although we know that it is much less convenient (one should parse parameters and handle errors become very non ergonomic).

What do you think?

[ivanovaleksey]

Oh, looks like there would be a problem.

I defined a method

fn create<'a>(params: Params, meta: Meta) -> Result<Value> {
    let conn = establish_connection!(meta.db_pool.unwrap());

    let req = params.parse::<CreateRequest>().unwrap();

    let room: models::Room = diesel::insert_into(rooms::table)
        .values(&req.data)
        .get_result(conn).unwrap();

    let resp = CreateResponse::new(&room);

    Ok(serde_json::to_value(resp).unwrap())
}

But I got an error

error[E0279]: the requirement `for<'de> 'de : ` is not satisfied (`expected bound lifetime parameter 'de, found concrete lifetime`)
  --> src/rpc/mod.rs:62:22
   |
62 |     let req = params.parse::<CreateRequest>().unwrap();
   |                      ^^^^^
   |
   = note: required because of the requirements on the impl of `for<'de> serde::Deserialize<'de>` for `messages::room::CreateRequest<'_>`
   = note: required because of the requirements on the impl of `serde::de::DeserializeOwned` for `messages::room::CreateRequest<'_>`

I guess it is because of how Params::parse is defined.

So, it is impossible to parse Params into a type that borrows?

[tomusdrw]

Yeah, unfortunately the whole library assumes that we own the objects being serialzed/deserialized. It would require changes on all layers to support that.

[ivanovaleksey]

Thanks for the reply @tomusdrw.
I guess I can close the issue.
Just for my understanding - is it really worth making those changes?

[tomusdrw]

I think it makes sense on the core/servers level to support borrowed data, not so much for macros unless we implement #66

[tomusdrw]

Closing since it probably would be pretty substantial.