associated type `Context` not found when implementing actix-web's Handler

Question

I have the following code based on the actix-web Database Integration sample.

extern crate actix;
extern crate actix_web;
extern crate serde_json;
#[macro_use]
extern crate serde_derive;
extern crate r2d2;
extern crate r2d2_mysql;

use actix::prelude::*;
use actix_web::{middleware::Logger, server, App, FutureResponse, HttpRequest, HttpResponse};

mod dbservices {

    use actix::prelude::*;
    use actix_web::dev::Handler;
    use model::DataDictionary;
    use r2d2::Pool;
    use r2d2_mysql::*;
    use std::io;

    pub struct MysqlConnection {
        db_pool: mysql::PooledConn,
    }

    impl Actor for MysqlConnection {
        type Context = SyncContext<Self>;
    }

    impl MysqlConnection {
        pub fn new(db_url: &str) -> MysqlConnection {
            unimplemented!();
        }
    }

    pub struct GetDD;
    impl Message for GetDD {
        type Result = io::Result<Vec<DataDictionary>>;
    }

    impl Handler<GetDD> for MysqlConnection {
        type Result = io::Result<Vec<DataDictionary>>;

        fn handle(&mut self, msg: GetDD, _: &mut Self::Context) -> Self::Result {
            unimplemented!();
        }
    }
}

mod model {
    #[derive(Debug, Clone, Serialize, Deserialize)]
    pub struct DataDictionary {
        id: i32,
        name: String,
    }
}

struct State {
    db: Addr<MysqlConnection>,
}

fn get_dd(req: HttpRequest<State>) -> FutureResponse<HttpResponse> {
    req.clone()
        .state()
        .db
        .send(GetDD)
        .from_err()
        .and_then(|result| Ok.json(result))
        .responder()
}

fn main() {
    std::env::set_var("RUST_LOG", "actix_web=debug,info");
    const db_url: str = "mysql://connstring";
    let addr = SyncArbiter::start(3, move || dbservices::MysqlConnection::new(db_url));

    server::new(|| {
        App::new()
            .middleware(Logger::default())
            .middleware(Logger::new("%a %{User-Agent}i"))
            .prefix("/api")
            .scope("/dd", |dp_scope| {
                dp_scope.resource("/", |r| r.h(dbservices::GetDD()))
            })
    }).bind("127.0.0.1:8088")
        .unwrap()
        .run();
}

I'm getting the following error when compiling. I'm not sure what I'm doing wrong:

error[E0220]: associated type `Context` not found for `Self`
  --> src/main.rs:43:50
   |
43 |         fn handle(&mut self, msg: GetDD, _: &mut Self::Context) -> Self::Result {
   |                                                  ^^^^^^^^^^^^^ associated type `Context` not found

Here are my dependencies from Cargo.toml:

[dependencies]
actix-web = "0.6.14"
actix = "0.6.1"
chrono = { version = "0.4.2", features = ["serde"] }
serde = "1.0.60"
serde_derive = "1.0.60"
serde_json = "1.0.17"
log = "0.4.1"
env_logger ="0.5.10"
futures = "0.2.1"
r2d2 = "*"

[dependencies.r2d2_mysql]
git = "https://github.com/outersky/r2d2-mysql"
version="*"

Answer 1

Creating a minimal MCVE almost always makes the problem easier to see:

extern crate actix;
extern crate actix_web;

use actix::prelude::*;
use actix_web::dev::Handler;
use std::io;

pub struct MysqlConnection;

impl Actor for MysqlConnection {
    type Context = SyncContext<Self>;
}

pub struct DummyMessage;
impl Message for DummyMessage {
    type Result = io::Result<String>;
}

impl Handler<DummyMessage> for MysqlConnection {
    type Result = io::Result<String>;

    fn handle(&mut self, _: DummyMessage, _: &mut Self::Context) -> Self::Result {
        unimplemented!();
    }
}

fn main() {}

error[E0220]: associated type `Context` not found for `Self`
  --> src/main.rs:22:53
   |
22 |     fn handle(&mut self, _: DummyMessage, _: &mut Self::Context) -> Self::Result {
   |                                                   ^^^^^^^^^^^^^ associated type `Context` not found

Problem 1 — multiple versions

actix-web = "0.6.14"
actix = "0.6.1"

This brings in two different versions of actix:

$ cargo tree -d

actix v0.5.8
└── actix-web v0.6.14
    └── repro v0.1.0 (file:///private/tmp/repro)

actix v0.6.1
└── repro v0.1.0 (file:///private/tmp/repro)

Pick a single one, actix 0.5.8.

See also:

• Why is a trait not implemented for a type that clearly has it implemented?

Problem 2 — wrong trait

You bring in actix_web::dev::Handler:

pub trait Handler<S>: 'static {
    type Result: Responder;
    fn handle(&mut self, req: HttpRequest<S>) -> Self::Result;
}

You are implementing actix::Handler:

pub trait Handler<M> 
where
    Self: Actor,
    M: Message, 
{
    type Result: MessageResponse<Self, M>;
    fn handle(&mut self, msg: M, ctx: &mut Self::Context) -> Self::Result;
}

actix::Handler has Actor as its supertrait, which means that it can access the associated type Context. actix_web::dev::Handler does not have the supertrait, so it doesn't know about Self::Context.

Pick the appropriate trait and implement it correctly.

Answer 2

You can replace Self::Context with SyncContext<Self>

Answer 3

This is actually strange since there is only single associated type Context but for some reason Rust wants you to specify specifically associated type: <Self as Actor>::Context which should not be required since you have single Context type

Are there any other errors that prevent your struct from compiling?