Invoke async code in ResponseError trait method error_response

[mellowagain]

In order to handle errors in my route, I created a custom struct which implements the ResponseError trait of actix-web.

struct CustomError {
     // ...
}

On error, I'd like to invoke a async function:

impl ResponseError for CustomError {
    fn error_response(&self) -> HttpResponse {
        render_template("error.html").await // <-- Not possible to call .await in a sync function
    }

    // ...
}

Sadly Rust doesn't have async traits (without the async_trait crate). I've already tried spawning a new Runtime but that obviously doesn't work as this method gets called from within the existing Runtime.

In tokio there is a Handle struct which allows one to retrieve a handle to the current runtime in which I could call .block_on for the async method, but I don't seem to find a way to acquire something similiar for the actix-web rt.

Any idea how to handle this?

[aliemjay]

I recommend rendering a simple response in ResponseError and then using a middleware for fully rendering the error page. With this approach you have the luxury of async as well as having HttpRequest.

In the middleware, you can downcast HttpResponse::error() to get the underlying error. You also have the option to pass arbitrary data from ResponseError::error_response() to the middleware using HttpResponse::extensions().

A related example https://github.com/svenstaro/miniserve/blob/f40e7d4c66775c8f89564ee1b2bbe4b2d7cfc660/src/errors.rs#L112

[mellowagain]

Thank you very much, this does work perfectly.

For anyone else stumbling upon this, here is my implementation of the answer provided by this comment.