Phase out async_trait macro

[HoLLy]

What's missing?

Rocket currently re-exports the async_trait crate for use with its Fairing (and possibly others). As of rust 1.75.0 this is no longer needed, therefore it would be nice to get rid of this dependency and use actual async functions for these traits.

This generally improves compile times, dependency count and IDE responsiveness.

Ideal Solution

For v0.6, remove async_trait (or put its usage behind a feature) and use async functions directly. This would raise the MSRV to 1.75.0.

Why can't this be implemented outside of Rocket?

This is an integral part of Rocket.

Are there workarounds usable today?

No. You bypass the need for the macro attribute by returning a Box<dyn Future<Output = T> + Send + 'a>, but this is unwieldy.

Alternative Solutions

No response

Additional Context

No response

System Checks

• I do not believe that this feature can or should be implemented outside of Rocket.
• I was unable to find a previous request for this feature.

[Sergio Benitez]

This is indeed the plan! The new interfaces for #1070 make use of async fn in traits. There's a new internal async_bound macro that lets us add Send bounds to the futures returned by the async functions as well. The idea is to transform all existing async traits into regular traits with async fns with the help of this macro. Once that lends, I or someone else can pick this task up, and it should be completable with relative ease.

[Sergio Benitez]

The async_bound macro is now in master and used by the new Listener trait's connect method:

Rocket/core/lib/src/listener/listener.rs

Lines 8 to 19 in fd29404

 pub trait Listener: Send + Sync { 
     type Accept: Send; 
  
     type Connection: Connection; 
  
     async fn accept(&self) -> io::Result<Self::Accept>; 
  
     #[crate::async_bound(Send)] 
     async fn connect(&self, accept: Self::Accept) -> io::Result<Self::Connection>; 
  
     fn socket_addr(&self) -> io::Result<Endpoint>; 
 } 

I would love help in moving Rocket away from async_trait to raw async fn in traits, with the help of the async_bound macro. This would mean transitioning at least the following traits and all of their implementations in the core, contrib, and examples:

pub trait FromData<'r>: Sized;
pub trait IoHandler: Send;
pub trait Handler: Cloneable + Send + Sync + 'static;
pub trait FromFormField<'v>: Send + Sized;
pub trait Fairing: Send + Sync + Any + 'static;
pub trait FromRequest<'r>: Sized;
pub trait FromForm<'r>: Send + Sized;

The process would be to:

1. Remove the #[async_trait] attribute from the declaration site.
2. Remove the #[async_trait] attribute from implementation sites.
3. Add #[async_bound(Send)] or #[async_bound(Send + Sync)] to trait methods as needed so that 1) the project compiles, and 2) we have the bounds we need. We want to restrict the returned futures a little as possible. As such, it would not be good to simply place all bounds on all methods unnecessarily. The compiler should help in figuring out which methods need which bounds.

Repeat this process once per trait. A commit per removal would be fantastic, finalizing the PR with a commit that removes the async_trait dependency entirely.

That's it! Any help would be appreciated.

Edit: we cannot make this change to traits that are object safe since doing so would make the trait not object safe. The following traits must be object safe, and thus (for the time being) must continue to use async_trait:

• Fairing
• IoHandler
• Handler

Edit 2: it is possible that we can't migrate any of these traits due to rust-lang/rust#100013. I've run into an issue attempting to migrate FromRequest, for example, due to the lifetimes. I imagine we'll see the issue recur on the remaining viable traits due to the same lifetime choices.