Osgiliath

Bridges the Anduin between the towers...

Disclaimer: The author of this repository is fairly new with Rust (especially macros) a lot of the implementation may be plain wrong or violate best practices. The idea of this repository is to demonstrate that it's possible and eventually gather feedback from the community.

This repository contains a prototype macro that generates code allowing to use async traits (via async_trait crate) as tower's tower::Service and vice versa.

The goals of this are two fold:

• easily use of any tower layers (timeout, rate limits, ...) with async traits.
• prototype external services quickly as part of the application via tower::Service which eventually can be replaced with clients.

The idea is to enable the following without having to write any glue code:

use async_trait::async_trait;
use osgiliath::tower_service;
use tower::Service;

#[tower_service] // includes #[async_trait]
trait TheTrait: Send {
    async fn bla(&mut self, value1: String, value2: usize);
    async fn blub(&mut self) -> f32;
}

struct TheStruct;

#[async_trait]
impl TheTrait for TheStruct {
    async fn bla(&mut self, _value1: String, _value2: usize) {}
    async fn blub(&mut self) -> f32 { return 4.20; }
}

fn use_as_trait(_s: &dyn TheTrait) {}

fn use_as_service(_s: &impl Service<TheTraitRequest>) {}

fn main() {
    // instantiate a wrapper around TheStruct that implements tower::Service
    let service = TheTraitService::new(TheStruct {});
    use_as_service(&service);
    // any Service<TheTraitRequest> also implements the original trait
    use_as_trait(&service);
}

Acknowledgments

This example would not have been possible without the excellent documentation of the syn, quote, async_trait, and proc_macro2 crates as well as the macro workshop template repository.