Combine generic parameters from impl block and method

[dtolnay]

#![feature(async_await)]

use async_trait::async_trait;

#[async_trait]
trait Trait {
    async fn f<U>(&self);
}

#[async_trait]
impl<T> Trait for Vec<T> {
    async fn f<U>(&self) {}
}

Currently this fails to compile because the nested freestanding async fn we generate is missing the T type parameter:

error[E0401]: can't use generic parameters from outer function
  --> src/main.rs:11:23
   |
7  |     async fn f<U>(&self);
   |              - try adding a local generic parameter in this method instead
...
11 | impl<T> Trait for Vec<T> {
   |      -                ^ use of generic parameter from outer function
   |      |
   |      type parameter from outer function

Expanded code:

impl<T> Trait for Vec<T> {
    fn f<'life0, 'async_trait, U>(
        &'life0 self,
    ) -> std::pin::Pin<
        std::boxed::Box<dyn std::future::Future<Output = ()> + std::marker::Send + 'async_trait>,
    >
    where
        U: 'async_trait,
        'life0: 'async_trait,
        Self: 'async_trait,
    {
        async fn __f<'async_trait, U>(_self: &Vec<T>) {}
        std::pin::Pin::from(std::boxed::Box::new(__f::<U>(self)))
    }
}

We need to merge the syn::Generics from the impl block with the syn::Generics on the method to produce the generics that go on the inner function. In this case it should be:

        async fn __f<'async_trait, T, U>(_self: &Vec<T>) {}

[withoutboats]

why dont you use an async move block?

[dtolnay]

@withoutboats for working code it would be fine and the implementation is simpler, but during development the error messages would be worse using async move. For example:

type Elided<'a> = &'a usize;

#[async_trait]
trait Trait {
    async fn f(elided: Elided) {
        println!("{}", *elided);
    }
}

Here is the error when using async move (#3). It only mentions 'static which is probably not the right fix in context.

error[E0621]: explicit lifetime required in the type of `elided`
 --> tests/dev.rs:7:1
  |
7 | #[async_trait]
  | ^^^^^^^^^^^^^^ lifetime `'static` required
8 | trait Trait {
9 |     async fn f(elided: Elided) {
  |                        ------ help: add explicit lifetime `'static` to the type of `elided`: `&'static usize`

Here is the error in the current implementation. Together with the elided lifetimes note in the readme, this should quickly get the author on the right track.

error[E0726]: implicit elided lifetime not allowed here
 --> tests/dev.rs:9:24
  |
9 |     async fn f(elided: Elided) {
  |                        ^^^^^^- help: indicate the anonymous lifetime: `<'_>`

error[E0621]: explicit lifetime required in the type of `elided`
 --> tests/dev.rs:9:16
  |
9 |     async fn f(elided: Elided) {
  |                ^^^^^^  ------ help: add explicit lifetime `'static` to the type of `elided`: `&'static usize`
  |                |
  |                lifetime `'static` required