Issue with capturing variables in an async block typed with with impl trait type alias

[icedrocket]

Hello,

In the code below, the pick3 function doesn't compile. I'm wondering if I'm missing something or if this is a limitation of the current compiler.

#![feature(type_alias_impl_trait)]
#![feature(async_closure)]
#![allow(dead_code, unused_imports)]

use std::fmt::Display;
use std::future::Future;

async fn pick1<T, I: Iterator<Item = T>>(input: &mut I) -> T {
    input.next().unwrap()
}

async fn pick2<T, I: Iterator<Item = T>>(input: &mut I) -> T {
    type __ReturnType<T> = T;

    let body = (async move || -> __ReturnType<T> { input.next().unwrap() })();

    return tokio::runtime::Builder::new_current_thread()
        .enable_all()
        .build()
        .expect("failed building the runtime")
        .block_on(body);
}

async fn pick3<'a, T, I: Iterator<Item = T>>(input: &'a mut I) -> T {
    type __FutureType<'a, T> = impl Future<Output = T> + 'a;

    let body: __FutureType<'a, T> = async move { input.next().unwrap() };

    return tokio::runtime::Builder::new_current_thread()
        .enable_all()
        .build()
        .expect("failed building the runtime")
        .block_on(body);
}

error: type parameter `I` is part of concrete type but not used in parameter list for the `impl Trait` type alias
  --> src\lib.rs:27:37
   |
27 |     let body: __FutureType<'a, T> = async move { input.next().unwrap() };
   |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Rust Playground

[jofas]

Do you need __FutureType? Without it, letting the compiler infer the type of body, your example compiles fine on the playground.

---

Otherwise, this works, too. I renamed the parameters of __FutureType to make it clearer that they are different from the ones you use in your function:

async fn pick3<'a, T, I: Iterator<Item = T>>(input: &'a mut I) -> T {
    type __FutureType<'b, U, J: Iterator<Item = U> + 'b> = impl Future<Output = U> + 'b;

    let body: __FutureType<'a, T, I> = async move { input.next().unwrap() };

    return tokio::runtime::Builder::new_current_thread()
        .enable_all()
        .build()
        .expect("failed building the runtime")
        .block_on(body);
}

Playground.

[icedrocket]

Ah, I didn't realize that. Thank you, jofas.