BoxFuture / LocalBoxFuture should be marked as must_use

[shepmaster]

use futures::future::{FutureExt, LocalBoxFuture}; // 0.3.1

async fn plain() {}

fn boxed() -> LocalBoxFuture<'static, ()> {
    async {}.boxed_local()
}

pub async fn example() {
    plain();

    boxed();
}

warning: unused implementer of `core::future::future::Future` that must be used
  --> src/lib.rs:10:5
   |
10 |     plain();
   |     ^^^^^^^^
   |
   = note: `#[warn(unused_must_use)]` on by default
   = note: futures do nothing unless you `.await` or poll them

Sadly, there's no error for the unused result of boxed, which has already caused me a few minutes of head-scratching and debugging.

[taiki-e]

related: #1691

[shepmaster]

Oh, those are type aliases... hmm.

[taiki-e]

Closing in favor of upstream issues: rust-lang/rust#67387, rust-lang/rust#73417