improve the "must use" lint for `Future` to suggest await

[nikomatsakis]

The following code gives a "must use" warning (which is awesome!), but the warning is not as clear as it could be:

#![feature(async_await)]
#![allow(dead_code)]

async fn foo() {
    bar();
}

async fn bar() {
}

fn main() { }

This results in:

warning: unused implementer of `std::future::Future` that must be used
 --> src/main.rs:5:5
  |
5 |     bar();
  |     ^^^^^^
  |
  = note: #[warn(unused_must_use)] on by default
  = note: futures do nothing unless polled

it should probably say "futures do nothing unless you await them" or something. Most people won't know what 'polled' is all about, I think.

[Centril]

The current note originates from:

rust/src/libcore/future/future.rs

Line 26 in a9ec99f

#[must_use = "futures do nothing unless polled"]

To change it, simply edit that.

[Schultzer]

What about #[must_use = "futures stays idle unless you await them"] ?

[aesedepece]

What about #[must_use = "futures stays idle unless you await them"] ?

In any case, #[must_use = "futures stay idle unless you await them"]. Just a typo 😉

I would actually expect something in the line of:

#[must_use = "a future does nothing unless you await or poll it. Did you miss using the `await!` macro on it?"]

Btw, I'd love to work on this issue and provide a PR. I'm really looking forward to help Rust just as much as it has helped me thus far!

[Centril]

We may want to work in the fact that you can use my_future.await somehow.
Other than that, #[must_use = "futures stay idle unless you await them"] sounds pretty good.

[aesedepece]

@Centril what trait does provide my_future.await? 😲

[Centril]

@aesedepece my_future.await is equivalent to await!(my_future) and is likely to replace it eventually. See https://boats.gitlab.io/blog/post/await-decision/.

I think for now #[must_use = "futures stay idle unless you await them"] might be good enough and we could tweak later.

[aesedepece]

That makes sense to me! Am I ok to work on this?

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