Suggest `async {}` for `async || {}`

[jyn514]

This code (playground):

fn main() {
    let my_future = async || {
        1
    };
}

prints out

error[E0658]: async closures are unstable
 --> src/main.rs:2:21
  |
2 |     let my_future = async || {
  |                     ^^^^^
  |
  = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information

error: aborting due to previous error

This is pretty unhelpful for beginners - normally this happens when they try to use await in a closure, which tells them they need async:

error[E0728]: `await` is only allowed inside `async` functions and blocks
 --> src/main.rs:3:9
  |
2 |     let my_future = || {
  |                     -- this is not `async`
3 |         1.await
  |         ^^^^^^^ only allowed inside `async` functions and blocks

But when they do they get an error about async being unstable, which they have no idea how to solve. The fix is to remove the pipe symbols:

async { }

However, this requires knowledge that async blocks desugar internally into closures, which is not at all clear from the syntax. It would be better for the compiler to suggest it:

error[E0658]: async closures are unstable
 --> src/main.rs:2:21
  |
2 |     let my_future = async || {
  |                     ^^^^^^^^^^ help: to use an async block, remove the `||`: `async {`
  |
  = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information

[rokob]

Hey, I saw this and it looked like it might be easy so I took a shot with #76580. I haven't fixed the tests yet because I wasn't sure if this approach is reasonable or not. You end up with

error[E0658]: async closures are unstable
 --> src/main.rs:2:21
  |
2 |     let my_future = async || {
  |                     ^^^^^
  |
  = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
  = help: to use an async block, remove the `||`: `async

error: aborting due to previous error

Re-ordering the note and help, or adding a nested span on async || to put the help would be considerably more involved.

[taiki-e]

Given the current async || is equivalent to || async {}, it may be preferable to suggest || async {} instead of async {}.

[jyn514]

@taiki-e the point of this is that the user didn't mean || async {} though. If you add || then you can no longer await it: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=083f8dbc81f44245734ff5c7a95e1c69

For context, the original code was something like https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=2f095b1aa4d8d3290b2ab41bed482469, where the user tried to use await outside of an async function.

[taiki-e]

@jyn514 Thanks for clarifying. What suggestion is preferable seems to depend on the expected return type.
Perhaps it is preferable to have both notes?

   = help: to use an async block, remove the `||`: `async {`
   = help: to use a closure that returns a future, use `|| async {`