MCP: Embed the hir_id of the _awaited expression_ into DesugaringKind

[henryboisdequin]

Proposal

Rust issue: rust-lang/rust#66731

This MCP proposes to embed the hir_id of a certain awaited expression into DesugaringKind. Take this code:

fn bar () -> impl futures::future::Future<Item=(), Error=()> {}

fn boo (){}

async fn foo() -> std::io::Result<()> {
    boo().await;
    bar().await;
    std::io::Result::Ok(())
}

This is the current output:

error[E0220]: associated type `Item` not found for `futures::Future`
 --> src/lib.rs:1:43
  |
1 | fn bar () -> impl futures::future::Future<Item=(), Error=()> {}
  |                                           ^^^^ associated type `Item` not found

error[E0220]: associated type `Error` not found for `futures::Future`
 --> src/lib.rs:1:52
  |
1 | fn bar () -> impl futures::future::Future<Item=(), Error=()> {}
  |                                                    ^^^^^ associated type `Error` not found

error[E0277]: `()` is not a future
 --> src/lib.rs:1:14
  |
1 | fn bar () -> impl futures::future::Future<Item=(), Error=()> {}
  |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `()` is not a future
  |
  = help: the trait `futures::Future` is not implemented for `()`

error[E0277]: `()` is not a future
 --> src/lib.rs:6:5
  |
6 |     boo().await;
  |     ^^^^^^^^^^^ `()` is not a future
  |
  = help: the trait `futures::Future` is not implemented for `()`
  = note: required by `futures::Future::poll`

error: aborting due to 4 previous errors

To enhance this error message, it would be helpful to mention that boo should be an async function in order to .await it. This would look something like this inside the diagnostic:

help: to `await` a function, use the `async` keyword:
3  |   async fn boo (){}

By passing the hir_id of the awaited expression into DesugaringKind like so: DesugaringKind::Await(expr.hir_id), we would be able to access the span of the awaited expression and produce the error message we are looking for. Also by having the hir_id, within the trait code we could inspect the desugaring kind even more and, in the future, make this diagnostic even more helpful.

(suggested by @nikomatsakis at this comment)

Mentors or Reviewers

• @nikomatsakis
• @estebank

Process

The main points of the Major Change Process is as follows:

• File an issue describing the proposal.
• A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

cc @rust-lang/compiler @rust-lang/compiler-contributors

[oli-obk]

@rustbot second

[apiraino]

@rustbot label -final-comment-period +major-change-accepted

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