Unhelpful span in async type inference error · Issue #62382 · rust-lang/rust (original) (raw)

The following snippet (playground):

#![feature(async_await)]

use std::future::Future;

fn get_future() -> impl Future<Output = ()> { panic!() }

async fn foo() { let a; get_future().await; }

fn main() {}

Gives the following error message:

error[E0698]: type inside `async` object must be known in this context
  --> infer_error.rs:11:5
   |
11 |     get_future().await;
   |     ^^^^^^^^^^^^^^^^^^ cannot infer type
   |
note: the type is part of the `async` object because of this `await`
  --> infer_error.rs:11:5
   |
11 |     get_future().await;
   |     ^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

This error is due to the unknown type of a. However, nothing in the span indicates that this is the case, making it extremely unclear what the proper fix is.