Poor errors from type-mismatched early return when the return type is `impl Future`

[kpreid]

Code

pub fn sort_of_async_function() -> impl Future<Output = ()> {
    if false {
        return;
    }
    
    async {}
}

Current output

error[E0277]: `()` is not a future
 --> src/lib.rs:1:36
  |
1 | pub fn sort_of_async_function() -> impl Future<Output = ()> {
  |                                    ^^^^^^^^^^^^^^^^^^^^^^^^ `()` is not a future
  |
  = help: the trait `Future` is not implemented for `()`

error[E0308]: mismatched types
 --> src/lib.rs:6:5
  |
6 |     async {}
  |     ^^^^^^^^ expected `()`, found `async` block
  |
  = note:  expected unit type `()`
          found `async` block `{async block@src/lib.rs:6:5: 6:10}`
note: return type inferred to be `()` here
 --> src/lib.rs:3:9
  |
3 |         return;
  |         ^^^^^^
help: you might have meant to return this value
  |
6 |     return async {};
  |     ++++++         +
help: you might have meant to return this value
  |
6 |     return async {};
  |     ++++++         +

error[E0277]: `()` is not a future
 --> src/lib.rs:6:5
  |
6 |     async {}
  |     ^^^^^^^^ `()` is not a future
  |
  = help: the trait `Future` is not implemented for `()`

Desired output

error[E0277]: `()` is not a future
 --> src/lib.rs:1:36
  |
3 |         return;
  |         ^^^^^^ `()` is not a future
  |
  = help: the trait `Future` is not implemented for `()`

error[E0308]: mismatched types
 --> src/lib.rs:6:5
  |
6 |     async {}
  |     ^^^^^^^^ expected `()`, found `async` block
  |
  = note:  expected unit type `()`
          found `async` block `{async block@src/lib.rs:6:5: 6:10}`
note: return type inferred to be `()` here
 --> src/lib.rs:3:9
  |
3 |         return;
  |         ^^^^^^

Rationale and extra context

The problems with the current output are:

• It prints “() is not a future” twice.
• Neither of those diagnostics point to the source of the (), and one of them points to the return value that is not of type ().
• The suggestion of adding a return to the block’s tail expression is not only unhelpful, but may mislead readers about how Rust works.

Other cases

Using a different trait than Future produces very different, and generally more sensible, results.

Rust Version

1.100.0-nightly (2026-09-02 2e2b193f8ada105f2760)

Also, stable 1.98.1 has a simpler but worse diagnostic output: it fails to mention the early return at all.

[Hamdan-Khan]

@rustbot claim