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.
Code
Current output
Desired output
Rationale and extra context
The problems with the current output are:
()is not a future” twice.(), and one of them points to the return value that is not of type().returnto the block’s tail expression is not only unhelpful, but may mislead readers about how Rust works.Other cases
Using a different trait than
Futureproduces very different, and generally more sensible, results.Rust Version
Also, stable 1.98.1 has a simpler but worse diagnostic output: it fails to mention the early return at all.