Wrong suggestion for Boxxing body of async closure when expected return type of closure to be Box

[fee1-dead]

Code

playground

#![feature(async_closure)]

fn uwu<X>(x: impl FnOnce() -> Box<X>) {}

fn main() {
    uwu(async move || {})
}

Current output

Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
 --> src/main.rs:6:23
  |
6 |     uwu(async move || {})
  |                       ^^ expected `Box<_>`, found `async` closure body
  |
  = note:            expected struct `Box<_>`
          found `async` closure body `[async closure body@src/main.rs:6:23: 6:25]`
  = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
help: store this in the heap by calling `Box::new`
  |
6 |     uwu(async move || Box::new({}))
  |                       +++++++++  +

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` (bin "playground") due to previous error

Desired output

Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
 --> src/main.rs:6:23
  |
6 |     uwu(async move || {})
  |                       ^^ expected `Box<_>`, found `async` closure body
  |
  = note:            expected struct `Box<_>`
          found `async` closure body `[async closure body@src/main.rs:6:23: 6:25]`
  = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` (bin "playground") due to previous error

Rationale and extra context

No response

Other cases

No response

Anything else?

No response

[JohnBobbo96]

Would this be better? This will fix the problem by returning a box with a unit type.

Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
 --> src/main.rs:6:23
  |
6 |     uwu(async move || {})
  |                       ^^ expected `Box<_>`, found `async` closure body
  |
  = note:            expected struct `Box<_>`
          found `async` closure body `[async closure body@src/main.rs:6:23: 6:25]`
  = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
help: store this in the heap by calling `Box::new`
  |
6 |     uwu(async move || Box::new(()))
  |                       ++++++++++++

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` (bin "playground") due to previous error

This also doesn't give any suggestion for regular blocks/closures: playground link

Compiling playground v0.0.1 (/playground)
error[[E0308]](https://doc.rust-lang.org/nightly/error_codes/E0308.html): mismatched types
 --> src/main.rs:5:9
  |
5 |     foo({});
  |         ^^ expected `Box<_>`, found `()`
  |
  = note: expected struct `Box<_>`
          found unit type `()`

error[[E0308]](https://doc.rust-lang.org/nightly/error_codes/E0308.html): mismatched types
 --> src/main.rs:6:12
  |
6 |     bar(|| {});
  |            ^^ expected `Box<_>`, found `()`
  |
  = note: expected struct `Box<_>`
          found unit type `()`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` (bin "playground") due to 2 previous errors

[JohnBobbo96]

im going to try and fix this

@rustbot claim

[JohnBobbo96]

Would this be better? This will fix the problem by returning a box with a unit type.

Actually this still doesn't work for async closures, weird.

[fee1-dead]

Async closures return futures, not boxes. The suggestion is simply wrong and there is no way to actually call uwu with an async closure.