Suggest that `Pin<Box<dyn Unpin + Future>>` is redundant

[jyn514]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=232bc66719a632fbcb8f5d400a58f74e

use core::future::Future;
use core::pin::Pin;
fn f() -> Pin<Box<dyn Unpin + Future<Output = ()>>> {
    Box::pin(async {})
}

The current output is:

error[E0277]: `from_generator::GenFuture<[static generator@src/lib.rs:4:20: 4:22]>` cannot be unpinned
  --> src/lib.rs:4:5
   |
4  |     Box::pin(async {})
   |     ^^^^^^^^^^^^^^^^^^ within `impl Future<Output = [async output]>`, the trait `Unpin` is not implemented for `from_generator::GenFuture<[static generator@src/lib.rs:4:20: 4:22]>`
   |
   = note: consider using `Box::pin`
   = note: required because it appears within the type `impl Future<Output = [async output]>`
   = note: required for the cast to the object type `dyn Future<Output = ()> + Unpin`

For more information about this error, try `rustc --explain E0277`.

Ideally the output should look like:

error[E0277]: async blocks cannot be unpinned
  --> src/lib.rs:4:5
   |
4  |     Box::pin(async {})
   |     ^^^^^^^^^^^^^^^^^^ within `impl Future<Output = ()>`, the trait `Unpin` is not implemented for `from_generator::GenFuture<[static generator@src/lib.rs:4:20: 4:22]>`
   |
   = note: async blocks never implement `Unpin`
   = note: required because it appears within the type `impl Future<Output = [async output]>`
   = note: required for the cast to the object type `dyn Future<Output = ()> + Unpin`

warning: `Pin<Box<dyn Pin + Future<Output = ()>>>` is redundant and most likely not what you intended
   = help: remove the `Unpin` requirement: `Pin<Box<dyn Future<Output = ()>>>`

For more information about this error, try `rustc --explain E0277`.

Pin<Box<T>> is always Unpin and therefore there's no need to require T: Unpin.
Also note the "[async output]" message in the first block; that looks like an outright bug.

[compiler-errors]

Also note the "[async output]" message in the first block; that looks like an outright bug.

That's actually just because the pretty-printing code sees an infer variable in the return type of the generator. Might be able to solve this by adding a few resolve_var_if_possible, but that's just the placeholder that the impl Future printing code outputs for unresolved generators.

[jyn514]

that's just the placeholder that the impl Future printing code outputs for unresolved generators.

Sure - but to a user it still looks like a bug, it doesn't give them any further information.