Do not suggest bounds restrictions for synthesized RPITITs

[spastorino]

Before this PR we were getting ...

warning: the feature `async_fn_in_trait` is incomplete and may not be safe to use and/or cause compiler crashes
 --> tests/ui/async-await/in-trait/missing-send-bound.rs:5:12
  |
5 | #![feature(async_fn_in_trait)]
  |            ^^^^^^^^^^^^^^^^^
  |
  = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
  = note: `#[warn(incomplete_features)]` on by default

error: future cannot be sent between threads safely
  --> tests/ui/async-await/in-trait/missing-send-bound.rs:17:20
   |
17 |     assert_is_send(test::<T>());
   |                    ^^^^^^^^^^^ future returned by `test` is not `Send`
   |
   = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `impl Future<Output = ()>`
note: future is not `Send` as it awaits another future which is not `Send`
  --> tests/ui/async-await/in-trait/missing-send-bound.rs:13:5
   |
13 |     T::bar().await;
   |     ^^^^^^^^ await occurs here on type `impl Future<Output = ()>`, which is not `Send`
note: required by a bound in `assert_is_send`
  --> tests/ui/async-await/in-trait/missing-send-bound.rs:21:27
   |
21 | fn assert_is_send(_: impl Send) {}
   |                           ^^^^ required by this bound in `assert_is_send`
help: consider further restricting the associated type
   |
16 | fn test2<T: Foo>() where impl Future<Output = ()>: Send {
   |                    ++++++++++++++++++++++++++++++++++++

error: aborting due to previous error; 1 warning emitted

and we want this output ...

warning: the feature `async_fn_in_trait` is incomplete and may not be safe to use and/or cause compiler crashes
  --> $DIR/missing-send-bound.rs:5:12
   |
LL | #![feature(async_fn_in_trait)]
   |            ^^^^^^^^^^^^^^^^^
   |
   = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
   = note: `#[warn(incomplete_features)]` on by default

error: future cannot be sent between threads safely
  --> $DIR/missing-send-bound.rs:17:20
   |
LL |     assert_is_send(test::<T>());
   |                    ^^^^^^^^^^^ future returned by `test` is not `Send`
   |
   = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `impl Future<Output = ()>`
note: future is not `Send` as it awaits another future which is not `Send`
  --> $DIR/missing-send-bound.rs:13:5
   |
LL |     T::bar().await;
   |     ^^^^^^^^ await occurs here on type `impl Future<Output = ()>`, which is not `Send`
note: required by a bound in `assert_is_send`
  --> $DIR/missing-send-bound.rs:21:27
   |
LL | fn assert_is_send(_: impl Send) {}
   |                           ^^^^ required by this bound in `assert_is_send`

error: aborting due to previous error; 1 warning emitted

r? @compiler-errors

[compiler-errors]

r=me fixed

[spastorino]

@bors r=compiler-errors

[bors]

📌 Commit 3b04ad2 has been approved by compiler-errors

It is now in the queue for this repository.