Help for missing trait method suggests unstable code

[Cerber-Ursi]

I tried this code:

use core::iter::Sum;

struct Summer;

impl Sum<i32> for Summer {
}

Playground

I expected to see this happen: an error which either just shows the error itself or the compilable sugestion for change.

Instead, this happened: the error is like following:

error[E0046]: not all trait items implemented, missing: `sum`
 --> src/lib.rs:5:1
  |
5 | impl Sum<i32> for Summer {
  | ^^^^^^^^^^^^^^^^^^^^^^^^ missing `sum` in implementation
  |
  = help: implement the missing item: `fn sum<I>(_: I) -> Self where I: Iterator, std::iter::Iterator::Item = A { todo!() }`

Now, if I add the code from the help block into the trait, I'll get another error:

error: equality constraints are not yet supported in `where` clauses
 --> src/lib.rs:6:44
  |
6 | fn sum<I>(_: I) -> Self where I: Iterator, std::iter::Iterator::Item = A { todo!() }
  |                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not supported
  |
  = note: [see issue #20041 <https://github.com/rust-lang/rust/issues/20041>](https://github.com/rust-lang/rust/issues/20041) for more information

error[E0412]: cannot find type `A` in this scope
 --> src/lib.rs:6:72
  |
6 | fn sum<I>(_: I) -> Self where I: Iterator, std::iter::Iterator::Item = A { todo!() }
  |        - similarly named type parameter `I` defined here               ^ help: a type parameter with a similar name exists: `I`

The first one is the one I'm worried about, since it shows that the suggest was using a nightly-only feature on stable.

Noting also that rust-analyzer seems to do this correctly - it creates an intention to insert fn sum<I: Iterator<Item = i32>>(_: I) { todo!(); }, which compiles immediately.