Add a suggestion when using a type alias instead of trait alias

[JohnTitor]

Fixes #43913

r? @estebank

[estebank]

I would change this to be something along the lines of

Alternatively, you could introduce a new trait with your desired restrictions as a super trait:

```
# trait Foo {}
# struct Bar;
# impl Foo for Bar {}
trait Qux: Foo {} // Anything that implements Qux also needs to implement Foo
fn baz<T: Qux>(t: T) {} // also ok!
```

Finally, if you are on nightly and want to use a trait alias instead of a type alias, you should use `#![feature(trait_alias)]`:

[JohnTitor]

Thanks, just applied your suggestion.

[JohnTitor]

@estebank Approving means r=you?

[Dylan-DPC-zz]

this looks fine to me, so assuming the approval to mean a r+, giving it a pass

@bors r=estebank rollup

[bors]

📌 Commit eea27b8 has been approved by estebank

[bors]

⌛ Testing commit eea27b8 with merge 3a5e0119313a0c45261e05e8ce9e826a9542ead9...

[Dylan-DPC-zz]

likely failure of rollup

@bors r-

[JohnTitor]

@Dylan-DPC Hm, why did you think it's related? I haven't figured out how the change here caused the failure.

[Dylan-DPC-zz]

@JohnTitor i'm not sure, a few rollups have failed because of the same error, and this one seems to be one of the few that are common between them. I don't have time right now to investigate, but if you can find the culprit PR would be helpful :)

[estebank]

Approving means r=you?

It was meant as "r+ after addressing the comment". Sorry I didn't see the PR again since.

---

Should we try r+ing this with rollup=never?

[JohnTitor]

Should we try r+ing this with rollup=never?

Let's try it.

@bors r=estebank rollup=never

[bors]

📌 Commit eea27b8 has been approved by estebank

[bors]

⌛ Testing commit eea27b8 with merge 8ad0821...

[bors]

☀️ Test successful - checks-actions
Approved by: estebank
Pushing 8ad0821 to master...