unconstrained type parameter makes no suggestions as to how to fix

[gilescope]

Code

trait MyTrait {}

struct S;

impl <U> MyTrait for S {}

Current output

error[E0207]: the type parameter `U` is not constrained by the impl trait, self type, or predicates
 --> src/lib.rs:5:7
  |
5 | impl <U> MyTrait for S {}
  |       ^ unconstrained type parameter

For more information about this error, try `rustc --explain E0207`.
error: could not compile `playground` due to previous error

Desired output

error[E0207]: the type parameter `U` is not constrained by the impl trait, self type, or predicates
 --> src/lib.rs:5:7
  |
5 | impl <U> MyTrait for S {}
  |       ^ unconstrained type parameter

Either remove the type parameter (<U>) or make use of it, for example ` for S<U>`.

For more information about this error, try `rustc --explain E0207`.
error: could not compile `playground` due to previous error

Rationale and extra context

If we have an unconstrained type parameter, we get an error stating that it is unconstrained, but no suggestions on what to do about it and force the user to google.

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=3ca555fca7f8220aaab497f87c26225b

Is there room to reduce the learning curve here with better diagnostics?

Other cases

No response

Anything else?

No response

[gilescope]

(btw the diagnostics is really coming on - it must be many months since I've come across something that really could do with improvement. rustc has improved so much over these past few years.)

[Noratrieb]

I think this diagnostics should explain all places where a type parameter can be constrained. I have definitely found this a little confusing before :)
It should also link to an explanation why this error exists in the first place, I think it can be a little confusing why the code isn't just accepted.

[chenyukang]

@rustbot claim

[chenyukang]

I don't have enough buffer to fix it, for anyone who would want to pick it up,
add a help message in this function:

rust/compiler/rustc_hir_analysis/src/impl_wf_check.rs

Line 173 in a9842c7

fn report_unused_parameter(tcx: TyCtxt<'_>, span: Span, kind: &str, name: Symbol) {

need to take care the scenarios of different number of parameters.

[jkvargas]

@rustbot claim

[Haramaty]

@rustbot claim