Suggest adding a type parameter for impls

[mibac138]

Add a new suggestion upon encountering an unknown type in a impl that suggests adding a new type parameter. This diagnostic suggests to add a new type parameter even though it may be a const parameter, however after adding the parameter and running rustc again a follow up error steers the user to change the type parameter to a const parameter.

struct X<const C: ()>();
impl X<C> {}

suggests

error[E0412]: cannot find type `C` in this scope
 --> bar.rs:2:8
  |
1 | struct X<const C: ()>();
  | ------------------------ similarly named struct `X` defined here
2 | impl X<C> {}
  |        ^
  |
help: a struct with a similar name exists
  |
2 | impl X<X> {}
  |        ^
help: you might be missing a type parameter
  |
2 | impl<C> X<C> {}
  |     ^^^

After adding a type parameter the code now becomes

struct X<const C: ()>();
impl<C> X<C> {}

and the error now fully steers the user towards the correct code

error[E0747]: type provided when a constant was expected
 --> bar.rs:2:11
  |
2 | impl<C> X<C> {}
  |           ^
  |
help: consider changing this type parameter to be a `const` generic
  |
2 | impl<const C: ()> X<C> {}
  |      ^^^^^^^^^^^

r? @estebank
Somewhat related #84946

[estebank]

We should likely filter cases like these, where we suggest the same type we are evaluating, but single letter structs should be uncommon enough that it isn't a problem in practice.

[estebank]

@bors r+

[bors]

📌 Commit 4c72efc has been approved by estebank

[bors]

⌛ Testing commit 4c72efc with merge 703f2e1...

[bors]

☀️ Test successful - checks-actions
Approved by: estebank
Pushing 703f2e1 to master...