Improve error message for `impl Self`

[sunjay]

Suppose you have the following reduced code example:

impl Self {   
}

This gives you the error message:

error[E0391]: cycle detected when processing `<impl at src/lib.rs:1:1: 3:2>`
 --> src/lib.rs:1:6
  |
1 | impl Self {
  |      ^^^^
  |
  = note: ...which again requires processing `<impl at src/lib.rs:1:1: 3:2>`, completing the cycle
note: cycle used when collecting item types in top-level module
 --> src/lib.rs:1:1
  |
1 | impl Self {
  | ^^^^^^^^^

This is confusing because there isn't actually any obvious cycle (unless you know things about compilers). E0391 actually describes itself with an example like this:

trait FirstTrait : SecondTrait {

}

trait SecondTrait : FirstTrait {

}

This looks nothing like the original code example.

We should probably detect this case and provide a more useful message that indicates that Self is a special keyword that can be used within the impl block, but not as the type of the impl block itself.

[Centril]

I'm curious how impl Self { ... } happened -- can you describe the scenario?

[petar-dambovaliev]

@Centril I am also curious. I don't know of a language that supports that.
We should improve error messages when there is an actual real world example.

[compiler-errors]

@BoxyUwU fixed this, the error message on nightly looks like this now:

error: `Self` is not valid in the self type of an impl block
 --> src/lib.rs:1:6
  |
1 | impl Self {   
  |      ^^^^
  |
  = note: replace `Self` with a different type

Further improvements on the error message would be nice as a new issue, so newcomers can discover the issue, since it'd probably be a good E-easy task.

[BoxyUwU]

Oh I didn't realise there was an issue open for this..