"cycle detected" with impl Self {}

[matthiaskrgr]

Given the following code:

impl Self {}

The current output is:

error[E0391]: cycle detected when computing type of `<impl at src/main.rs:1:1: 1:13>`
 --> src/main.rs:1:6
  |
1 | impl Self {}
  |      ^^^^
  |
  = note: ...which immediately requires computing type of `<impl at src/main.rs:1:1: 1:13>` again
note: cycle used when collecting item types in top-level module
 --> src/main.rs:1:1
  |
1 | impl Self {}
  | ^^^^^^^^^

For more information about this error, try `rustc --explain E0391`.

Ideally the output should look like:

Something more like

error[E0412]: cannot find type `Self` in this scope
 --> src/main.rs:1:6
  |
1 | impl Self {}
  |      ^^^^ not found in this scope

Or even a better message explaining that there should be an actual type there instead of just "Self" ..?

[ibraheemdev]

A potentially confusing case:

struct Foo;

impl Foo {
    fn foo() {
        impl Self { fn bar() {} }
    }
}

error[E0391]: cycle detected when computing type of `main::<impl at src/main.rs:4:1: 8:2>::foo::<impl at src/main.rs:6:9: 6:34>`
 --> src/main.rs:6:14
  |
6 |         impl Self { fn bar() {} }
  |              ^^^^
  |
  = note: ...which immediately requires computing type of `main::<impl at src/main.rs:4:1: 8:2>::foo::<impl at src/main.rs:6:9: 6:34>` again
note: cycle used when collecting item types in top-level module
 --> src/main.rs:1:1
  |
1 | fn main() {
  | ^^^^^^^^^

[compiler-errors]

@rustbot claim

[edward-shen]

No longer repros in 1.68.0-nightly (2023-01-09 3020239) as it now returns a a better message (playground):

error: `Self` is not valid in the self type of an impl block
 --> src/lib.rs:5:14
  |
5 |         impl Self { fn bar() {} }
  |              ^^^^
  |
  = note: replace `Self` with a different type

error: could not compile `playground` due to previous error

Also found this issue via a similar usecase, which also no longer repros:

impl TryFrom<String> for Self {
    type Error = String;
    fn try_from(s: String) -> Result<Self, Self::Error> {
        Self::new(s)
    }
}

[compiler-errors]

Yeah @BoxyUwU fixed this