E0283 error without location information

[jorendorff]

Given the following code: (playground)

pub trait Gen<'source> {
    type Output;

    fn gen<T>(&self) -> T
    where
        Self: for<'s> Gen<'s, Output = T>;
}

The current output is:

error[[E0283]](https://doc.rust-lang.org/nightly/error-index.html#E0283): type annotations needed: cannot satisfy `Self: Gen<'source>`
  |
  = note: cannot satisfy `Self: Gen<'source>`

For more information about this error, try `rustc --explain E0283`.
error: could not compile `playground` due to previous error

The error doesn't contain any location information (filename, line number, spans). The location of fn gen or the bad Self: bound should be included.

[jorendorff]

Here's an example where rustc does have nice spans:

pub trait Gen<'source> {
    type Output;

    fn gen(&self) -> Self::Output
    where
        Self: for<'s> Gen<'s, Output = Self::Output>;
}

The output is:

error[[E0391]](https://doc.rust-lang.org/nightly/error-index.html#E0391): cycle detected when computing the bounds for type parameter `Self`
 --> src/lib.rs:6:40
  |
6 |         Self: for<'s> Gen<'s, Output = Self::Output>;
  |                                        ^^^^^^^^^^^^
  |
  = note: ...which immediately requires computing the bounds for type parameter `Self` again
note: cycle used when computing function signature of `Gen::gen`
 --> src/lib.rs:4:22
  |
4 |     fn gen(&self) -> Self::Output
  |                      ^^^^^^^^^^^^

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