Given the following code:

trait StringExt {
    fn ext(&self);
}
impl StringExt for String {
    fn ext(&self) {
        <Self as str>::trim(self);
        // str::trim(self); - this works
    }
}

Playground

The current output is:

error[E0576]: cannot find method or associated constant `trim` in `str`
 --> src/lib.rs:6:24
  |
6 |         <Self as str>::trim(self);
  |                        ^^^^ not found in `str`

For more information about this error, try `rustc --explain E0576`.

However, str is not a trait, therefore the fully-qualified trait syntax should be simply invalid in this case - therefore, the error would better say the real problem ("str is not a trait"), probably with the help message suggesting the commented-out line.

Initially found in this SO question.