Misleading error when using fully qualified trait syntax without trait

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.

Did you mean the ideal output should like:

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

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

?

Not sure, to be honest. This might even better not be E0576, since the problem is not that "something wasn't found for the type" - it's "there's no type to search in".

Current output:

error[E0404]: expected trait, found builtin type `str`
 --> src/lib.rs:6:18
  |
6 |         <Self as str>::trim(self);
  |                  ^^^ not a trait

@rustbot

I would like to give it a try and add the test for this.

@rustbot claim

@rustbot

@rustbot review

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Rollup merge of rust-lang#131991 - jannden:issue-98565-test, r=jieyouxu

Auto merge of rust-lang#132005 - matthiaskrgr:rollup-ced4upi, r=matth…

Unrolled build for rust-lang#131991

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Misleading error when using fully qualified trait syntax without trait #98565

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Misleading error when using fully qualified trait syntax without trait #98565

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lyming2007 commented on Jul 12, 2022

Cerber-Ursi commented on Jul 12, 2022

estebank commented on Oct 4, 2024

jannden commented on Oct 20, 2024

jannden commented on Oct 21, 2024

rustbot commented on Oct 21, 2024

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