Formatting raw string literals in doc comments is broken in nightly

[wiktor-k]

I found an issue with cargo 1.79.0-nightly (d438c80c4 2024-03-19) and format_code_in_doc_comments = true with raw string literals:

/// # Examples
///
/// ```
/// use std::io::Write;
///
/// # fn main() -> testresult::TestResult {
/// let source = r#"```sh
/// echo x
/// ```"#;
/// let mut sink = Vec::new();
///
/// tangler::extract(source.as_bytes(), &mut sink, &["sh"])?;
///
/// assert_eq!(&b"echo x\n\n"[..], &sink[..]);
/// # Ok(()) }
/// ```

(Original source at https://github.com/wiktor-k/tangler/blob/main/src/lib.rs#L24-L26 . The tool works with markdown documents, that's why the doc comment uses raw strings).

It shows:

$ cargo +nightly fmt --all -- --check
error[E0748]: unterminated raw string
 --> <stdin>:5:18
  |
5 |     let source = r#"```sh
  |                  ^ unterminated raw string
  |
  = note: this raw string should be terminated with `"#`

but cargo test (nightly and stable) and cargo doc are OK with this fragment of code.

I'm not sure if I should report it as a separate ticket or just appending it here is OK...?

Originally posted by @wiktor-k in #3348 (comment)

[ytmimi]

Thank you for the report. Linking the tracking issue for format_code_in_doc_comments #3348

[ytmimi]

I think the fix for this would involve updating this logic. We might also want check that the line ends with "```" to make sure that we're actually looking at a closing code fence.

[ytmimi]

After looking into this one a little more I think the error message that's popping up is related to #6109, which was a result of nightly changes in the compiler. The issue has been fixed on nightly for a while now.

@wiktor-k when you have a moment can you confirm if a more recent nightly version still causes the error message to pop up.

That all said, I still think rustfmt can do better here and not treat the end of the string literal (/// ```"#;) as the end of the code block, and I think my suggestion in #6142 (comment) is still where we need to make the change.

[wiktor-k]

@wiktor-k when you have a moment can you confirm if a more recent nightly version still causes the error message to pop up.

Indeed I can't reproduce it now.

That all said, I still think rustfmt can do better here and not treat the end of the string literal (/// ```"#;) as the end of the code block, and I think my suggestion in #6142 (comment) is still where we need to make the change.

Just for the record I'm okay with closing this issue and if it pops up again I can create a new one.

Thanks for your help and time! 🙇

[ytmimi]

Going to close this since the original issue is resolved.