Extra blank line inserted before code blocks in doc comments

[shulaoda]

Summary

Doc comment formatting inconsistency when using wrap_comments = true with max_width = 20: an extra blank line is inserted before code blocks.

# rustfmt.toml
wrap_comments = true
max_width = 20

/// Some doc comment with code snippet:
///```
/// '\u{1F}
/// ```
pub struct Code {}

Additional Context

This issue was discovered while attempting to fix other issues in rust-lang/rustfmt.

The latest rustfmt (rustfmt 1.8.0-nightly (6ac1fcd7a3 2025-10-09)) produces the correct output without the extra blank line. This issue is being filed to track this regression and verify whether it gets fixed after the next sync from rust-lang/rustfmt to rust-lang/rust.

[ytmimi]

@siddhardh-codemonk Maybe you can take a look at this one. To test is out I'd create an input file (issue_6695.rs):

/// Some doc comment with code snippet:
///```
/// '\u{1F}
/// ```
pub struct Code {}

You can run cargo run --bin rustfmt -- --emit=stdout --config=max_width=20,wrap_comments=true path/to/issue_6695.rs from the root of the rustfmt repo to build run and from source. I'm setting --emit=stdout so that we don't override the original input file and --config=max_width=20,wrap_comments=true just passes all the configuration options needed to reproduce.

With all that, rustfmt produces this:

Output:

/// Some doc comment
/// with code snippet:
/// 
/// ```
/// '\u{1F}
/// ```
pub struct Code {}

error[internal]: line formatted, but exceeded maximum width (maximum: 20 (see `max_width` option), found: 22)
 --> path/to/issue_6695.rs:2:2:21
  |
2 | /// with code snippet:
  |                     ^^
  |
  = note: set `error_on_unformatted = false` to suppress the warning against comments or string literals

error[internal]: left behind trailing whitespace
 --> path/to/issue_6695.rs:3:3:4
  |
3 | /// 
  |    ^
  |
  = note: set `error_on_unformatted = false` to suppress the warning against comments or string literals

warning: rustfmt has failed to format. See previous 2 errors.

If you're having issues / have questions you can leave comments here or message me on Zulip

[siddhardh-codemonk]

Thanks again for the repro! I dug into this and found there are actually two separate bugs causing the two errors:

Trailing whitespace — traced to how handle_line pushes a space for continuation, then separately pushes a newline when a code block starts, without cleaning up the earlier space. Opened a PR fixing this: #7083
Width overflow — this one's in break_string's MIN_STRING threshold rejecting a valid break point and falling through to a forward search that doesn't re-check the width limit. It's more involved since it's shared wrapping logic used elsewhere, so I wanted to flag it here first rather than guess at a fix. Happy to take a swing at it if you can confirm the right direction (e.g. should the forward-search fallback verify width before accepting, or should MIN_STRING behave differently in this case?).