A line comment with a no-break space gains an empty // line on every run

[abashev]

What

Reported upstream as google/google-java-format#562 ("Running with --replace does not converge", 2021). The reporter's comment has a no-break space (U+00A0) right after the opening quote, which is easy to miss. It reproduces here on main:

public class T {
    //String testString = "<U+00A0>thisisnotaHYPERLINKandsoitshouldntbetruncated… \"http://…\"";
}

where the word after the no-break space is longer than the line. The first run splits the comment in front of the no-break space, and every later run adds one more empty // line above the second half:

    // String testString = "
    //
    //<U+00A0>thisisnotaHYPERLINKandsoitshouldntbetruncated…

So the file never becomes stable, and a format check fails after every --replace.

Why it happens

JavaCommentsHelper.wrapLineComments looks for a break with CharMatcher.whitespace(), which counts U+00A0 as whitespace, so the continuation line starts with // and the no-break space. On the next run LINE_COMMENT_MISSING_SPACE_PREFIX, whose \s does not match U+00A0, reads that as a comment with no space after the slashes and inserts one. The wrap then breaks at the no-break space again, the inserted space stays behind on a line of its own, and trim() turns that line into an empty //.

Fix

Break line comments only at breaking whitespace, CharMatcher.breakingWhitespace(), which leaves out U+00A0, U+2007 and U+202F. A no-break space is by definition not a place to break a line. Comments without a no-break space are not affected.

Done when

• the comment above formats in one run the way the next run leaves it;
• a golden covers a long line comment whose only break opportunity is a no-break space.