Function rustfmt_nightly::comment::combine_strs_with_missing_comments

pub(crate) fn combine_strs_with_missing_comments(
    context: &RewriteContext<'_>,
    prev_str: &str,
    next_str: &str,
    span: Span,
    shape: Shape,
    allow_extend: bool
) -> Option<String>

Combine prev_str and next_str into a single String. span may contain comments between two strings. If there are such comments, then that will be recovered. If allow_extend is true and there is no comment between the two strings, then they will be put on a single line as long as doing so does not exceed max width.