Comment single-line to multi-line formatting is botched

Given source like this:

fn foo(x: impl Iterator<Item = &'a PathBuf>) -> io::Result<impl Iterator<Item = &'a PathBuf>> {
    for foo in bar {
        for line in BufReader::new(File::open(banana)?)
            // A fairly long comment with https://google.github.io/flatbuffers/flatbuffers_grammar.html
            // an URL
            .lines()
        {
            let peach = match (loop {}) {
                Some(idx) => idx + open_quote, // get the absolute position from the original string
                None => loop {},
            };
        }
    }
}

rustfmt will produce the following diff:

             .lines()
         {
             let peach = match (loop {}) {
-                Some(idx) => idx + open_quote, // get the absolute position from the original string
+                Some(idx) => idx + open_quote, /* get the absolute position from the original
+                                                 * string */
                 None => loop {},
             };
         }

In this diff an incorrectly aligned 2nd line of the block comment can be seen.

Start by running rustfmt on the Rust reproducer in the issue and compare the single-line comment's conversion with the shown diff. No source file or test is named; locate the comment-formatting entry point and add coverage that preserves correct alignment when the comment becomes multi-line.