Panic in src/tools/rustfmt/src/lists.rs:678:54 on a bad comment

[kvark]

Describe the bug

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/tools/rustfmt/src/lists.rs:678:54
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

To Reproduce

fn main() {
    panic!("Foo {:?}", /*"*/
                    1
                );
}

Expected behavior

Formatting succeeds

Meta

• rustfmt version: rustfmt 1.4.18-stable (8157a3f 2020-07-15)
• From where did you install rustfmt?: rustup
• How do you run rustfmt: cargo fmt

[calebcartwright]

There was a similar report in #4312 that was addressed by 98efebd on master (hasn't been backported to a release version). That update does prevent the panic in this case when the comment is within a mac call, but the formatting still isn't correct with that change on master either, as the comment gets moved before the comma thus likely losing the arg association.

[davidBar-On]

... as the comment gets moved before the comma thus likely losing the arg association

Submitted PR #4581 with a proposed fix for this issue.

[karyon]

The backport of 98efebd has been done in #4796. I have tested the snippet in the OP and it does not panic and produces this here, which looks good to me:

fn main() {
    panic!("Foo {:?}" /*"*/, 1);
}