Non-idempotence for macros in match arms

The following example takes two formatting attempts to settle, and furthermore settles into a horrible state:

fn x() {
    match 3 {
        _ => return println!("String long enough to reach `max_width + 13`......................................"),
    }
}

formats once to:

fn x() {
    match 3 {
        _ => return println!(
            "String long enough to reach `max_width + 13`......................................"
        ),
    }
}

and a second time to:

fn x() {
    match 3 {
        _ => {
            return println!(
            "String long enough to reach `max_width + 13`......................................"
        )
        }
    }
}

It seems that the second transformation is where the bug lies - the first formatting is what I'd expect, and already falls under max_width. Plus, the final formatting just looks awful.

A few things to help anyone tracking this down (which could be me when I get some time):

• It needs to be a macro, but any macro will do. A function with a name of equivalent length settles after the first step.
• It needs both the match and return statement, but any match pattern has the same effect.

I'm very happy to work on this when I get some time.

Start by reproducing the two formatting passes with the Rust snippet in the issue, then trace rustfmt's handling of macros inside match arms and return statements. Done means formatting this example is idempotent after one pass and does not produce the malformed-looking second output.