Trailing commas handling in macro calls

fn main() {
    foo("abcdefghijklmnopqrstuvwxyz".to_string(), "abcdefghijklmnopqrstuvwxyz".to_string());
}

gets, as expected, formatted to:

fn main() {
    foo(
        "abcdefghijklmnopqrstuvwxyz".to_string(),
        "abcdefghijklmnopqrstuvwxyz".to_string(),
    );
}

However,

fn main() {
    assert_eq!("abcdefghijklmnopqrstuvwxyz".to_string(), "abcdefghijklmnopqrstuvwxyz".to_string());
}

gets formatted as:

fn main() {
    assert_eq!(
        "abcdefghijklmnopqrstuvwxyz".to_string(),
        "abcdefghijklmnopqrstuvwxyz".to_string()
    );
}

No trailing comma after the second parameter.

And even worse, function calls in one of the assert_eq! parameters get the same treatment:

fn main() {
    assert_eq!(foo("abcdefghijklmnopqrstuvwxyz".to_string(), "abcdefghijklmnopqrstuvwxyz".to_string()), "abcdefghijklmnopqrstuvwxyz".to_string());
}

gets formatted as:

fn main() {
    assert_eq!(
        foo(
            "abcdefghijklmnopqrstuvwxyz".to_string(),
            "abcdefghijklmnopqrstuvwxyz".to_string()
        ),
        "abcdefghijklmnopqrstuvwxyz".to_string()
    );
}

No trailing comma after the second parameter of foo.

And if you have a trailing comma and a parameter gets shorter, the trailing comma stays.

fn main() {
    assert_eq!(
        foo(
            "abcdef".to_string(),
            "abcdef".to_string(),
        ),
        "abcdefghijklmnopqrstuvwxyz".to_string()
    );
}

gets formatted as:

fn main() {
    assert_eq!(
        foo("abcdef".to_string(), "abcdef".to_string(),),
        "abcdefghijklmnopqrstuvwxyz".to_string()
    );
}

I understand that trailings commas can change the meaning of macros, but it seems assert_eq! is already special cased.

Even more than the trailing comma after assert_eq!'s second parameter, the handling of trailing commas in function calls in one of assert_eq!'s parameter seems to me pretty inconvenient.

Thanks for the hard work on such a useful tool.

Start by reproducing the three assert_eq! examples with rustfmt and compare their handling of trailing commas in macro arguments and nested function calls. Trace the formatting entry point for macro calls, then inspect how assert_eq! is special-cased. Done means the reported examples have consistent trailing-comma behavior without changing macro semantics.