No formatting in arguments passed to macros defined with `=>`

rustfmt (both stable 1.42.0 and nightly of 12-Apr-2020) does not format arguments of macros defined with =>.

For example, test_arrow!(...) usage below is left as is (with extra whitespaces) after rustfmt, while similar test_comma!(...) is formatted correctly (i.e. original extra space is removed):

macro_rules! test_arrow {
    ($x:expr => $y:expr) => {
        println!("{} {}", $x, $y);
    };
}

macro_rules! test_comma {
    ($x:expr, $y:expr) => {
        println!("{} {}", $x, $y);
    };
}

fn main() {
    test_arrow!("hello" =>   2    + 2);
    test_comma!("hello", 2 + 2);
}

Reproduce the issue with the supplied test_arrow! and test_comma! macro examples using rustfmt, then trace the macro-formatting entry point that handles their arguments. Done means the test_arrow! invocation formats its extra whitespace consistently with test_comma!.