Block comment removed from function signature when using unit

When I write a function like one of the following

fn foo() -> (/* some comment */) {}
fn bar(x: (/* some comment */)) {}
fn baz() -> Result<(/* some comment */), ()> {}

The comment inside the unit in the return/parameter type gets removed:

fn foo() -> () {}
fn bar(x: ()) {}
fn baz() -> Result<(), ()> {}

This does not happen inside of the function body itself:

fn qux() {
    (/* comment */) // this is fine
}

Reproduce the three function-signature examples and compare them with the function-body example using rustfmt. Locate the formatting path for comments inside unit types, add regression coverage for the affected signatures, and verify that the comments are preserved without changing the existing body behavior.