Check for args like fn foo(ref x: u8), where the ref is useless

[Twisol]

ref on an entire argument (as opposed to a substructure of an argument) is effectively ignored by rustc, as you only receive a reference to the value already copied as a parameter.

fn bleh(ref mut x: u8) {
}

warning: `ref` directly on a function argument is ignored. Have you considered using a reference type instead?, #[warn(clippy_toplevel_ref_arg)] on by default
fn bleh(ref mut x: u8) {
        ^~~~~~~~~