An unused type parameter and an impl in the argument position allows you to define a function that cannot be called

[IBUzPE9]

use std::fmt::Debug;

// Function that can not be called
fn xxx<A>(x:impl Debug){ println!("Done! {:?}", x) }

// Should an unused type parameter generate an error or warning?
fn yyy<A, B:Debug>(x:B){ println!("Done! {:?}", x) }

fn main(){
    // xxx(123u8); //type annotations needed
    // xxx::<()>(123u8); //too few type parameters provided: expected 2 type parameters, found 1 type parameter
    // xxx::<(), _>(123u8); //cannot provide explicit type parameters when `impl Trait` is used in argument position.
    // xxx::<(), u8>(123u8); //cannot provide explicit type parameters when `impl Trait` is used in argument position.
    yyy::<(), u8>(123u8);
    yyy::<(), _>(123u8);
}

playground

[ChayimFriedman2]

This is "fixed" on nightly thanks to explicit_generic_args_with_impl_trait stabilization.

use std::fmt::Debug;

fn xxx<A>(x: impl Debug) {
    println!("Done! {:?}", x)
}

fn main() {
    xxx::<()>(123u8);
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=e43997ce75350ad49e6fc177eb75a37b