Explain why fn pointers for different fn items are different types

[estebank]

Given

fn foo() {}
fn bar() {}

fn main() {
    let mut x = foo;
    x = bar;
}

The compiler should explain why foo and bar are not the same type and should suggest first changing x's type to fn() -> ().

taken from https://twitter.com/ekuber/status/1273753857542111237