An error should be raised for bad implementations of Display in an unit struct context

[c410-f3r]

The following overflows the stack:

use core::fmt;

struct Foo;

impl fmt::Display for Foo {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}", self)
    }
}

fn main() {
    println!("{}", Foo);
}

[jonas-schievink]

The first one shows

error[E0277]: `fn(i32) -> Foo {Foo}` doesn't implement `std::fmt::Display`

because you're passing the type constructor function, not an instance of Foo. Change it to Foo(123) and it also overflows the stack.

The unconditional recursion lint not triggering on Display impls is a well known issue tracked (primarily) by #57965, so closing in favor of that.