Should warn when calling blanket implementation of a method rather than our own

[gdesmott]

The following snippet crashes:

thread 'main' has overflowed its stack
fatal runtime error: stack overflow
Aborted (core dumped)

use std::fmt;

struct Badger;

impl Badger {
    fn to_string<'a>(self) -> &'a str {
        "badger"
    }
}

impl fmt::Display for Badger {
    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
        f.write_str(&self.to_string())
    }
}

fn main() {
    let b = Badger {};
    println!("{}", b);
}

fmt() calls the blanket implementation of ToString, which is using Display, instead of our own implementation resulting in an infinite recursion.

We wouldn't have the problem if our function was taking &self but that's still pretty confusing and prone to errors. It would be good to have at least a warning (or a clippy lint?) for such patterns.

[jonas-schievink]

This is effectively #57965