Implementing fmt::Display With "{}" Causes Stack Overflow

[grippy]

While implementing fmt::Display on this enum below, I accidentally used "{}" instead of "{:?}" or "{:#}".

use std::fmt;

#[derive(Debug)]
pub enum Lang {
    English
}

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

pub fn test_1() {
    println!("{}", Lang::English);    
}

fn main() {
    test_1()
}

This program bails with a fatal runtime error: stack overflow. There's no exception output provided while running w/ RUST_BACKTRACE=1 so it makes it hard to dig into the root cause.

Interestingly, if I remove the custom fmt function the compiler gives a nice explanation of what you need to do here:

error[E0277]: `Lang` doesn't implement `std::fmt::Display`
  --> src/main.rs:15:20
   |
15 |     println!("{}", Lang::English);    
   |                    ^^^^^^^^^^^^^ `Lang` cannot be formatted with the default formatter
   |
   = help: the trait `std::fmt::Display` is not implemented for `Lang`
   = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
   = note: required by `std::fmt::Display::fmt`

My issue here is that it would've been nice for the compiler to warn on the bad string binding pattern in the fmt function (similar to [E0277]).

[jonas-schievink]

Maybe the unconditional recursion lint could catch this if it would inspect entire SCCs of the call graph (currently I think it just catches a function directly calling itself)

[estebank]

Duplicate of #45838.