Consider adding fmt::fmt_fn or fmt::from_fn to the standard library

[EFanZh]

Similar to iter::from_fn and future::poll_fn, I find it useful to have a fmt::fmt_fn or fmt::from_fn function that converts a function into a value that implements Debug and Display:

use std::fmt::{self, Debug, Display, Formatter};

pub struct FmtFn<F>(F)
where
    F: ?Sized;

impl<F> Display for FmtFn<F>
where
    F: Fn(&mut Formatter) -> fmt::Result + ?Sized,
{
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        (self.0)(f)
    }
}

impl<F> Debug for FmtFn<F>
where
    F: Fn(&mut Formatter) -> fmt::Result + ?Sized,
{
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        (self.0)(f)
    }
}

pub fn fmt_fn<F>(f: F) -> FmtFn<F>
where
    F: Fn(&mut Formatter) -> fmt::Result,
{
    FmtFn(f)
}

fn main() {
    assert_eq!(format!("{}", fmt_fn(|f| f.write_str("abc"))), "abc");
    assert_eq!(format!("{:?}", fmt_fn(|f| f.write_str("def"))), "def");

    let dst_fmt_fn: &FmtFn<dyn Fn(&mut Formatter) -> fmt::Result> = &fmt_fn(|f| f.write_str("ghi"));

    assert_eq!(format!("{}", dst_fmt_fn), "ghi");
}

Or maybe we can have two different functions: display_fn and debug_fn, where display_fn returns a value that implements Display, and debug_fn returns a value that implements Debug.

[Rageking8]

@rustbot label +C-feature-request

[EFanZh]

Superseded by #117729.