#![allow(dead_code, deprecated, unused_variables, unused_mut)]
use std::fmt::{Error, Write};
fn writer<W: Write>(f: &mut W, c: char) -> Result<(), Error> {
    f.write_char(c)
}
fn main() -> Result<(), impl core::fmt::Debug> {
    let mut buf = String::new();
    writer(&mut buf, 'a')?;
    writer(&mut buf, 'b')?;
    assert_eq!(& buf, "ab");
    std::fmt::Result::Ok(())
}