Converting a `Display` into another `Display`

uppercase.rs

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

pub struct Uppercase<T>(T);

impl<T: Display> Display for Uppercase<T> {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        struct Adapter<'a, 'b: 'a>(&'a mut Formatter<'b>);

        impl<'a, 'b: 'a> Write for Adapter<'a, 'b> {
            fn write_str(&mut self, s: &str) -> fmt::Result {
                s.chars().try_for_each(|c| self.write_char(c))
            }

            fn write_char(&mut self, c: char) -> fmt::Result {
                self.0.write_char(c.to_ascii_uppercase())
            }
        }

        write!(Adapter(f), "{}", self.0)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test() {
        assert_eq!("HELLO, WORLD!", Uppercase("Hello, world!").to_string());
    }
}