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// A `fmt` body may write repeatedly, so a formatter write appends rather than
// assigns. `UpperHex` here writes once per element.
use core::fmt;

struct Bytes<'a>(&'a [u8]);

impl fmt::UpperHex for Bytes<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        for &b in self.0 {
            write!(f, "{:02X}", b)?;
        }
        Ok(())
    }
}

impl fmt::LowerHex for Bytes<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        for &b in self.0 {
            write!(f, "{:02x}", b)?;
        }
        Ok(())
    }
}

impl fmt::Display for Bytes<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str("<")?;
        write!(f, "{self:x}")?;
        f.write_str(">")
    }
}

fn main() {
    let raw: Vec<u8> = vec![0xab, 0xcd, 0x01, 0x00];
    println!("{:X}", Bytes(&raw));
    println!("{:x}", Bytes(&raw));
    println!("{}", Bytes(&raw));
    println!("[{}]", Bytes(&[]));

    // `debug_assert*` fires in debug builds, which is the profile modelled.
    assert!(raw.len() == 4);
    assert_eq!(raw.len(), 4);
    assert_ne!(raw.len(), 5);
    debug_assert_eq!(raw[0], 0xab);
    println!("asserts passed");
}