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// A `Display` impl becomes a proc returning the string the formatter is
// written with, since the observable result of `{}` is exactly those bytes.
use core::fmt;

#[derive(Debug, PartialEq)]
pub enum Error {
    InvalidEncoding,
    InvalidLength,
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Error::InvalidEncoding => f.write_str("invalid Base16 encoding"),
            Error::InvalidLength => f.write_str("invalid Base16 length"),
        }
    }
}

// A marker trait with no items: we do not model trait resolution, so it
// generates nothing.
impl core::error::Error for Error {}

struct Point {
    x: i32,
    y: i32,
}

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

impl Point {
    fn sum(&self) -> i32 {
        self.x + self.y
    }
}

fn main() {
    println!("{}", Error::InvalidEncoding);
    println!("{}", Error::InvalidLength);
    println!("{:?}", Error::InvalidLength);
    let p = Point { x: 3, y: -4 };
    println!("{} {}", p, p.sum());
}