This repository demonstrates the following Rust compiler bug on the AVR target: rust-lang/rust#107293

The included Rust crate in worduino-avr/ passes the feature flag score to worduino-engine/. In worduino-engine, the only thing changed by this feature flag is the presence or abscence of the score field in struct Player; otherwise, this field is unused:

#[derive(Clone, Copy)]
struct Player {
    e: Entity,
    lives: u8,
    #[cfg(feature = "score")] score: u16,
    bullet: Option<Entity>,
}

impl Player {
    fn new() -> Player {
        Player {
            e: Self::start_pos(),
            lives: 2,
            #[cfg(feature = "score")] score: 0,
            bullet: None,
        }
    }
}

As we can see with the included AVR simulator, the observable behaviour of the two programs differ: they end up sending different data via SPI.