Error code E0665

Note: this error code is no longer emitted by the compiler.

The Default trait was derived on an enum.

Erroneous code example:

#![allow(unused)]
fn main() {
#[derive(Default)]
enum Food {
    Sweet,
    Salty,
}
}

The Default cannot be derived on an enum for the simple reason that the compiler doesn't know which value to pick by default whereas it can for a struct as long as all its fields implement the Default trait as well.

If you still want to implement Default on your enum, you'll have to do it "by hand":

#![allow(unused)]
fn main() {
enum Food {
    Sweet,
    Salty,
}

impl Default for Food {
    fn default() -> Food {
        Food::Sweet
    }
}
}