Error code E0665

The Default trait was derived on an enum without specifying the default variant.

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.

For the case where the desired default variant has no payload, you can annotate it with #[default] to derive it:

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

In the case where the default variant does have a payload, you will have to implement Default on your enum manually:

#![allow(unused)]
fn main() {
enum Food {
    Sweet(i32),
    Salty,
}

impl Default for Food {
    fn default() -> Food {
        Food::Sweet(1)
    }
}
}