Rust error E0665: `#[derive(Default)]` on enum with no `#[default]`

What it meansThe Default trait was derived on an enum without specifying the default variant.

What rustc prints

error[E0665]: `#[derive(Default)]` on enum with no `#[default]`
 --> main.rs:1:10
  |
1 |   #[derive(Default)]
  |            ^^^^^^^
2 | / enum Food {
3 | |     Sweet,
4 | |     Salty,
5 | | }
  | |_- this enum needs a unit variant marked with `#[default]`
  |
help: make this unit variant default by placing `#[default]` on it
  |
3 |     #[default] Sweet,
  |     ++++++++++
help: make this unit variant default by placing `#[default]` on it
  |
4 |     #[default] Salty,
  |     ++++++++++

Real output of rustc 1.99.0 compiling the official example below (2021 edition). Your names and line numbers will differ; the first line is the part to match.

E0665Types and traits

Activein rustc 1.99.0

8related errors linked below

The compiler team's explanation

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

Erroneous code example:

#[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:

#[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:

enum Food {
    Sweet(i32),
    Salty,
}

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

From rustc --explain E0665 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.

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Rust error E0665: `#[derive(Default)]` on enum with no `#[default]`

What rustc prints

The compiler team's explanation

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