Derive Macro fixed_map::Key

#[derive(Key)]
{
    // Attributes available to this derive:
    #[key]
}

Derive to implement the Key trait.

Requires that fixed_map is in scope.

This derive implements the Key trait for a given type.

The Key trait is what allows fixed_map to set up storage for a type that will be the key in a fixed map.

Given the following enum:

#[derive(Clone, Copy, fixed_map::Key)]
pub enum Key {
    First,
    Second,
    Third,
}

It performs the following simplified expansion:

/// Build a storage struct containing an item for each key:
pub struct KeyStorage<V> {
    /// Storage for `Key::First`.
    f1: Option<V>,
    /// Storage for `Key::Second`.
    f2: Option<V>,
    /// Storage for `Key::Third`.
    f3: Option<V>,
}

/// Implement storage for `KeyStorage`.
impl<V> fixed_map::Storage<Key, V> for KeyStorage<V> {
    fn get(&self, key: Key) -> Option<&V> {
        match *self {
            Key::First => self.f1.as_ref(),
            Key::Second => self.f2.as_ref(),
            Key::Third => self.f3.as_ref(),
        }
    }

    /* other methods skipped */
}

/// Implement the `Key` trait to point out storage.
impl<V> fixed_map::Key<Key, V> for Key {
    type Storage = KeyStorage<V>;
}