Redesign entry traits

[apasel422]

The current design is nonsensical. With some future-Rust features, we could have:

pub trait EntryMap: Map {
    type Occupied<'a>: Occupied<'a = 'a, Key = Self::Key, Value = Self::Value>;
    type Vacant<'a>: Vacant<'a = 'a, Key = Self::Key, Value = Self::Value>;

    fn entry<'a>(&'a mut self, key: Self::Key)
        -> Entry<Self::Occupied<'a>, Self::Vacant<'a>>;
}

pub enum Entry<O, V>
where
    O: Occupied,
    V: Vacant<'a = O::'a, Key = O::Key, Value = O::Value>,
{
    Occupied(O),
    Vacant(V),
}

pub trait Occupied {
    lifetime 'a;
    type Key: Self::'a;
    type Value: Self::'a;

    fn into_mut(self) -> &Self::'a mut Self::Value;
    // ...
}

pub trait Vacant {
    lifetime 'a;
    type Key: Self::'a;
    type Value: Self::'a;

    fn insert(self, value: Self::Value) -> &Self::'a mut Self::Value;
    // ...
}

[apasel422]

I'd also be fine keeping the lifetime parameter on EntryMap but removing it from {Occupied, Vacant}, at least until we get higher-kinded types:

pub trait EntryMap<'a>: Map {
    type Occupied: Occupied<'a = 'a, Key = Self::Key, Value = Self::Value>;
    type Vacant: Vacant<'a = 'a, Key = Self::Key, Value = Self::Value>;

    fn entry(&'a mut self, key: Self::Key) -> Entry<Self::Occupied, Self::Vacant>;
}

pub enum Entry<O, V>
where
    O: Occupied,
    V: Vacant<'a = O::'a, Key = O::Key, Value = O::Value>,
{
    Occupied(O),
    Vacant(V),
}

pub trait Occupied {
    lifetime 'a;
    type Key: Self::'a;
    type Value: Self::'a;

    fn into_mut(self) -> &Self::'a mut Self::Value;
    // ...
}

pub trait Vacant {
    lifetime 'a;
    type Key: Self::'a;
    type Value: Self::'a;

    fn insert(self, value: Self::Value) -> &Self::'a mut Self::Value;
    // ...
}

[apasel422]

This also needs to take object-safety into account. It would be nice to be able to use these traits with objects.

[apasel422]

The current formulation only works for maps that contain 'static data.

[apasel422]

The new formulation solves the problems of the original without higher-kinded types or associated lifetimes, but instead uses trait objects.