# Objective
With the current `MapEntities` `impl`s, it is not possible to derive
things like this:
```rust
#[derive(Component)]
pub struct Inventory {
#[entities]
slots: Vec<Option<Entity>>,
}
```
This is because `MapEntities` is only implemented for `Vec<Entity>` &
`Option<Entity>`, and not arbitrary combinations of those.
It would be nice to also support those types.
## Solution
I replaced the `impl`s of the following types
- `Option<Entity>`: replaced with `Option<T>`
- `Vec<Entity>`: replaced with `Vec<T>`
- `HashSet<Entity, S>`: replaced with `HashSet<T, S>`
- `T` also had to be `Eq + core:#️⃣:Hash` here. **Not sure if this is
too restrictive?**
- `IndexSet<Entity, S>`: replaced with `IndexSet <T, S>`
- `T` also had to be `Eq + core:#️⃣:Hash` here. **Not sure if this is
too restrictive?**
- `BTreeSet<Entity>`: replaced with `BTreeSet<T>`
- `VecDeque<Entity>`: replaced with `VecDeque<T>`
- `SmallVec<A: smallvec::Array<Item = Entity>>`: replaced with
`SmallVec<A: smallvec::Array<Item = T>>`
(in all of the above, `T` is a generic type that implements
`MapEntities` (`Entity` being one of them).)
## Testing
I did not test any of this, but extended the `Component::map_entities`
doctest with an example usage of the newly supported types.
---
## Showcase
With these changes, this is now possible:
```rust
#[derive(Component)]
pub struct Inventory {
#[entities]
slots: Vec<Option<Entity>>,
}
```
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