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# Objective
The existing `RelationshipSourceCollection` uses `Vec` as the only
possible backing for our relationships. While a reasonable choice,
benchmarking use cases might reveal that a different data type is better
or faster.
For example:
- Not all relationships require a stable ordering between the
relationship sources (i.e. children). In cases where we a) have many
such relations and b) don't care about the ordering between them, a hash
set is likely a better datastructure than a `Vec`.
- The number of children-like entities may be small on average, and a
`smallvec` may be faster
## Solution
- Implement `RelationshipSourceCollection` for `EntityHashSet`, our
custom entity-optimized `HashSet`.
-~~Implement `DoubleEndedIterator` for `EntityHashSet` to make things
compile.~~
- This implementation was cursed and very surprising.
- Instead, by moving the iterator type on `RelationshipSourceCollection`
from an erased RPTIT to an explicit associated type we can add a trait
bound on the offending methods!
- Implement `RelationshipSourceCollection` for `SmallVec`
## Testing
I've added a pair of new tests to make sure this pattern compiles
successfully in practice!
## Migration Guide
`EntityHashSet` and `EntityHashMap` are no longer re-exported in
`bevy_ecs::entity` directly. If you were not using `bevy_ecs` / `bevy`'s
`prelude`, you can access them through their now-public modules,
`hash_set` and `hash_map` instead.
## Notes to reviewers
The `EntityHashSet::Iter` type needs to be public for this impl to be
allowed. I initially renamed it to something that wasn't ambiguous and
re-exported it, but as @Victoronz pointed out, that was somewhat
unidiomatic.
In
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Bevy Text
