# Objective
- Fixes#18690
- Closes [#2065](https://github.com/bevyengine/bevy-website/pull/2065)
- Alternative to #18691
The changes to the Hash made in #15801 to the
[BuildHasher](https://doc.rust-lang.org/std/hash/trait.BuildHasher.html)
resulted in serious migration problems and downgraded UX for users of
Bevy's re-exported hashmaps. Once merged, we need to go in and remove
the migration guide added as part of #15801.
## Solution
- Newtype `HashMap` and `HashSet` instead of type aliases
- Added `Deref/Mut` to allow accessing future `hashbrown` methods
without maintenance from Bevy
- Added bidirectional `From` implementations to provide escape hatch for
API incompatibility
- Added inlinable re-exports of all methods directly to Bevy's types.
This ensures `HashMap::new()` works (since the `Deref` implementation
wont cover these kinds of invocations).
## Testing
- CI
---
## Migration Guide
- If you relied on Bevy's `HashMap` and/or `HashSet` types to be
identical to `hashbrown`, consider using `From` and `Into` to convert
between the `hashbrown` and Bevy types as required.
- If you relied on `hashbrown/serde` or `hashbrown/rayon` features, you
may need to enable `bevy_platform_support/serialize` and/or
`bevy_platform_support/rayon` respectively.
---
## Notes
- Did not replicate the Rayon traits, users will need to rely on the
`Deref/Mut` or `From` implementations for those methods.
- Did not re-expose the `unsafe` methods from `hashbrown`. In most cases
users will still have access via `Deref/Mut` anyway.
- I have added `inline` to all methods as they are trivial wrappings of
existing methods.
- I chose to make `HashMap::new` and `HashSet::new` const, which is
different to `hashbrown`. We can do this because we default to a
fixed-state build-hasher. Mild ergonomic win over using
`HashMap::with_hasher(FixedHasher)`.
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