Make the pointers in Fetch impls unions #5085
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# Objective Other than the exposed functions for reading matched tables and archetypes, a `QueryState` does not actually need both internal Vecs for storing matched archetypes and tables. In practice, it will only use one of the two depending on if it uses dense or archetypal iteration. Same vein as #12474. The goal is to reduce the memory overhead of using queries, which Bevy itself, ecosystem plugins, and end users are already fairly liberally using. ## Solution Add `StorageId`, which is a union over `TableId` and `ArchetypeId`, and store only one of the two at runtime. Read the slice as if it was one ID depending on whether the query is dense or not. This follows in the same vein as #5085; however, this one directly impacts heap memory usage at runtime, while #5085 primarily targeted transient pointers that might not actually exist at runtime. --- ## Changelog Changed: `QueryState::matched_tables` now returns an iterator instead of a reference to a slice. Changed: `QueryState::matched_archetypes` now returns an iterator instead of a reference to a slice. ## Migration Guide `QueryState::matched_tables` and `QueryState::matched_archetypes` does not return a reference to a slice, but an iterator instead. You may need to use iterator combinators or collect them into a Vec to use it as a slice. --------- Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
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Objective
Every
Fetchstruct initializes pointers for both storages, even though only one is ever really used. This both makes the struct bigger, and adds a miniscule amount of overhead when initializing aFetchas it needs to zero out the unused Fetch impl.Solution
Extend #4800. Add a compile-time discriminated
unioncalledStorageSwitchwhich is a debug-build checked union of different ways a Fetch could represent a pointer to the respective storage.As sparse sets always have a reference populated when making a fetch. They're no longer wrapped in an Option, and do not need any unwrapping.
This can be done (more) safely if GATs was stablized, but it seems like rust-lang/rust#96709 may be stalled for the foreseeable future.
As this is based on #4800, this won't come out of draft until some form of that PR is merged.
Co-Authored-By: Boxy supbscripter@gmail.com