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# Objective The type `&World` is currently in an awkward place, since it has two meanings: 1. Read-only access to the entire world. 2. Interior mutable access to the world; immutable and/or mutable access to certain portions of world data. This makes `&World` difficult to reason about, and surprising to see in function signatures if one does not know about the interior mutable property. The type `UnsafeWorldCell` was added in #6404, which is meant to alleviate this confusion by adding a dedicated type for interior mutable world access. However, much of the engine still treats `&World` as an interior mutable-ish type. One of those places is `SystemParam`. ## Solution Modify `SystemParam::get_param` to accept `UnsafeWorldCell` instead of `&World`. Simplify the safety invariants, since the `UnsafeWorldCell` type encapsulates the concept of constrained world access. --- ## Changelog `SystemParam::get_param` now accepts an `UnsafeWorldCell` instead of `&World`. This type provides a high-level API for unsafe interior mutable world access. ## Migration Guide For manual implementers of `SystemParam`: the function `get_item` now takes `UnsafeWorldCell` instead of `&World`. To access world data, use: * `.get_entity()`, which returns an `UnsafeEntityCell` which can be used to access component data. * `get_resource()` and its variants, to access resource data. |
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