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# Objective - Fix a type inference regression introduced by #3001 - Make read only bounds on world queries more user friendly ptrification required you to write `Q::Fetch: ReadOnlyFetch` as `for<'w> QueryFetch<'w, Q>: ReadOnlyFetch` which has the same type inference problem as `for<'w> QueryFetch<'w, Q>: FilterFetch<'w>` had, i.e. the following code would error: ```rust #[derive(Component)] struct Foo; fn bar(a: Query<(&Foo, Without<Foo>)>) { foo(a); } fn foo<Q: WorldQuery>(a: Query<Q, ()>) where for<'w> QueryFetch<'w, Q>: ReadOnlyFetch, { } ``` `for<..>` bounds are also rather user unfriendly.. ## Solution Remove the `ReadOnlyFetch` trait in favour of a `ReadOnlyWorldQuery` trait, and remove `WorldQueryGats::ReadOnlyFetch` in favor of `WorldQuery::ReadOnly` allowing the previous code snippet to be written as: ```rust #[derive(Component)] struct Foo; fn bar(a: Query<(&Foo, Without<Foo>)>) { foo(a); } fn foo<Q: ReadOnlyWorldQuery>(a: Query<Q, ()>) {} ``` This avoids the `for<...>` bound which makes the code simpler and also fixes the type inference issue. The reason for moving the two functions out of `FetchState` and into `WorldQuery` is to allow the world query `&mut T` to share a `State` with the `&T` world query so that it can have `type ReadOnly = &T`. Presumably it would be possible to instead have a `ReadOnlyRefMut<T>` world query and then do `type ReadOnly = ReadOnlyRefMut<T>` much like how (before this PR) we had a `ReadOnlyWriteFetch<T>`. A side benefit of the current solution in this PR is that it will likely make it easier in the future to support an API such as `Query<&mut T> -> Query<&T>`. The primary benefit IMO is just that `ReadOnlyRefMut<T>` and its associated fetch would have to reimplement all of the logic that the `&T` world query impl does but this solution avoids that :) --- ## Changelog/Migration Guide The trait `ReadOnlyFetch` has been replaced with `ReadOnlyWorldQuery` along with the `WorldQueryGats::ReadOnlyFetch` assoc type which has been replaced with `<WorldQuery::ReadOnly as WorldQueryGats>::Fetch` - Any where clauses such as `QueryFetch<Q>: ReadOnlyFetch` should be replaced with `Q: ReadOnlyWorldQuery`. - Any custom world query impls should implement `ReadOnlyWorldQuery` insead of `ReadOnlyFetch` Functions `update_component_access` and `update_archetype_component_access` have been moved from the `FetchState` trait to `WorldQuery` - Any callers should now call `Q::update_component_access(state` instead of `state.update_component_access` (and `update_archetype_component_access` respectively) - Any custom world query impls should move the functions from the `FetchState` impl to `WorldQuery` impl `WorldQuery` has been made an `unsafe trait`, `FetchState` has been made a safe `trait`. (I think this is how it should have always been, but regardless this is _definitely_ necessary now that the two functions have been moved to `WorldQuery`) - If you have a custom `FetchState` impl make it a normal `impl` instead of `unsafe impl` - If you have a custom `WorldQuery` impl make it an `unsafe impl`, if your code was sound before it is going to still be sound |
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Compile fail tests for bevy_ecs
This crate is separate from bevy_ecs and not part of the Bevy workspace in order to not fail crater tests for Bevy. The tests assert on the exact compiler errors and can easily fail for new Rust versions due to updated compiler errors (e.g. changes in spans).
The CI workflow executes these tests on the stable rust toolchain (see tools/ci).