# Objective
Improve the performance of `FilteredEntity(Ref|Mut)` and
`Entity(Ref|Mut)Except`.
`FilteredEntityRef` needs an `Access<ComponentId>` to determine what
components it can access. There is one stored in the query state, but
query items cannot borrow from the state, so it has to `clone()` the
access for each row. Cloning the access involves memory allocations and
can be expensive.
## Solution
Let query items borrow from their query state.
Add an `'s` lifetime to `WorldQuery::Item` and `WorldQuery::Fetch`,
similar to the one in `SystemParam`, and provide `&'s Self::State` to
the fetch so that it can borrow from the state.
Unfortunately, there are a few cases where we currently return query
items from temporary query states: the sorted iteration methods create a
temporary state to query the sort keys, and the
`EntityRef::components<Q>()` methods create a temporary state for their
query.
To allow these to continue to work with most `QueryData`
implementations, introduce a new subtrait `ReleaseStateQueryData` that
converts a `QueryItem<'w, 's>` to `QueryItem<'w, 'static>`, and is
implemented for everything except `FilteredEntity(Ref|Mut)` and
`Entity(Ref|Mut)Except`.
`#[derive(QueryData)]` will generate `ReleaseStateQueryData`
implementations that apply when all of the subqueries implement
`ReleaseStateQueryData`.
This PR does not actually change the implementation of
`FilteredEntity(Ref|Mut)` or `Entity(Ref|Mut)Except`! That will be done
as a follow-up PR so that the changes are easier to review. I have
pushed the changes as chescock/bevy#5.
## Testing
I ran performance traces of many_foxes, both against main and against
chescock/bevy#5, both including #15282. These changes do appear to make
generalized animation a bit faster:
(Red is main, yellow is chescock/bevy#5)
## Migration Guide
The `WorldQuery::Item` and `WorldQuery::Fetch` associated types and the
`QueryItem` and `ROQueryItem` type aliases now have an additional
lifetime parameter corresponding to the `'s` lifetime in `Query`. Manual
implementations of `WorldQuery` will need to update the method
signatures to include the new lifetimes. Other uses of the types will
need to be updated to include a lifetime parameter, although it can
usually be passed as `'_`. In particular, `ROQueryItem` is used when
implementing `RenderCommand`.
Before:
```rust
fn render<'w>(
item: &P,
view: ROQueryItem<'w, Self::ViewQuery>,
entity: Option<ROQueryItem<'w, Self::ItemQuery>>,
param: SystemParamItem<'w, '_, Self::Param>,
pass: &mut TrackedRenderPass<'w>,
) -> RenderCommandResult;
```
After:
```rust
fn render<'w>(
item: &P,
view: ROQueryItem<'w, '_, Self::ViewQuery>,
entity: Option<ROQueryItem<'w, '_, Self::ItemQuery>>,
param: SystemParamItem<'w, '_, Self::Param>,
pass: &mut TrackedRenderPass<'w>,
) -> RenderCommandResult;
```
---
Methods on `QueryState` that take `&mut self` may now result in
conflicting borrows if the query items capture the lifetime of the
mutable reference. This affects `get()`, `iter()`, and others. To fix
the errors, first call `QueryState::update_archetypes()`, and then
replace a call `state.foo(world, param)` with
`state.query_manual(world).foo_inner(param)`. Alternately, you may be
able to restructure the code to call `state.query(world)` once and then
make multiple calls using the `Query`.
Before:
```rust
let mut state: QueryState<_, _> = ...;
let d1 = state.get(world, e1);
let d2 = state.get(world, e2); // Error: cannot borrow `state` as mutable more than once at a time
println!("{d1:?}");
println!("{d2:?}");
```
After:
```rust
let mut state: QueryState<_, _> = ...;
state.update_archetypes(world);
let d1 = state.get_manual(world, e1);
let d2 = state.get_manual(world, e2);
// OR
state.update_archetypes(world);
let d1 = state.query(world).get_inner(e1);
let d2 = state.query(world).get_inner(e2);
// OR
let query = state.query(world);
let d1 = query.get_inner(e1);
let d1 = query.get_inner(e2);
println!("{d1:?}");
println!("{d2:?}");
```
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