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Shorten the 'world lifetime returned from QueryLens::query(). #17694

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Merged
merged 2 commits into from
Feb 12, 2025
Merged

Shorten the 'world lifetime returned from QueryLens::query(). #17694

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63a5e84
Shorten the 'world lifetime returned from `QueryLens::query()`.
chescock Feb 5, 2025
80147ff
Add compile-fail test.
chescock Feb 6, 2025
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24 changes: 24 additions & 0 deletions crates/bevy_ecs/compile_fail/tests/ui/query_lens_lifetime_safety.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,24 @@
use bevy_ecs::prelude::*;
use bevy_ecs::system::SystemState;

#[derive(Component, Eq, PartialEq, Debug)]
struct Foo(u32);

fn main() {
let mut world = World::default();
let e = world.spawn(Foo(10_u32)).id();

let mut system_state = SystemState::<Query<&mut Foo>>::new(&mut world);
{
let mut query = system_state.get_mut(&mut world);
let mut lens = query.as_query_lens();
dbg!("hi");
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Did you mean to leave these in?

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I copied the skeleton from https://github.com/bevyengine/bevy/blob/f2a65c2dd3a3ef75964a8aaed67e1f24618bf19f/crates/bevy_ecs/compile_fail/tests/ui/query_lifetime_safety.rs . So, I did mean to leave them there, but not for any particularly good reason :). I'm inclined to leave it like this for consistency with the other file, but I'm happy to change it if there are strong objections.

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Huh, I must have got my search filtering wrong because I thought none of the other compile-fail tests had dbg in them, feel free to leave it then.

{
let mut data: Mut<Foo> = lens.query().get_inner(e).unwrap();
let mut data2: Mut<Foo> = lens.query().get_inner(e).unwrap();
//~^ E0499
assert_eq!(&mut *data, &mut *data2); // oops UB
}
dbg!("bye");
}
}
5 changes: 5 additions & 0 deletions crates/bevy_ecs/compile_fail/tests/ui/query_lens_lifetime_safety.stderr
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@@ -0,0 +1,5 @@
error[E0499]: cannot borrow `lens` as mutable more than once at a time
--> tests/ui\query_lens_lifetime_safety.rs:18:39
|
17 | let mut data: Mut<Foo> = lens.query().get_inner(e).unwrap();
| ---- first mutable borrow occurs here
20 changes: 17 additions & 3 deletions crates/bevy_ecs/src/system/query.rs
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Expand Up @@ -2097,7 +2097,21 @@ pub struct QueryLens<'w, Q: QueryData, F: QueryFilter = ()> {

impl<'w, Q: QueryData, F: QueryFilter> QueryLens<'w, Q, F> {
/// Create a [`Query`] from the underlying [`QueryState`].
pub fn query(&mut self) -> Query<'w, '_, Q, F> {
pub fn query(&mut self) -> Query<'_, '_, Q, F> {
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@cBournhonesque cBournhonesque Feb 12, 2025 •
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So the new flow is

  1. query -> Query<'w, s'>
  2. lens = &'a query.as_query_lens() -> QueryLens<'a>
  3. lens.query() -> Query<'a, 'a>
  4. get_inner() -> QueryItem<'a>

and the old one is

  1. lens.query() -> Query<'w, 'a>
  2. get_inner() -> QueryItem<'w>

So in the old case, the item we get only depends on the 'w lifetime, so the compiler doesn't stop us from creating a second object with the 'a lifetime via lens.query().

I think the change makes sense; out of curoisity, would this PR #15396 also work to fix your compile-fail test?
Since the QueryItem we would get would have been QueryItem<'w, 'a>, we might not be able to call lens.query() a second time because the 'a lifetime is present in the return value

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I did't completely follow your notation, but your conclusion sounds right! We had an &'a mut QueryLens<'w>, and currently that gives a Query<'w, 'a> which we can use with get_inner() to get a QueryItem<'w>, but with this change it gives a Query<'a, 'a> that we can only use to get a QueryItem<'a>.

I believe the compile-fail test would still compile with #15396, because the compiler would see that the concrete type of QueryItem<'w, 'a> is Mut<'w, Foo>, which doesn't involve the 'a lifetime.

Query {
world: self.world,
state: &self.state,
last_run: self.last_run,
this_run: self.this_run,
}
}
}

impl<'w, Q: ReadOnlyQueryData, F: QueryFilter> QueryLens<'w, Q, F> {
/// Create a [`Query`] from the underlying [`QueryState`].
/// This returns results with the actual "inner" world lifetime,
/// so it may only be used with read-only queries to prevent mutable aliasing.
pub fn query_inner(&self) -> Query<'w, '_, Q, F> {
Query {
world: self.world,
state: &self.state,
Expand All @@ -2108,9 +2122,9 @@ impl<'w, Q: QueryData, F: QueryFilter> QueryLens<'w, Q, F> {
}

impl<'w, 's, Q: QueryData, F: QueryFilter> From<&'s mut QueryLens<'w, Q, F>>
for Query<'w, 's, Q, F>
for Query<'s, 's, Q, F>
{
fn from(value: &'s mut QueryLens<'w, Q, F>) -> Query<'w, 's, Q, F> {
fn from(value: &'s mut QueryLens<'w, Q, F>) -> Query<'s, 's, Q, F> {
value.query()
}
}
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