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[Merged by Bors] - Fix unsound EntityMut::remove_children. Add EntityMut::world_scope #6464

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[Merged by Bors] - Fix unsound EntityMut::remove_children. Add EntityMut::world_scope #6464

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4dc6e04
Well, that was a lie.
tim-blackbird Nov 1, 2022
06f2774
Remove unhelpful safety comments.
tim-blackbird Nov 1, 2022
5fe9581
reworded
tim-blackbird Nov 1, 2022
9483f1a
reworded again
tim-blackbird Nov 4, 2022
4453650
remove unnecessary unsafe
tim-blackbird Nov 4, 2022
201a3f9
Add `EntityMut::world_scope`
tim-blackbird Nov 4, 2022
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8 changes: 7 additions & 1 deletion crates/bevy_ecs/src/world/entity_ref.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -526,7 +526,7 @@ impl<'w> EntityMut<'w> {

/// Returns this `EntityMut`'s world.
///
/// See [`EntityMut::into_world_mut`] for a safe alternative.
/// See [`EntityMut::world_scope`] or [`EntityMut::into_world_mut`] for a safe alternative.
///
/// # Safety
/// Caller must not modify the world in a way that changes the current entity's location
Expand All @@ -543,6 +543,12 @@ impl<'w> EntityMut<'w> {
self.world
}

/// Gives mutable access to this `EntityMut`'s [`World`] in a temporary scope.
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Might benefit from explaining why you'd want to use it.

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It's the only way of safely getting a &mut World from an EntityMut without consuming self. Which sort of speaks for itself?
I'm not sure what to add to the docs that could make that clearer (Aside from being very pedantic) :)

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You could discuss update_location here, but that's very gory-internals, and likely to go out of date in dangerous ways. If people care, they can read the code.

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I was thinking: someone reading the doc doesn't necessarily see the source code. If they encounter this method, they may ask themselves why its here. But I agree with alice, it's really a minor thing and really depends on internals.

pub fn world_scope(&mut self, f: impl FnOnce(&mut World)) {
f(self.world);
self.update_location();
}

/// Updates the internal entity location to match the current location in the internal
/// [`World`]. This is only needed if the user called [`EntityMut::world`], which enables the
/// location to change.
Expand Down
35 changes: 11 additions & 24 deletions crates/bevy_hierarchy/src/child_builder.rs
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Expand Up @@ -456,31 +456,23 @@ pub trait BuildWorldChildren {

impl<'w> BuildWorldChildren for EntityMut<'w> {
fn with_children(&mut self, spawn_children: impl FnOnce(&mut WorldChildBuilder)) -> &mut Self {
{
let entity = self.id();
let entity = self.id();
self.world_scope(|world| {
let mut builder = WorldChildBuilder {
current_entity: None,
parent_entities: vec![entity],
// SAFETY: self.update_location() is called below. It is impossible to make EntityMut
// function calls on `self` within the scope defined here
world: unsafe { self.world_mut() },
world,
};

spawn_children(&mut builder);
}
self.update_location();
});
self
}

fn push_children(&mut self, children: &[Entity]) -> &mut Self {
let parent = self.id();
{
// SAFETY: parent entity is not modified and its location is updated manually
let world = unsafe { self.world_mut() };
self.world_scope(|world| {
update_old_parents(world, parent, children);
// Inserting a bundle in the children entities may change the parent entity's location if they were of the same archetype
self.update_location();
}
});
if let Some(mut children_component) = self.get_mut::<Children>() {
children_component
.0
Expand All @@ -494,14 +486,9 @@ impl<'w> BuildWorldChildren for EntityMut<'w> {

fn insert_children(&mut self, index: usize, children: &[Entity]) -> &mut Self {
let parent = self.id();
{
// SAFETY: parent entity is not modified and its location is updated manually
let world = unsafe { self.world_mut() };
self.world_scope(|world| {
update_old_parents(world, parent, children);
// Inserting a bundle in the children entities may change the parent entity's location if they were of the same archetype
self.update_location();
}

});
if let Some(mut children_component) = self.get_mut::<Children>() {
children_component
.0
Expand All @@ -515,9 +502,9 @@ impl<'w> BuildWorldChildren for EntityMut<'w> {

fn remove_children(&mut self, children: &[Entity]) -> &mut Self {
let parent = self.id();
// SAFETY: This doesn't change the parent's location
let world = unsafe { self.world_mut() };
remove_children(parent, children, world);
self.world_scope(|world| {
remove_children(parent, children, world);
});
self
}
}
Expand Down
8 changes: 2 additions & 6 deletions crates/bevy_hierarchy/src/hierarchy.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -104,7 +104,7 @@ impl<'w, 's, 'a> DespawnRecursiveExt for EntityCommands<'w, 's, 'a> {

impl<'w> DespawnRecursiveExt for EntityMut<'w> {
/// Despawns the provided entity and its children.
fn despawn_recursive(mut self) {
fn despawn_recursive(self) {
let entity = self.id();

#[cfg(feature = "trace")]
Expand All @@ -114,11 +114,7 @@ impl<'w> DespawnRecursiveExt for EntityMut<'w> {
)
.entered();

// SAFETY: EntityMut is consumed so even though the location is no longer
// valid, it cannot be accessed again with the invalid location.
unsafe {
despawn_with_children_recursive(self.world_mut(), entity);
}
despawn_with_children_recursive(self.into_world_mut(), entity);
}

fn despawn_descendants(&mut self) {
Expand Down