# Objective

`flush_and_reserve_invalid_assuming_no_entities` was made for the old
rendering world (which was reset every frame) and is usused since the
0.15 retained rendering world, but wasn't removed yet. It is pub, but is
undocumented apart from the safety comment.

## Solution

Remove `flush_and_reserve_invalid_assuming_no_entities` and the safety
invariants this method required for `EntityMeta`, `EntityLocation`,
`TableId` and `TableRow`. This reduces the amount of unsafe code &
safety invariants and makes #16047 easier.

## Alternatives
- Document `flush_and_reserve_invalid_assuming_no_entities` and keep it
unchanged
- Document `flush_and_reserve_invalid_assuming_no_entities` and change
it to be based on `EntityMeta::INVALID`


## Migration Guide
- exchange `Entities::flush_and_reserve_invalid_assuming_no_entities`
for `reserve` and `flush_as_invalid` and notify us if that's
insufficient

---------

Co-authored-by: Benjamin Brienen <[email protected]>

# Objective

`flush_and_reserve_invalid_assuming_no_entities` was made for the old
rendering world (which was reset every frame) and is usused since the
0.15 retained rendering world, but wasn't removed yet. It is pub, but is
undocumented apart from the safety comment.

## Solution

Remove `flush_and_reserve_invalid_assuming_no_entities` and the safety
invariants this method required for `EntityMeta`, `EntityLocation`,
`TableId` and `TableRow`. This reduces the amount of unsafe code &
safety invariants and makes bevyengine#16047 easier.

## Alternatives
- Document `flush_and_reserve_invalid_assuming_no_entities` and keep it
unchanged
- Document `flush_and_reserve_invalid_assuming_no_entities` and change
it to be based on `EntityMeta::INVALID`


## Migration Guide
- exchange `Entities::flush_and_reserve_invalid_assuming_no_entities`
for `reserve` and `flush_as_invalid` and notify us if that's
insufficient

---------

Co-authored-by: Benjamin Brienen <[email protected]>

# Objective

Expand `track_change_detection` feature to also track entity spawns and
despawns. Use this to create better error messages.

# Solution

Adds `Entities::entity_get_spawned_or_despawned_by` as well as `{all
entity reference types}::spawned_by`.

This also removes the deprecated `get_many_entities_mut` & co (and
therefore can't land in 0.15) because we don't yet have no Polonius.

## Testing

Added a test that checks that the locations get updated and these
updates are ordered correctly vs hooks & observers.

---

## Showcase

Access location:
```rust
let mut world = World::new();
let entity = world.spawn_empty().id();
println!("spawned by: {}", world.entity(entity).spawned_by());
```
```
spawned by: src/main.rs:5:24
```
Error message (with `track_change_detection`):
```rust
world.despawn(entity);
world.entity(entity);
```
```
thread 'main' panicked at src/main.rs:11:11:
Entity 0v1#4294967296 was despawned by src/main.rs:10:11
```
and without:
```
thread 'main' panicked at src/main.rs:11:11:
Entity 0v1#4294967296 does not exist (enable `track_change_detection` feature for more details)
```
Similar error messages now also exists for `Query::get`,
`World::entity_mut`, `EntityCommands` creation and everything that
causes `B0003`, e.g.
```
error[B0003]: Could not insert a bundle (of type `MaterialMeshBundle<StandardMaterial>`) for entity Entity { index: 7, generation: 1 }, which was despawned by src/main.rs:10:11. See: https://bevyengine.org/learn/errors/#b0003
```

---------

Co-authored-by: kurk070ff <[email protected]>
Co-authored-by: Freya Pines <[email protected]>
Co-authored-by: Freya Pines <[email protected]>
Co-authored-by: Matty Weatherley <[email protected]>

# Objective

Expand `track_change_detection` feature to also track entity spawns and
despawns. Use this to create better error messages.

# Solution

Adds `Entities::entity_get_spawned_or_despawned_by` as well as `{all
entity reference types}::spawned_by`.

This also removes the deprecated `get_many_entities_mut` & co (and
therefore can't land in 0.15) because we don't yet have no Polonius.

## Testing

Added a test that checks that the locations get updated and these
updates are ordered correctly vs hooks & observers.

---

## Showcase

Access location:
```rust
let mut world = World::new();
let entity = world.spawn_empty().id();
println!("spawned by: {}", world.entity(entity).spawned_by());
```
```
spawned by: src/main.rs:5:24
```
Error message (with `track_change_detection`):
```rust
world.despawn(entity);
world.entity(entity);
```
```
thread 'main' panicked at src/main.rs:11:11:
Entity 0v1#4294967296 was despawned by src/main.rs:10:11
```
and without:
```
thread 'main' panicked at src/main.rs:11:11:
Entity 0v1#4294967296 does not exist (enable `track_change_detection` feature for more details)
```
Similar error messages now also exists for `Query::get`,
`World::entity_mut`, `EntityCommands` creation and everything that
causes `B0003`, e.g.
```
error[B0003]: Could not insert a bundle (of type `MaterialMeshBundle<StandardMaterial>`) for entity Entity { index: 7, generation: 1 }, which was despawned by src/main.rs:10:11. See: https://bevyengine.org/learn/errors/#b0003
```

---------

Co-authored-by: kurk070ff <[email protected]>
Co-authored-by: Freya Pines <[email protected]>
Co-authored-by: Freya Pines <[email protected]>
Co-authored-by: Matty Weatherley <[email protected]>