# Objective
The method `World::increment_change_tick` currently takes `&self` as the
method receiver, which is semantically strange. Even though the interior
mutability is sound, the existence of this method is strange since we
tend to think of `&World` as being a read-only snapshot of a world, not
an aliasable reference to a world with mutability. For those purposes,
we have `UnsafeWorldCell`.
## Solution
Change the method signature to take `&mut self`. Use exclusive access to
remove the need for atomic adds, which makes the method slightly more
efficient. Redirect users to [`UnsafeWorldCell::increment_change_tick`]
if they need to increment the world's change tick from an aliased
context.
In practice I don't think there will be many breakages, if any. In cases
where you need to call `increment_change_tick`, you usually already have
either `&mut World` or `UnsafeWorldCell`.
---
## Migration Guide
The method `World::increment_change_tick` now requires `&mut self`
instead of `&self`. If you need to call this method but do not have
mutable access to the world, consider using
`world.as_unsafe_world_cell_readonly().increment_change_tick()`, which
does the same thing, but is less efficient than the method on `World`
due to requiring atomic synchronization.
```rust
fn my_system(world: &World) {
// Before
world.increment_change_tick();
// After
world.as_unsafe_world_cell_readonly().increment_change_tick();
}
```
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