Fix soundness of UnsafeWorldCell usage example

[SludgePhD]

Objective

• The example in the docs is unsound.

Demo:

#[derive(Resource)]
struct MyRes(u32);

fn main() {
    let mut w = World::new();
    w.insert_resource(MyRes(0));

    let (mut res, comp) = split_world_access(&mut w);
    let mut r1 = res.get_resource_mut::<MyRes>().unwrap();
    let mut r2 = res.get_resource_mut::<MyRes>().unwrap();

    *r1 = MyRes(1);
    *r2 = MyRes(2);
}

The API in the example allows aliasing mutable references to the same resource. Miri also complains when running this.

Solution

• Change the example API to make the returned Mut borrow from the OnlyResourceAccessWorld instead of borrowing from the world via 'w. This prevents obtaining more than one Mut at the same time from it.

[alice-i-cecile]

Great catch, thank you.