Require read-only queries in QueryState::par_iter

[JoJoJet]

Objective

The method QueryState::par_iter does not currently force the query to be read-only. This means you can unsoundly mutate a world through an immutable reference in safe code.

fn bad_system(world: &World, mut query: Local<QueryState<&mut T>>) {
    query.par_iter(world).for_each_mut(|mut x| *x = unsoundness);
}

Solution

Use read-only versions of the WorldQuery types.

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Migration Guide

The function QueryState::par_iter now forces any world accesses to be read-only, similar to how QueryState::iter works. Any code that previously mutated the world using this method was unsound. If you need to mutate the world, use par_iter_mut instead.