Problem

Given a &World, I cannot issue a query that only reads from the world.

There are two possible methods on World for this: query, query_filtered, plus SystemState::new(). All of them require &mut World, as they could provide mutating type parameters.

Proposed Solutions

Quick-and-dirty

Make read-only variants of all three methods.

Ergonomic

Add a system_state and read_only_system_state method to World, which generates a SystemState and then immediately calls .get_mut / .get on it.

This allows users to do things like:

let query = world.read_only_system_state::<Query<(&Foo, &Bar), With<Bat>>>();

for (foo, bar) in query.iter(){

}