Consider letting `SystemState::get_manual` take an immutable reference to self? Similar to `QueryState::get_manual` take an immutable reference

[alice-i-cecile]

          > Is there a way we can make `SystemStare::get_manual` take an immutable reference to self? Similar to `QueryState::get_manual`.

At first glance, this seems very difficult to generalize. We'd have to change SystemParam::get_param to take an immutable refence to the state, which would break params that need to modify their state such as Commands or Local or EventReader.

One way of doing this would be to add a new trait for SystemParams that do not mutably access their state in get_param, and then add SystemState::get_readonly which is constrained to using this trait. This would allow most SystemParams to be used with a read-only SystemState, while excluding the param types that don't make sense. I am not exactly sure what this would look like, though.

Originally posted by @JoJoJet in #7084 (comment)

[alice-i-cecile]

Based on the follow-up discussion in #7084, I think this is probably worth doing at some point. May be tricky to get write, but I think that a ReadOnlyState trait like @JoJoJet proposes is a very plausible solution.