Third Rule of Hooks: "Side effects may only be caused in useEffect and event handlers"

I'd like to propose a third rule of hooks.

In the section "Advanced Guides > Strict Mode" it states that

Because the above methods might be called more than once, it’s important that they do not contain side-effects. Ignoring this rule can lead to a variety of problems, including memory leaks and invalid application state.

If I'm not mistaken, the inverse of this statement is that side effects based on a certain state of states shall be triggered in a useEffect, and that side effects from user interaction shall be triggered directly in the event handler. Everything else shall be side effect free. I think that adding this to the "rules of hooks" section makes it easier to remember and gives it much more visibility. I think the "variety of problems" will especially grow when concurrent mode is introduced. By having a "checklist" that can be used during code reviews etc. makes it easier to avoid this kind of problems.

Also one could maybe repeat the "Do not mutate state" rule there, cause this is still a very common problem and having it multiple times in the docs does no harm in my eyes.

What do you think?

Start with the linked Strict Mode guide and the Rules of Hooks section. Compare the proposed side-effect guidance and repeated state-mutation rule with the existing documentation; done means the wording and placement are agreed on and the relevant docs are updated.