eslint-plugin-react-hooks: lint rule to enforce that `useMemo` returns a value

Nothing prevents developers from calling useMemo without a return:

import { useMemo } from "react";

useMemo(() => {
    console.log("Yippee!");
}, []);

[TypeScript playground showing no errors]

Calling useMemo without a returned value is somewhat equivalent to calling useEffect without a returned value. Judging by the few times I've seen it happen, it's likely a case of a developer either confusing the two hooks or forgetting to return the memoized value.

Suggestion: can we add a lint rule to flag useMemo calls that don't definitely return a value? It could have a suggestion fixer to switch to useEffect.

Start in the eslint-plugin-react-hooks package by reading its existing hook rules and their tests. Determine how the rule should identify useMemo callbacks that do not definitely return a value, and whether the optional useEffect suggestion is in scope. Done means covered cases are reported consistently with tests for valid and invalid callbacks.