What's the point of this change? I'm not sure that this code is better than the original... it requires understanding _memoize, and the original code seemed clear enough.

_memoize makes sense to use when we need to memoize multiple different calculations. In this case, we only need to cache one thing at a time, so I'm not sure it's useful.

I could see an argument that we already need to understand _memoize to understand this file. But my intuition is that this diff doesn't really result in any meaningful improvements. Curious for other reviewers' takes.

My primary motivation for this diff was to align with the rules.react.js style. We use _memoize for almost all variables/functions defined in this file. I felt that storing getDefaultTextMessageRules in a variable, instead of following this file convention, was making this piece of code somewhat outdated. While using _memoize might not lead to significant improvements, it does, in my opinion, present a better aesthetic. However, I'm open to other reviewers' perspectives and suggestions.

Although this change on its own doesn't have important disadvantages, the following diff D8846 makes it worse. The consequence is that we're wasting memory because _memoize remembers the result of each computation, while we only care about the most recent one. So it's better to reintroduce the previous approach.