A data race that doesn't compile

“The second condition falls out of the first”. Hi claude!

You're right about the terminology, my title oversells it. There's actually no concurrent mutation at all: each reducer reads the shared state and returns a fresh slice, and the pieces are stitched back together after. What the compile-time check really guarantees is that every state slice has exactly one reducer. That's what makes running them in parallel safe without a lock.

1. Well no actually, the internal project was in Python, I wanted a playground project inspired by my experience to learn Rust. On macros: I know derive could cut the boilerplate, but I wanted to understand the underlying mechanism first, so I wrote the type-level part by hand. The compile-time guarantee comes from those bounds, not from the macro, so learning that part felt more useful than hiding it. Derive is on my list for later.

2. On the writing: I drafted with Claude (noted at the bottom of the post) and I agree that I didn't edit the tone down enough. Not an excuse, but with a full-time job and two kids I'd rather spend my time on the ideas than the prose. I'll be more careful next time.

I stopped reading at "Here’s the observation that started everything:".

No kidding, it drives me crazy. The tone is always so authoritative and dramatic, like every fact is a mic drop moment.

I need to get back to writing some of my own articles I've been meaning to write, as a palate cleanser.