> But isn't it an anti-pattern to wrap something in an `Arc` when some of your use cases for it are purely single-threaded, and as such you're wasting a certain

> But isn't it an anti-pattern to wrap something in an `Arc` when some of your use cases for it are purely single-threaded, and as such you're wasting a certain (potentially very significant) amount of performance on upholding memory visibility and such even when it's not needed? Yes. You have Rc and Rc::make_mut for the single threaded case. Pick what you need. I interpreted the GP comment as talking about the general pattern of make_mut, not Arc::make_mut itself. Stylo's multithreaded, we pick Arc. Singlethreaded systems can get by with Rc, but there are often legit reasons for picking Arc, and it's not that bad a hit.