Right. I advise people who don't think that anything should be owned by the lock make a marker type - call it for example struct MacGuffin - with no data in it,

Right. I advise people who don't think that anything should be owned by the lock make a marker type - call it for example struct MacGuffin - with no data in it, just to mark that we took the lock. Firstly they can now write APIs which express ideas like "You can't call this unless you have the lock" (that's a MacGuffin parameter) but then also very often they discover actually there is mutable data that they only want when they've taken the lock, and hence have MacGuffin, and hey, we can just put it in the MacGuffin type. So long as MacGuffin doesn't have any data in it, it essentially evaporates at runtime. This is a zero size type (ZST) so we can store that in no bytes of RAM, we can use no registers at all to pass it into a function, and so on. If you're a C programmer this doesn't make sense because you don't have ZSTs, all your types take up at least one byte, but Rust isn't like that.