> Rust closures that capture data are fat pointers { fn, data }

This isn’t fully accurate. In your example, `&mut C` actually has the same layout as usize. It’s not a fat pointer. `C` is a concrete type and essentially just an anonymous struct with FnMut implemented for it.

You’re probably thinking of `&mut dyn FnMut` which is a fat pointer that pairs a pointer to the data with a pointer to a VTable.

So in your specific example, the double indirection is unnecessary.

The following passes miri: https://play.rust-lang.org/?version=nightly&mode=debug&editi...

(did this on mobile, so please excuse any messiness).