`ref` and `mutable` were unified in F# 4.0. You no longer need to use a `ref` when closing over a mutable value, you can use `let mutable` in all cases and the compiler will figure it out for you. The generated code is the same as before, it's just that there's a unified syntax now.
There is an optional warning one can enable which emits when a `let mutable` is converted to a `ref` behind the scenes - for those who want to by hyper-aware of hidden allocations.
