In #1277 we added a warning for use mutable x = ...

However it turns out that use mutable ... is useful in F# when the disposable type is a struct. This occurs when an iterator is a struct type, e.g.

open System.Collections.Generic

let f (x: List<'T>) = 
    use mutable d = x.GetEnumerator()
    while (d.MoveNext() ) do 
       ()
    ()

We should revert the warning on use mutable for this reason