A mutable reference is unique, so it can indeed not be copied, and the for loop will consume it. However, it is possible to reborrow a mutable reference, which creates a sub-mutable reference to the same thing. While the reborrow exists, the original is not usable to enforce the uniqueness.
To reborrow, use &mut *the_mut_ref.
for n in &mut *v {
...
}
Note that reborrows happens automatically whenever you use a mutable reference in a place that can only accept a mutable reference, but for loops can accept many types, including non-mutable references, so it doesn't happen here.