pub fn iter_mut(&mut self) -> IterMut<'_, T> {
        // SAFETY: The internal `IterMut` safety invariant is established because the
        // `ring` we create is a dereferencable slice for lifetime '_.
        IterMut {
            tail: self.tail,
            head: self.head,
            ring: ptr::slice_from_raw_parts_mut(self.ptr(), self.cap()),
            phantom: PhantomData,
        }
    }
Returns the number of elements in the `VecDeque`.

# Examples

```
use std::collections::VecDeque;

let mut v = VecDeque::new();
assert_eq!(v.len(), 0);
v.push_back(1)
benchmarks/haystacks/code/rust-library.rs