/// Returns a mutable reference into the given `Arc`, if there are
/// no other `Arc` or [`Weak`] pointers to the same allocation.
///
/// Returns [`None`] otherwise, because it is not safe to
/// mutate a shared value.
///
/// See also [`make_mut`][make_mut], which will [`clone`][clone]
/// the inner value when there are other `Arc` pointers.
///
/// [make_mut]: Arc::make_mut
/// [clone]: Clone::clone
///
/// # Examples
///
/// ```
/// use std::sync::Arc;
///
/// let mut x = Arc::new(3);
/// *Arc::get_mut(&mut x).unwrap() = 4;
/// assert_eq!(*x, 4);
///
/// let _y = Arc::clone(&x);
/// assert!(Arc::get_mut(&mut x).is_none());
/// ```
#[inline]
#[stable(feature = "arc_unique", since = "1.4.0")]
pub fn get_mut(this: &mut Self) -> Option<&mut T> {
if this.is_unique() {
// This unsafety is ok because we're guaranteed that the pointer
// returned is the *only* pointer that will ever be returned to T. Our
// reference count is guaranteed to be 1 at this point, and we required
// the Arc itself to be `mut`, so we're returning the only possible
// reference to the inner data.
unsafe { Some(Arc::get_mut_unchecked(this)) }
} else {
None
}
}