Arc get_mut

/// 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
        }
    }