Insert

There are two ways to add elements to Vec<T>: insert() and push(). The former takes two parameters: an index of where to insert the element and the data. Before inserting, the position on the index will be freed by moving all succeeding elements towards the end (to the right). Therefore, if an element is inserted at the front, every element has to be shifted by one. Vec<T> code shows the following:

#[stable(feature = "rust1", since = "1.0.0")]
pub fn insert(&mut self, index: usize, element: T) {
    let len = self.len();
    assert!(index <= len);

    // space for the new element
    if len == self.buf.cap() {
        self.reserve(1);
    }

    unsafe {
        // infallible
        // The spot to put the new value
        {
            let p = self.as_mut_ptr().add(index);
            // Shift everything over to make space. (Duplicating the
            // `index`th element into two consecutive places.)
            ptr::copy(p, p.offset(1), len - index)...