nomicon/src/vec/vec-insert-remove.md

Insert and Remove

Something not provided by slice is insert and remove, so let's do those next.

Insert needs to shift all the elements at the target index to the right by one. To do this we need to use ptr::copy, which is our version of C's memmove. This copies some chunk of memory from one location to another, correctly handling the case where the source and destination overlap (which will definitely happen here).

If we insert at index i, we want to shift the [i .. len] to [i+1 .. len+1] using the old len.

pub fn insert(&mut self, index: usize, elem: T) {
    // Note: `<=` because it's valid to insert after everything
    // which would be equivalent to push.
    assert!(index <= self.len, "index out of bounds");
    if self.cap == self.len { self.grow(); }

    unsafe {
        // ptr::copy(src, dest, len): "copy from src to dest len elems"
        ptr::copy(self.ptr.as_ptr().add(index),
                  self.ptr.as_ptr().add(index + 1),
                  self.len - index);
        ptr::write(self.ptr.as_ptr().add(index), elem);
        self.len += 1;
    }
}

Remove behaves in the opposite manner. We need to shift all the elements from [i+1 .. len + 1] to [i .. len] using the new len.

pub fn remove(&mut self, index: usize) -> T {
    // Note: `<` because it's *not* valid to remove after everything
    assert!(index < self.len, "index out of bounds");
    unsafe {
        self.len -= 1;
        let result = ptr::read(self.ptr.as_ptr().add(index));
        ptr::copy(self.ptr.as_ptr().add(index + 1),
                  self.ptr.as_ptr().add(index),
                  self.len - index);
        result
    }
}