nomicon/src/vec/vec-deref.md

Deref

Alright! We've got a decent minimal stack implemented. We can push, we can pop, and we can clean up after ourselves. However there's a whole mess of functionality we'd reasonably want. In particular, we have a proper array, but none of the slice functionality. That's actually pretty easy to solve: we can implement Deref<Target=[T]>. This will magically make our Vec coerce to, and behave like, a slice in all sorts of conditions.

All we need is slice::from_raw_parts. It will correctly handle empty slices for us. Later once we set up zero-sized type support it will also Just Work for those too.

use std::ops::Deref;

impl<T> Deref for Vec<T> {
    type Target = [T];
    fn deref(&self) -> &[T] {
        unsafe {
            std::slice::from_raw_parts(self.ptr.as_ptr(), self.len)
        }
    }
}

And let's do DerefMut too:

use std::ops::DerefMut;

impl<T> DerefMut for Vec<T> {
    fn deref_mut(&mut self) -> &mut [T] {
        unsafe {
            std::slice::from_raw_parts_mut(self.ptr.as_ptr(), self.len)
        }
    }
}

Now we have len, first, last, indexing, slicing, sorting, iter, iter_mut, and all other sorts of bells and whistles provided by slice. Sweet!