Prepend trait + impl for `Vec<T>` and `String`

prepend.rs

trait Prepend<T> {
    fn prepend(&mut self, t: T);    
}

impl<S: Str> Prepend<S> for String {
    fn prepend(&mut self, s: S) {
        unsafe {
            self.as_mut_vec().prepend(s.as_slice().as_bytes());    
        }      
    }
}

impl<'a, T> Prepend<&'a [T]> for Vec<T> {
    fn prepend(&mut self, slice: &[T]) {
        use std::ptr::copy_memory;

        let old_len = self.len();

        self.reserve(slice.len());

        unsafe {
            self.set_len(old_len + slice.len());
            copy_memory(self[mut slice.len()..].as_mut_ptr(), self[..old_len].as_ptr(), old_len);
            copy_memory(self.as_mut_ptr(), slice.as_ptr(), slice.len());
        }
    }    
}

#[test]
fn test_prepend() {
    let mut vec = vec![3u64, 4, 5];
    vec.prepend([1u64, 2]);
    assert_eq!(vec[], [1u64, 2, 3, 4, 5][]);
}

#[test]
fn test_prepend_string() {
    let mut string = "World!".into_string();
    string.prepend("Hello, ");
    assert_eq!(&*string, "Hello, World!");
}