Clone on Resize - An alternative to the Cow type, keep a mutable slice until you really need to grow or shrink it.

cor.rs

pub enum Cor<'a, T: 'a> {
    Borrowed(&'a mut [T]),
    Owned(Vec<T>),
}

impl<T: Clone> Cor<'_, T> {
    pub fn as_resizable(&mut self) -> &mut Vec<T> {
        match self {
            Cor::Borrowed(slice) => {
                *self = Cor::Owned(slice.to_vec());
                match self {
                    Cor::Borrowed(..) => unreachable!(),
                    Cor::Owned(ref mut vec) => vec,
                }
            },
            Cor::Owned(ref mut vec) => vec,
        }
    }
  
    pub fn into_resizable(self) -> Vec<T> {
        match self {
            Cor::Borrowed(slice) => slice.to_vec(),
            Cor::Owned(vec) => vec,
        }
    }
}

impl<T> AsRef<[T]> for Cor<'_, T> {
    fn as_ref(&self) -> &[T] {
        match self {
            Cor::Borrowed(slice) => slice,
            Cor::Owned(vec) => vec.as_ref(),
        }
    }
}

impl<T> AsMut<[T]>  for Cor<'_, T> {
    fn as_mut(&mut self) -> &mut [T] {
        match self {
            Cor::Borrowed(slice) => slice,
            Cor::Owned(vec) => vec.as_mut(),
        }
    }
}