Cow::clone should do shallow copies?

[brson]

In a recent PR it was pointed out that cloning a Cow shouldn't ever need to allocate - the cloned Cow could borrow a pointer to the original.

[bluss]

No, this is not possible

impl<'a, T> Clone for Cow<'a, T> {
    fn clone<'b>(&'b self) -> Cow<'a, T> { ... }
}

The owned variant can only be borrowed for the scope 'b, not the scope 'a that is required.

[alexcrichton]

Aha excellent point!