fn clone_into(&self, target: &mut Self::Owned) {
        *target = self.to_owned();
    }
A clone-on-write smart pointer.

The type `Cow` is a smart pointer providing clone-on-write functionality: it
can enclose and provide immutable access to borrowed data, and clone the
data lazily when mutation or ownership is required. The type is designed to
work with general borrowed data via the `Borrow` trait.

`Cow` implements `Deref`, which means that you can call
non-mutating methods directly on the data it encloses. If mutation
is desired, `to_mut` will obtain a mutable reference to an owned
value, cloning if necessary.

If you need reference-counting pointers, note that
[`Rc::make_mut`][crate::rc::Rc::make_mut] and
[`Arc::make_mut`][crate::sync::Arc::make_mut] can provide clone-on-write
functionality as well.

# Examples

```
use std::borrow::Cow;

fn abs_all(input: &mut Cow<[i32]>) {
for i in 0..input.len() {
let v = input[i];
if v < 0 {
// Clones into a vector if not already owned.
input.to_mut()[i] = -v;
}
}
}

// No clone occurs because `input` doesn't need to be mutated.
let slice = [0, 1, 2];
let mut input = Cow::from(&slice[..]);
abs_all(&mut input);

// Clone occurs because `input` needs to be mutated.
let slice = [-1, 0, 1];
let mut input = Cow::from(&slice[..]);
abs_all(&mut input);

// No clone occurs because `input` is already owned.
let mut input = Cow::from(vec![-1, 0, 1]);
abs_all(&mut input);
```

Another example showing how to keep `Cow` in a struct:

```
use std::borrow::Cow;

struct Items<'a, X: 'a> where [X]: ToOwned<Owned = Vec<X>> {
values: Cow<'a, [X]>,
}

impl<'a, X: Clone + 'a> Items<'a, X> where [X]: ToOwned<Owned = Vec<X>> {
fn new(v: Cow<'a, [X]>) -> Self {
Items { values: v }
}
}

// Creates a container from borrowed values of a slice
let readonly = [1, 2];
let borrowed = Items::new((&readonly[..]).into());
match borrowed {
Items { values: Cow::Borrowed(b) } => println!("borrowed {:?}", b),
_ => panic!("expect borrowed value"),
}

let mut clone_on_write = borrowed;
// Mutates the data from slice into owned vec and pushes a new value on top
clone_on_write.values.to_mut().push(3);
println!("clone_on_w
benchmarks/haystacks/code/rust-library.rs