beef

Alternative implementation of Cow that's more compact in memory.

use beef::Cow;

let borrowed = Cow::borrowed("Hello");
let owned = Cow::from(String::from("World"));

assert_eq!(
    format!("{} {}!", borrowed, owned),
    "Hello World!",
);

// beef::Cow is 3 word sized, while std::borrow::Cow is 4 word sized
assert!(std::mem::size_of::<Cow<str>>() < std::mem::size_of::<std::borrow::Cow<str>>());

How does it work?

The standard library Cow is an enum with two variants:

pub enum Cow<'a, B> where
    B: 'a + ToOwned + ?Sized,
{
    Borrowed(&'a B),
    Owned(<B as ToOwned>::Owned),
}

For the most common pairs of values - &str and String, or &[u8] and Vec<u8> - this means that the entire enum is 4 words wide:

                                             Padding
                                                |
                                                v
          +----------+----------+----------+----------+
Borrowed: | Tag      | Pointer  | Length   | XXXXXXXX |
          +----------+----------+----------+----------+

          +----------+----------+----------+----------+
Owned:    | Tag      | Pointer  | Length   | Capacity |
          +----------+----------+----------+----------+

Instead of being an enum with a tag, beef::Cow uses capacity to determine whether the value it's holding is owned (capacity is greater than 0), or borrowed (capacity is 0).