The Complete Rust Programming Reference Guide

Cow

Cow is a smart pointer type that provides two versions of strings. It stands for Clone on Write. It has the following type signature:

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

First, we have the two variants:

• Borrowed that represents the borrowed version of some type B. This B has to implement the ToOwned trait.
• There is also owned variant which contains the owned version of the type.

This type is suitable for cases where one needs to avoid allocations where it's not needed. A real world example is the JSON parser crate called serde_json.