How to Use Cow<T> (Clone on Write) in Rust

Rust does not have a built-in Cow<T> type for Clone-on-Write; you must use the standard library's std::borrow::Cow to achieve this pattern. Import Cow and use Cow::Borrowed for read-only access or Cow::Owned when you need to modify the data, letting the compiler handle the clone automatically if you mutate a borrowed value.

use std::borrow::Cow;

fn main() {
    let mut s: Cow<str> = Cow::Borrowed("hello");
    s.to_mut(); // Clones to owned if needed
    s.push_str(" world");
    println!("{s}");
}

Cow stands for Clone-on-Write and lets you hold data that might be borrowed or owned without knowing which one it is until you try to change it. It saves memory by avoiding a copy if you only read the data, but automatically makes a copy if you need to edit it. Think of it like a library book: you can read it without owning it, but if you want to write notes in it, you must buy your own copy first.