In Rust, what is the difference between clone() and to_owned()?

Solution 1:

.clone() returns its receiver. clone() on a &str returns a &str. If you want a String, you need a different method, which in this case is .to_owned().

For most types, clone() is sufficient because it's only defined on the underlying type and not on the reference type. But for str and [T], clone() is implemented on the reference type (&str and &[T]), and therefore it has the wrong type. It's also implemented on the owned types (String and Vec<T>), and in that case clone() will return another owned value.

Your first example works because c1 and s1 (and c2 and s2) have the same types. Your second example fails because they don't (c1 is String whereas s1 is &str). That's a perfect example of why the separate methods are necessary.

As of current Rust, both now compile, but in test_clone() c1 is a String and in test_to_owned() it's a &str. I'm pretty sure it compiles as Rust is now more lenient about automatically referencing and dereferencing values. In this particular example I believe the c1 == s1 line is compiled as though it said &*c1 == s1. If you wish to prove the types involved you can add a deliberate type error, such as let _: i32 = c1; and the error message will show the type.