Answer by PitaJ for Create owned value on demand

Here's how to use Cow in your case:

use std::borrow::Cow;let condition = false;let a: String = "a".to_string();let r: Cow<str> = if condition {    Cow::from(&a)} else {    let b: String = "b".to_string();    Cow::from(b)};

For your second example, you have to add Clone. For an arbitrary clone type, you also have to instantiate the variants directly.

use std::borrow::Cow;#[derive(Clone)]struct S(i32);let condition = false;let a: S = S(0);let r: Cow<S> = if condition {    Cow::Borrowed(&a)} else {    let b: S = S(1);    Cow::Owned(b)};

And this does mean that you can use Cow<String> in the same way.

use std::borrow::Cow;let condition = false;let a: String = "a".to_string();let r: Cow<String> = if condition {    Cow::Borrowed(&a)} else {    let b: String = "b".to_string();    Cow::Owned(b)};

It's better to use Cow<str> for the same reason that it's better to use &str instead of &String: &str has a smaller representation in memory (one fewer pointer-sized field), and has all of the same capabilities.