Function that accepts either owned data of type T or a ref to data of type T and returns original data or clone of referenced data respectively

Question

How can you write a function that accepts either data of type T or a reference to data of type T, and returns either the original data or a clone of the referenced data respectively? Something like this:

fn foo(input: T or &T) -> T {
    if input is &T {
        input.clone()
    } else {
        input
    }
}

Answer 1

You want to pass in something that can be converted into Cow:

use std::borrow::Cow;

fn main() {
    let owned = String::from("Hello world.");
    let _clone = owned.clone();
    let borrowed = &_clone;
    assert_eq!(borrowed_or_owned(owned), "Hello world.".to_string());
    assert_eq!(borrowed_or_owned(borrowed), "Hello world.".to_string());
}

fn borrowed_or_owned<'a, T>(value: impl Into<Cow<'a, T>>) -> <T as ToOwned>::Owned
where
    T: ToOwned + 'a + ?Sized,
{
    value.into().into_owned()
}

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