How do I convert the "largest value in a Vec" example in the Rust book to not use the Copy trait?

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I'm trying to accomplish an exercise "left to the reader" in the 2018 Rust book. The example they have, 10-15, uses the Copy trait. However, they recommend implementing the same without Copy and I've been really struggling with it.

Without Copy, I cannot use largest = list[0]. The compiler recommends using a reference instead. I do so, making largest into a &T. The compiler then complains that the largest used in the comparison is a &T, not T, so I change it to *largest to dereference the pointer. This goes fine, but then stumbles on largest = item, with complaints about T instead of &T. I switch to largest = &item. Then I get an error I cannot deal with:

error[E0597]: `item` does not live long enough
 --> src/main.rs:6:24
  |
6 |             largest = &item;
  |                        ^^^^ borrowed value does not live long enough
7 |         }
8 |     }
  |     - borrowed value only lives until here
  |
note: borrowed value must be valid for the anonymous lifetime #1 defined on the function body at 1:1...

I do not understand how to lengthen the life of this value. It lives and dies in the list.iter(). How can I extend it while still only using references?

Here is my code for reference:

fn largest<T: PartialOrd>(list: &[T]) -> &T {
    let mut largest = &list[0];

    for &item in list.iter() {
        if item > *largest {
            largest = &item;
        }
    }

    largest
}
rust vector
1 Answers

When you write for &item, this destructures each reference returned by the iterator, making the type of item T. You don't want to destructure these references, you want to keep them! Otherwise, when you take a reference to item, you are taking a reference to a local variable, which you can't return because local variables don't live long enough.

fn largest<T: PartialOrd>(list: &[T]) -> &T {
    let mut largest = &list[0];

    for item in list.iter() {
        if item > largest {
            largest = item;
        }
    }

    largest
}

Note also how we can compare references directly, because references to types implementing PartialOrd also implement PartialOrd, deferring the comparison to their referents (i.e. it's not a pointer comparison, unlike for raw pointers).

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