Ownership error masks missing implementation of trait

[Leopard2A5]

I would expect the main function to not compile because clone() has not been implemented for struct Test. However this error is masked by:

src/main.rs:26:2: 26:7 error: cannot borrowtestsas mutable because it is also borrowed as immutable

The error here is the missing implementation though, because it solves the ownership issue. rustc doesn't complain about the missing implementation at all.

#[derive(Debug)]
struct Test {
    name: String
}

// impl std::clone::Clone for Test {
//  fn clone(&self) -> Test {
//      Test {
//          name: self.name.clone()
//      }
//  }
// }

fn main() {
    let mut tests = Vec::new();
    tests.push(Test{ name: "test1".to_string() });
    tests.push(Test{ name: "test2".to_string() });
    tests.push(Test{ name: "test3".to_string() });

    tests.sort_by(|a, b| a.name.cmp(&b.name));
    println!("{:?}", tests);

    let test = tests.last().unwrap().clone();
    println!("{:?}", test);

    tests.sort_by(|a, b| b.name.cmp(&a.name));
    println!("{:?}", tests);
}

[mitaa]

This compiles because Copy and Clone (Make Copy a subtrait of Clone) are implemented for any &T. So in this situation just the pointer is copied, which retainins the borrow.

Maybe a lint which warns when this impl/method is invoked could help?
Because I don't think this is ever intended when someone explicitly uses .clone().

[Leopard2A5]

(I have to point out that i've only just started with Rust.)

So if i get you right i should have dereferenced tests.last().unwrap() before calling clone and that would have been a compiler error because it doesn't use the Clone impl's for &T?

Maybe like so?
let test: &Test = tests.last().unwrap():
let test: Test = (*test).clone();

[mitaa]

Well, yes, if you do that you will get the desired compiler error, but I don't think the error message is worth the additional verbosity in general.

<anon>:23:41: 23:48 error: no method named `clone` found for type `Test` in the current scope
<anon>:23     let test = (*tests.last().unwrap()).clone();
                                                  ^~~~~~~
<anon>:23:41: 23:48 help: items from traits can only be used if the trait is implemented and in scope; the following trait defines an item `clone`, perhaps you need to implement it:
<anon>:23:41: 23:48 help: candidate #1: `core::clone::Clone`

[Leopard2A5]

So to wrap it up it was my bad, because i didn't know that references can be cloned by default... kinda hard to put my finger on the actual problem here 📦
Anyways, thanks!

[steveklabnik]

Thanks for reporting! Always better to have too many reports than not enough.

Since this isn't a bug, I'm closing it.