> Even if those values contain owned references into the heap, the programs can ...

weberc2 · on Aug 16, 2017

> Passing data to a function when it's not a reference means that you're giving ownership to that function.

Only in Rust, but there's no apparent reason to move ownership. If Rust didn't move ownership during pass-by-value, it would still be trivial to prove the following code correct:

    fn main() {
        let x = String::new();
        foo(x);
        bar(x);
        // x is destroyed
    }

> Maybe I'm misunderstanding the problem you're having. Do you have an example of another language that lets you do what you want?

Here's the equivalent in C++:

   int main() {
       std::string x = "";
       foo(x);
       bar(x);
       // x is destroyed
   }

wtetzner · on Aug 16, 2017

> Here's the equivalent in C++: >

   int main() {
       std::string x = "";
       foo(x);
       bar(x);
       // x is destroyed
   }

Doesn't C++ do a copy in this case?

weberc2 · on Aug 17, 2017

It does; that's the point: you can copy without losing safety guarantees, so there's no obvious reason to use an affine type system.

wtetzner · on Aug 17, 2017

You can copy in Rust too. You just call .clone(), or if you want your type to always be copied, have it implement the Copy trait. In that case, it does exactly what you want.

But Rust is more explicit about copying in general, to make it harder for the performance cost to be hidden.