Struct ownership

Question

struct Haha {
    pub a: u32,
    pub b: Vec<u32>,
}

let example = Haha {
    a: 32,
    b: vec![1],
}; 
let new_a = example.a;
let new_b = example.b;

My understanding is:

1. new_a is a copy of example.a so example still owns example.a.
2. new_b now owns example.b since example.b was moved.

Does rust implicitly copy example.a because it has Copy trait? And since example.b, which is a Vec, does not implement Copy trait, ownership of example.b is moved rather than copied?

Answer 1

Your understanding is correct. a is copied while b is moved. You can confirm this by trying to access the two fields afterwards.

println!("{:?}", example.a);

This prints 32. example.a is still accessible because it was copied, not moved.

println!("{:?}", example.b);

Accessing example.b fails to compile with the error message:

error[E0382]: borrow of moved value: `example.b`
  --> src/main.rs:13:22
   |
12 |     let _new_b = example.b;
   |                  --------- value moved here
13 |     println!("{:?}", example.b);
   |                      ^^^^^^^^^ value borrowed here after move
   |
   = note: move occurs because `example.b` has type `std::vec::Vec<u32>`, which does not implement the `Copy` trait

Which confirms exactly what you said, that example.b was moved because it doesn't implement the Copy trait.