为什么移动闭包不拥有变量的所有权？

Question

The Rust Programming Language says :

If you want to force the closure to take ownership of the values it uses in the environment, you can use the move keyword before the parameter list

What I have noticed with my code is that it won't take ownership of these values. The differences between my code and the given example are:

• using an integer instead of a Vec
• making x mutable instead of immutable

Example 1: The Rust Programming Language

fn main() {
    let x = vec![1, 2, 3];

    let equal_to_x = move |z| z == x;

    println!("can't use x here: {:?}", x);

    let y = vec![1, 2, 3];

    assert!(equal_to_x(y));
}

Example 2: My Code

fn main() {
    let mut x = 1;

    let equal_to_x = move |z| z == x;

    println!("can use x here: {:?}", x);

    let y = 1;

    assert!(equal_to_x(y));
}

1. Why will example 2 compile but example 1 won't?

2. AWhy is the ownership of x not moved even if I explicitly write move in front of the closure? Why is x accessible after moving it into the closure?

Answer 1

The answer is given in the error message of your first example

error[E0382]: borrow of moved value: `x`
 --> src/main.rs:6:40
  |
2 |     let x = vec![1, 2, 3];
  |         - move occurs because `x` has type `std::vec::Vec<i32>`, which does not implement the `Copy` trait
3 | 
4 |     let equal_to_x = move |z| z == x;
  |                      --------      - variable moved due to use in closure
  |                      |
  |                      value moved into closure here
5 | 
6 |     println!("can't use x here: {:?}", x);
  |                                        ^ value borrowed here after move

"move occurs because x has type std::vec::Vec<i32> , which does not implement the Copy trait"

This means, when a type implements the Copy trait (like an i32 does), move copies the variable into the scope of the closure.