About ownership of Rust

Question

I have a question about ownership of Rust.

I don't have used Rust but I know that Rust have a ownership concept and Rust manages memory by ownership. I learned that Rust prevents to access to invalid memory in compile time.

If there is code as below

let s1 = String::from("hello");
let s2 = s1;

println!("{}, world!", s1);

It may display error because s1 is invalid memory.

Here I wonder about the result in the below case.

let s1 = String::from("hello");
if(The condition that can't know in compile time)
   let s2 = s1;

println!("{}, world!", s1);

The if condition may be rand or input from user etc.

if perform the if statement s1 will move to s2 and it will deleted when the if statement end. in this case accessing to s1 on println is will generated error.

but if the if statement is not performed, accessing to s1 is valid. How Rust operate in this case?

Thanks for reading.

Answer 1

The condition doesn't affect much. Since the if may move s1, the compiler has to consider it moved for the purposes of the rest of the scope.

On the playground:

fn main() {
    let s1 = String::from("hello");
    if std::env::args().next() == None { // runtime check
       let _s2 = s1;
    }
    
    println!("{}, world!", s1);
}

error[E0382]: borrow of moved value: `s1`
  --> src/lib.rs:10:28
   |
5  |     let s1 = String::from("hello");
   |         -- move occurs because `s1` has type `std::string::String`, which does not implement the `Copy` trait
6  |     if std::env::args().next() == None {
7  |        let _s2 = s1;
   |                  -- value moved here
...
10 |     println!("{}, world!", s1);
   |                            ^^ value borrowed here after move

If you have a situation where you need a value that may have been moved from, use an Option.