CODEWITHSUNDEEP
rustownership
doki
doki

Reputation: 105

Ownership of values whose variables are shadowed

See some code below:

#[derive(Debug)]
struct MyString {
    a: String,
}

impl MyString {
    fn new(s: &str) -> Self {
        MyString { a: String::from(s) }
    }
}

impl Deref for MyString {
    type Target = String;

    fn deref(&self) -> &Self::Target {
        &self.a
    }
}

impl Drop for MyString {
    fn drop(&mut self) {
        println!("{} dropped!", self.a);
    }
}

fn main() {
    let mut test1 = MyString::new("test1");
    let mut test1 = MyString::new(&test1);
    println!("shadow variable test1");
    test1.a = String::from("test2");
}

I thought there should be a compile error because test1 is still borrowed.

However, there's no compile error, and the result is

shadow variables test1
test2 dropped
test1 dropped

So if a variable is shadowed, the value is still valid in the current scope. But who is its owner?

Upvotes: 1

Views: 193

Answers (1)

Jmb
Jmb

Reputation: 23264

From an ownership standpoint, your code is exactly equivalent to:

fn main() {
    let mut test1_1 = MyString::new("test1");
    let mut test1_2 = MyString::new(&test1_1);
    println!("shadow variable test1");
    test1_2.a = String::from("test2");
}

So test1_1 is dropped when it goes out of scope at the end of the main function.

Note also that test1_2 does not borrow test1_1, the borrow only lasts for as long as the call to MyString::new(&test1). Afterwards there is no connection between the two variables.

Upvotes: 1

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