CODEWITHSUNDEEP
rustscopelifetime
Scriptodude
Scriptodude

Reputation: 83

Why does this code with string literals compile when one of them appears to be out of scope?

I came across the section talking about lifetimes and was a little bit confused at first. So I decided to give it a go by coding a little example. (Playground)

fn main() {
    let b = "hello";
    let mut d = "Unassigned";
    {
        let a = "hi";
        let c = lifetime(&a, &b);
        d = &c;
    }

    // My confusion happens here, I was expecting a compile-time error
    // since the lifetime of 'c' is the same as 'a' which is in the previous block
    // from my understanding. But this compiles just fine
    println!("{}", d);
}

fn lifetime<'a, 'b>(test: &'a str, test2: &'b str) -> &'a str {
    println!("{}, {}", test, test2);
    return "returned value of lifetime";
}

From my understanding, the lifetime function binds the lifetime 'a to the returned reference value. So normally I would expect the line println!("{}", d); to break at compile time with an error referencing the lifetime 'a being out of scope, which I was wrong.

What did I understand wrong? Why is this code compiling?

I've seen this and that which basically just confused me further, because they somewhat say what I was expecting as a result in the first place.

Upvotes: 2

Views: 85

Answers (1)

Scriptodude
Scriptodude

Reputation: 83

After finding out about contravariance as pointed out in the comment above, I wrote this little snippet (similar to the question)

struct Potato(i32);

fn doesnt_work() {
    let b = Potato(1);
    let d;
    {
        let a = Potato(2);
        let c = lifetime2(&b, &a);
        d = c;
    }

    // Compile-time error ! Borrowed value does not live long enough.
    println!("{}", d.0);
}

fn lifetime2<'a, 'b>(test: &'a Potato, test2: &'b Potato) -> &'a Potato {
    return &Potato(test.0);
}

Which now gives the actual expected compile-time error.

It finds out in my original question the lifetime was being inferred as 'static which is the highest common lifetime of str references.

Upvotes: 2

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