Can I create a binding with the type `Option<&str>`?

Question

Can I create a binding with the type Option<&str>? Tiny non-working example:

fn main() {
    let a: Option<&str> = {
        Some(&{"a".to_string() + "b"}) // Let's say the string is not static
    };
}

This does not work, I need to add lifetime (or use a Option<String> without &). So how can I declare lifetime here? I know I can return a Option<String> and everything will be fine, but that's not what I want — I'm trying to understand some Rust mechanics. I can declare lifetime in a function, but don't know how to do this in a simple let binding.

score 2 · Accepted Answer · edited May 23 '17 at 12:32

Absolutely:

fn main() {
    let s = "a".to_string() + "b";
    let a: Option<&str> = Some(&s);
}

The problem is not in creating a Option<&str>, it's that you are trying to take a reference to something that has gone out of scope. This is (a part of) the error message for the original code:

error: borrowed value does not live long enough
  |>         Some(&{"a".to_string() + "b"})
  |>               ^^^^^^^^^^^^^^^^^^^^^^^ does not live long enough

See Return local String as a slice (&str) for further information.

score 1 · Answer 2 · answered Jul 18 '16 at 21:37

1

You need to extend lifetime of the string beyond a's initializer expression by binding it to another name:

fn main() {
    let x: String = "a".to_string() + "b";
    let a: Option<&str> = {
        Some(&x)
    };
}

answered Jul 18 '16 at 21:37

yuri kilochek

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score 1 · Answer 3 · answered Jul 18 '16 at 21:38

You are trying to keep a reference to a value that does not live long enough, like the error says:

3 |>         Some(&{"a".to_string() + "b"}) // Let's say the string is not static
  |>               ^^^^^^^^^^^^^^^^^^^^^^^ does not live long enough

You can have a bind of type Option<&str>, but the reference must live longer than the bind. In your example, you create a bind to the result string, an than take a reference to it:

fn main() {
    let x: String = "a".to_string() + "b";
    let a: Option<&str> = Some(&x);
}

Can I create a binding with the type `Option<&str>`?

3 Answers3