'returns a value referencing data owned by the current function' on match with `&str`

1

I have this code

enum Enum {
    V1,
    V2
}

fn decoded() -> Result<Enum, String> {
    // found from external function
    let string = Some(String::from("v1")); // Return type Option<String>
    
    // how to match String with &str
    match string.map(|s| s.as_str()) {
        Some("v1") => Ok(Enum::V1),
        Some("v2") => Ok(Enum::V1),
        Some(v) => Err(format!("Unrecognized {:?}", v).into()),
        None => Err("Unexpected null".into())
    }
}

fn main() {
    decoded();
}

The thing is, although I am matching on &str, I am returning completely different type. I guess rust is not able to deduce that.

Playground link: Rust Playground

2

The error is not about your decoded function. It's about the map callback function.

You need to change string.map(|s| s.as_str()) to string.as_ref().map(|s| s.as_str()), which can be abbreviated as string.as_deref().

That's because map() on Option<String> is expected to destroy the String in the process (it maps one type to another, and after map the old type ceases to exist).

By adding as_ref() you make it Option<&String> and destroying of &String doesn't do anything, so the string data keeps existing.

1 Like
3

Perfect. Thank you, @kornel. It works now!

That's because map() on Option<String> is expected to destroy the String in the process (it maps one type to another, and after map the old type ceases to exist).

This being the key phrase.

4

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