How to match against a String in a nested pattern

Question

I want to check whether a particular nested field is equal to a certain String and only run the arm if it is. Something like the following:

struct Bar {
    baz: usize,
    value: Option<String>
}

struct Foo {
    bar: Bar
}

let value = Foo { bar: Bar { baz: 1, value: Some("foobar".to_string()) } };

match value {
    Foo {
        bar: Bar {
            baz,
            value: Some("foobar") // does not work since &str != String
        }
    } => println!("Matched! baz: {}", baz),
    _ => panic!("No Match")
}

Adding a match guard works but constant Strings aren't a thing so I can't guard using that, the only possibility I see is to guard against a new String like so:

struct Bar {
    value: Option<String>
}

struct Foo {
    bar: Bar
}

let value = Foo { bar: Bar { value: Some("foobar".to_string()) } };

match value {
    Foo {
        bar: Bar { baz, value },
    } if value == Some("foobar".to_string()) => println!("Matched! baz: {}", baz),
    _ => panic!("No Match"),
}

Is this my only option or is there a better way?

Note: I have seen How to match a String against string literals? but my match is nested and more complicated (the above is only an example) so I can't just match against a reference to the String.

If I understood correctly, there is a feature that is WIP in the compiler that will allow you to match the nested string with a literal `&str`. This is part of a bigger project to allow nested matching over values that are obtained by dereferencing the matched value (this would be possible in your case because `String: Deref`). This is partly possible right now, but they [the Rust lang team] are waiting to have a fully general solution before making it available. — jthulhu, May 31 '23 at 09:30

Dogbert · Accepted Answer · 2023-05-31T06:29:26.727

You can extract the value inside the Some, which would give you a value of type String, which you can then compare with == in an if guard:

    match value {
        Foo {
            bar: Bar {
                baz,
                value: Some(value),
            },
        } if value == "foobar" => println!("Matched! baz: {}", baz),
        _ => panic!("No Match"),
    }

Output:

Matched! baz: 1

Playground

As @drewtato pointed out in a comment below, this will move the whole value. If you want to reuse value again, you can match with Some(ref value) and the comparison will still work.

How to match against a String in a nested pattern

1 Answers1