Add value of a method to serde serialization output

Question

Is there a way to add the value of a method to the serialization output of serde when the struct derives Serialize? I'm looking for something like a "virtual field".

I know I can define my own Serializer / Visitor or use serde_json::builder to get a Value, I just wanted to check first if there was any way to do this using serde_macro magic.

To be clear I want something like this:

#[derive(Serialize, Deserialize, Debug)]
struct Foo {
    bar: String,
    #[serde(call="Foo::baz")]
    baz: i32 // but this is not a real field
}

impl Foo {
    fn baz(&self) -> i32 { self.bar.len() as i32 }
}

Answer 1

Here is what I am using now. It's still verbose, and I don't know if it is the best way to handle this, but I thought I would add it here for the record:

#[derive(Deserialize, Debug)]
struct Foo {
    bar: String
}

impl Foo {
    fn baz(&self) -> i32 { self.bar.len() as i32 }
}

impl ::serde::Serialize for Foo {
    fn serialize<S>(&self, serializer: &mut S) -> Result<(), S::Error> 
        where S: ::serde::Serializer {

        #[derive(Serialize)]
        struct Extended<'a> {
            bar: &'a String,
            baz: i32
        }

        let ext = Extended {
            bar: &self.bar,
            baz: self.baz()
        };

        Ok(try!(ext.serialize(serializer)))
    }
}