How to move one field out of a struct that implements Drop trait?

You can use std::mem::replace() to swap the field with a new blank value in order to transfer ownership to you:

extern crate hyper;

fn just_the_headers() -> Result<hyper::header::Headers, hyper::error::Error> {
    let c = hyper::client::Client::new();
    let result = c.get("http://www.example.com").send();
    match result {
        Err(e) => Err(e),
        Ok(mut response) => Ok(std::mem::replace(&mut response.headers, hyper::header::Headers::new())),
    }
}

fn main() {
    println!("{:?}", just_the_headers());
}

Here, we’re replacing response.headers with a new empty set of headers. replace() returns the value that was stored in the field before we replaced it.