pub trait Summary {
    fn summarize (&self) -> String;
}

pub struct NewsArticle {
    pub author: String,
    pub headline: String,
    pub location: String,
    pub content: String,
}

impl Summary for NewsArticle {
    fn summarize(&self) -> String {     // return String ownership. &self means self: &Self
        format!("{} by {} ({})", self.headline, self.author, self.location)
    }
}

pub struct Tweet {
    pub username: String,
    pub content: String,
    pub retweet: bool,
    pub reply: bool,
}

impl Summary for Tweet {
    fn summarize(&self) -> String {     // &Self is the type owns impl
        format!("{}: {}",self.username, self.content)
    }
}