pub trait Summarizable {
    fn summary(&self) -> String {
        String::from("Read more...")
    }
}

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

impl Summarizable for NewsArticle {
    fn summary(&self) -> String {
        format!("{}, by {} ({})", self.headline, self.author, self.location)
    }
}

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

impl Summarizable for Tweet {
    fn summary(&self) -> String {
        format!("{}: {}", self.username, self.content)
    }
}

#[cfg(test)]
mod tests {
    #[test]
    fn it_works() {
    }
}

pub trait Summarizable {
    fn summary(&self) -> String;
}

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

impl Summarizable for NewsArticle {
    fn summary(&self) -> String {
        format!("{}, by {} ({})", self.headline, self.author, self.location)
    }
}

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

impl Summarizable for Tweet {
    fn summary(&self) -> String {
        format!("{}: {}", self.username, self.content)
    }
}

#[cfg(test)]
mod tests {
    #[test]
    fn it_works() {
    }
}

--- 
+++ 
@@ -1,5 +1,7 @@
 pub trait Summarizable {
-    fn summary(&self) -> String;
+    fn summary(&self) -> String {
+        String::from("Read more...")
+    }
 }
 
 pub struct NewsArticle {
Add default implementation for summary method on Summarizable trait
