Challenge 1: Describable With a Default — Possible Solution
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trait Describable {
    fn describe(&self) -> String {
        String::from("No description available.")
    }
}

struct Book {
    title: String,
}

impl Describable for Book {
    fn describe(&self) -> String {
        format!("A book titled '{}'", self.title)
    }
}

struct Widget;

impl Describable for Widget {} // empty impl — uses the default

fn main() {
    let book = Book { title: String::from("The Rust Book") };
    let widget = Widget;

    println!("{}", book.describe());   // A book titled 'The Rust Book'
    println!("{}", widget.describe()); // No description available.
}

WHY THIS WORKS AS AN ANSWER
------------------------------
The trait's describe method includes a default body returning "No
description available." — exactly this chapter's default-
implementation pattern — meaning any type can implement Describable
with an EMPTY impl block and still have a working describe() method.

Book overrides the default with its own implementation using its
title field, following this chapter's own Article/Tweet example
pattern of providing a genuinely different summary per type.

Widget uses `impl Describable for Widget {}` with nothing inside the
braces — this is valid syntax specifically because the trait's method
already has a default body, so there's nothing Widget is REQUIRED to
provide. If describe had no default at all, this same empty impl block
would be a compile error demanding an implementation be supplied.