Fix solution for Storing Books exercise (#1237)

As pointed out by @njr0 in #1233, we were making the exercise confusing
by showing people code that cannot work — and then expecting the course
participants to somehow fix this, without setting clear boundaries for
what can and cannot be modified.

This PR should align the exercise with the other exercises in the course
and avoid the “brain teaser” here.

This also has the advantage of having a full working solution, with no
commented code which will bit-rot over time.

src/exercises/day-2/book-library.md

@@ -25,14 +25,24 @@ Use this to model a library's book collection. Copy the code below to
     }
 
 {{#include book-library.rs:Library_len}}
+        todo!("Return the length of `self.books`")
+    }
 
 {{#include book-library.rs:Library_is_empty}}
+        todo!("Return `true` if `self.books` is empty")
+    }
 
 {{#include book-library.rs:Library_add_book}}
+        todo!("Add a new book to `self.books`")
+    }
 
 {{#include book-library.rs:Library_print_books}}
+        todo!("Iterate over `self.books` and print each book's title and year")
+    }
 
 {{#include book-library.rs:Library_oldest_book}}
+        todo!("Return a reference to the oldest book (if any)")
+    }
 }
 
 {{#include book-library.rs:main}}

src/exercises/day-2/book-library.rs

@@ -33,8 +33,9 @@ impl Book {
     }
 }
 
-// Implement the methods below. Update the `self` parameter to
-// indicate the method's required level of ownership over the object:
+// Implement the methods below. Notice how the `self` parameter
+// changes type to indicate the method's required level of ownership
+// over the object:
 //
 // - `&self` for shared read-only access,
 // - `&mut self` for unique and mutable access,
@@ -49,49 +50,34 @@ impl Library {
     }
 
     // ANCHOR: Library_len
-    //fn len(self) -> usize {
-    //    todo!("Return the length of `self.books`")
-    //}
-    // ANCHOR_END: Library_len
     fn len(&self) -> usize {
+        // ANCHOR_END: Library_len
         self.books.len()
     }
 
     // ANCHOR: Library_is_empty
-    //fn is_empty(self) -> bool {
-    //    todo!("Return `true` if `self.books` is empty")
-    //}
-    // ANCHOR_END: Library_is_empty
     fn is_empty(&self) -> bool {
+        // ANCHOR_END: Library_is_empty
         self.books.is_empty()
     }
 
     // ANCHOR: Library_add_book
-    //fn add_book(self, book: Book) {
-    //    todo!("Add a new book to `self.books`")
-    //}
-    // ANCHOR_END: Library_add_book
     fn add_book(&mut self, book: Book) {
+        // ANCHOR_END: Library_add_book
         self.books.push(book)
     }
 
     // ANCHOR: Library_print_books
-    //fn print_books(self) {
-    //    todo!("Iterate over `self.books` and print each book's title and year")
-    //}
-    // ANCHOR_END: Library_print_books
     fn print_books(&self) {
+        // ANCHOR_END: Library_print_books
         for book in &self.books {
             println!("{}, published in {}", book.title, book.year);
         }
     }
 
     // ANCHOR: Library_oldest_book
-    //fn oldest_book(self) -> Option<&Book> {
-    //    todo!("Return a reference to the oldest book (if any)")
-    //}
-    // ANCHOR_END: Library_oldest_book
     fn oldest_book(&self) -> Option<&Book> {
+        // ANCHOR_END: Library_oldest_book
         // Using a closure and a built-in method:
         // self.books.iter().min_by_key(|book| book.year)
 
@@ -108,30 +94,31 @@ impl Library {
 }
 
 // ANCHOR: main
-// This shows the desired behavior. Uncomment the code below and
-// implement the missing methods. You will need to update the
-// method signatures, including the "self" parameter! You may
-// also need to update the variable bindings within main.
 fn main() {
-    let library = Library::new();
-
-    //println!("The library is empty: library.is_empty() -> {}", library.is_empty());
-    //
-    //library.add_book(Book::new("Lord of the Rings", 1954));
-    //library.add_book(Book::new("Alice's Adventures in Wonderland", 1865));
-    //
-    //println!("The library is no longer empty: library.is_empty() -> {}", library.is_empty());
-    //
-    //
-    //library.print_books();
-    //
-    //match library.oldest_book() {
-    //    Some(book) => println!("The oldest book is {}", book.title),
-    //    None => println!("The library is empty!"),
-    //}
-    //
-    //println!("The library has {} books", library.len());
-    //library.print_books();
+    let mut library = Library::new();
+
+    println!(
+        "The library is empty: library.is_empty() -> {}",
+        library.is_empty()
+    );
+
+    library.add_book(Book::new("Lord of the Rings", 1954));
+    library.add_book(Book::new("Alice's Adventures in Wonderland", 1865));
+
+    println!(
+        "The library is no longer empty: library.is_empty() -> {}",
+        library.is_empty()
+    );
+
+    library.print_books();
+
+    match library.oldest_book() {
+        Some(book) => println!("The oldest book is {}", book.title),
+        None => println!("The library is empty!"),
+    }
+
+    println!("The library has {} books", library.len());
+    library.print_books();
 }
 // ANCHOR_END: main