3 files changed, 90 insertions, 0 deletions
diff --git a/src/doc/book/listings/ch15-smart-pointers/listing-15-22/Cargo.lock b/src/doc/book/listings/ch15-smart-pointers/listing-15-22/Cargo.lock
new file mode 100644
index 000000000..4dc2226a5
--- /dev/null
+++ b/src/doc/book/listings/ch15-smart-pointers/listing-15-22/Cargo.lock
@@ -0,0 +1,6 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "limit-tracker"
+version = "0.1.0"
+
diff --git a/src/doc/book/listings/ch15-smart-pointers/listing-15-22/Cargo.toml b/src/doc/book/listings/ch15-smart-pointers/listing-15-22/Cargo.toml
new file mode 100644
index 000000000..98c3f537c
--- /dev/null
+++ b/src/doc/book/listings/ch15-smart-pointers/listing-15-22/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "limit-tracker"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
diff --git a/src/doc/book/listings/ch15-smart-pointers/listing-15-22/src/lib.rs b/src/doc/book/listings/ch15-smart-pointers/listing-15-22/src/lib.rs
new file mode 100644
index 000000000..a77ffa41c
--- /dev/null
+++ b/src/doc/book/listings/ch15-smart-pointers/listing-15-22/src/lib.rs
@@ -0,0 +1,78 @@
+pub trait Messenger {
+    fn send(&self, msg: &str);
+}
+
+pub struct LimitTracker<'a, T: Messenger> {
+    messenger: &'a T,
+    value: usize,
+    max: usize,
+}
+
+impl<'a, T> LimitTracker<'a, T>
+where
+    T: Messenger,
+{
+    pub fn new(messenger: &'a T, max: usize) -> LimitTracker<'a, T> {
+        LimitTracker {
+            messenger,
+            value: 0,
+            max,
+        }
+    }
+
+    pub fn set_value(&mut self, value: usize) {
+        self.value = value;
+
+        let percentage_of_max = self.value as f64 / self.max as f64;
+
+        if percentage_of_max >= 1.0 {
+            self.messenger.send("Error: You are over your quota!");
+        } else if percentage_of_max >= 0.9 {
+            self.messenger
+                .send("Urgent warning: You've used up over 90% of your quota!");
+        } else if percentage_of_max >= 0.75 {
+            self.messenger
+                .send("Warning: You've used up over 75% of your quota!");
+        }
+    }
+}
+
+// ANCHOR: here
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::cell::RefCell;
+
+    struct MockMessenger {
+        sent_messages: RefCell<Vec<String>>,
+    }
+
+    impl MockMessenger {
+        fn new() -> MockMessenger {
+            MockMessenger {
+                sent_messages: RefCell::new(vec![]),
+            }
+        }
+    }
+
+    impl Messenger for MockMessenger {
+        fn send(&self, message: &str) {
+            self.sent_messages.borrow_mut().push(String::from(message));
+        }
+    }
+
+    #[test]
+    fn it_sends_an_over_75_percent_warning_message() {
+        // --snip--
+        // ANCHOR_END: here
+        let mock_messenger = MockMessenger::new();
+        let mut limit_tracker = LimitTracker::new(&mock_messenger, 100);
+
+        limit_tracker.set_value(80);
+        // ANCHOR: here
+
+        // ANCHOR: here
+        assert_eq!(mock_messenger.sent_messages.borrow().len(), 1);
+    }
+}
+// ANCHOR_END: here