Add new section on trait bounds.

exercises/04_traits/12_copy/Cargo.toml

@@ -0,0 +1,4 @@
+[package]
+name = "copy"
+version = "0.1.0"
+edition = "2021"

exercises/04_traits/12_copy/src/lib.rs

@@ -0,0 +1,25 @@
+// TODO: implement the necessary traits to make the test compile and pass.
+//  You *can't* modify the test.
+
+pub struct WrappingU32 {
+    value: u32,
+}
+
+impl WrappingU32 {
+    pub fn new(value: u32) -> Self {
+        Self { value }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_ops() {
+        let x = WrappingU32::new(42);
+        let y = WrappingU32::new(31);
+        let z = WrappingU32::new(u32::MAX);
+        assert_eq!(x + y + y + z, WrappingU32::new(103));
+    }
+}