Refining Ch03.

projects/data_types/src/main.rs

@@ -1,3 +1,7 @@
 fn main() {
-    let guess = "42".parse().expect("这不是个数字！");
+    let c = 'z';
+    let z: char = 'ℤ'; // 带有显式的类型注解
+    let heart_eyed_cat = '😻';
+
+    println! ("c 为 {c}, z 为 {z}, 爱心猫{heart_eyed_cat}");
 }