reorganized the directories

projects/lifetimes_demo/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "lifetimes_demo"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

projects/lifetimes_demo/src/main.rs

@@ -0,0 +1,23 @@
+use std::fmt::Display;
+
+fn longest_with_an_announcement<'a, T>(
+    x: &'a str,
+    y: &'a str,
+    ann: T,
+) -> &'a str 
+where 
+    T: Display,
+{
+    println! ("通知！{}", ann);
+    if x.len() > y.len() {
+        x
+    } else { 
+        y 
+    }
+}
+
+fn main() {
+    let result = longest_with_an_announcement("abc", "测试", "计算结果已出来。");
+
+    println! ("{}", result);
+}