oh interesting. Dangling reference not allowed.

```
error[E0597]: `x` does not live long enough
```

src/main.rs

@@ -8,14 +8,19 @@ fn main() {
     println!("t 1st: {}", t.0);
     println!("t 2nd: {}", t.1);
 
-    let mut x: i32 = 10; 
-    let mut z: i32 = 5; 
-    println!("x: {x}");
-    println!("z: {z}");
-    let mut ref_x: &mut i32 = &mut x;
-    *ref_x = 20; 
-    println!("x: {x}");
-    ref_x = &mut z;
-    *ref_x = 20; 
-    println!("z: {z}");
+    let pre_ref_x: &mut i32;
+    {
+        let mut x: i32 = 10;
+        let mut z: i32 = 5;
+        println!("x: {x}");
+        println!("z: {z}");
+        let mut ref_x: &mut i32 = &mut x;
+        *ref_x = 20;
+        pre_ref_x = &mut x;
+        println!("x: {x}");
+        ref_x = &mut z;
+        *ref_x = 20;
+        println!("z: {z}");
+    }
+    println!("pre_ref_x: {pre_ref_x}");
 }

target/debug/.fingerprint/basics00-837a95fe33c7c079/bin-basics00

@@ -1 +0,0 @@
-7cbe46f1ccde8504