Make example compilable

Author: Mark-Simulacrum

posts/2020-07-30-Rust-1.45.1.md

@@ -40,11 +40,17 @@ practice: the code must have inputs consisting of entirely constant values and
 no control flow or function calls in between.
 
 ```rust
-let mut foo = Foo { x: 42 };
-let x = &mut foo.x;
-*x = 13;
-let y = foo;
-println!("{}", y.x); // -> 42; expected result: 13
+struct Foo {
+    x: u32,
+}
+
+fn main() {
+    let mut foo = Foo { x: 42 };
+    let x = &mut foo.x;
+    *x = 13;
+    let y = foo;
+    println!("{}", y.x); // -> 42; expected result: 13
+}
 ```
 
 ## Contributors to 1.45.1