Replace one of the examples of the aliasing rule

src/borrowing/examples.md

@@ -17,21 +17,53 @@ fn main() {
 }
 ```
 
-Similarly, consider the case of iterator invalidation:
+We can also look at a case where these rules prevent incorrect optimizations:
 
 ```rust,editable,compile_fail
+fn swap_and_increment(a: &mut i32, b: &mut i32) {
+    *a = *a + 1;
+
+    let tmp = *a;
+    *a = *b;
+    *b = tmp;
+
+    *b = *a + 1;
+}
+
 fn main() {
-    let mut vec = vec![1, 2, 3, 4, 5];
-    for elem in &vec {
-        vec.push(elem * 2);
-    }
+    let mut x = 1;
+    swap_and_increment(&mut x, &mut x);
 }
 ```
 
 <details>
 
-- In both of these cases, modifying the collection by pushing new elements into
-  it can potentially invalidate existing references to the collection's elements
-  if the collection has to reallocate.
+- In the first case, modifying the collection by pushing new elements into it
+  can potentially invalidate existing references to the collection's elements if
+  the collection has to reallocate.
+
+- In the second case, the aliasing rule prevents mis-compilation: In the C
+  equivalent of this function, the program produces different results when
+  compiled with optimizations enabled.
+
+  - Show students [the C version of this on Godbolt][unoptimized]. By default,
+    with no optimizations enabled it will print `x = 3`. But
+    [enabling optimizations][optimized] causes it to instead print `x = 2`.
+
+  - Note the use of the `restrict` keyword in the C example. This tells the
+    compiler that `a` and `b` cannot alias, but nothing prevents you from
+    violating that requirement. The compiler will silently produce the wrong
+    behavior.
+
+  - In the Rust version, using `&mut` automatically implies the same
+    restriction, and the compiler statically prevents violations of this
+    requirement.
+
+  - Rust's aliasing rules provide strong guarantees about how references can
+    alias, allowing the compiler to apply optimizations without breaking the
+    semantics of your program.
 
 </details>
+
+[unoptimized]: https://godbolt.org/z/9EGh6eMxE
+[optimized]: https://godbolt.org/z/Kxsf8sahT