From d157b54fd4e9062705a2f40049d1dffa02aa13df Mon Sep 17 00:00:00 2001
From: Fabian Bornhofen <fbornhofen@google.com>
Date: Tue, 10 Jan 2023 15:04:17 +0100
Subject: [PATCH] Add speaker notes about RVO in borrowing.md

---
 src/ownership/borrowing.md | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/src/ownership/borrowing.md b/src/ownership/borrowing.md
index 36cf27c0..cb104645 100644
--- a/src/ownership/borrowing.md
+++ b/src/ownership/borrowing.md
@@ -21,3 +21,31 @@ fn main() {
 
 * The `add` function _borrows_ two points and returns a new point.
 * The caller retains ownership of the inputs.
+
+<details>
+
+Notes on stack returns:
+* Demonstrate that the return from `add` is cheap because the compiler can eliminate the copy operation. Change the above code to print stack addresses and run it on the [Playground](https://play.rust-lang.org/). In the "DEBUG" optimization level, the addresses should change, while the stay the same when changning to the "RELEASE" setting:
+
+  ```rust,editable
+  #[derive(Debug)]
+  struct Point(i32, i32);
+
+  fn add(p1: &Point, p2: &Point) -> Point {
+      let p = Point(p1.0 + p2.0, p1.1 + p2.1);
+      println!("&p.0: {:p}", &p.0);
+      p
+  }
+
+  fn main() {
+      let p1 = Point(3, 4);
+      let p2 = Point(10, 20);
+      let p3 = add(&p1, &p2);
+      println!("&p3.0: {:p}", &p3.0);
+      println!("{p1:?} + {p2:?} = {p3:?}");
+  }
+  ```
+* The Rust compiler can do return value optimization (RVO).
+* In C++, copy elision has to be defined in the language specification because constructors can have side effects. In Rust, this is not an issue at all.
+
+</details>