From: Ralf Jung <post@ralfj.de>
Date: Fri, 16 Aug 2024 05:50:56 +0000 (+0200)
Subject: add link to stabilization PR and explain why the code has UB
X-Git-Url: https://git.ralfj.de/web.git/commitdiff_plain/cce7a39d255170e4315c5da78949491e07c1c1f9?ds=sidebyside;hp=7a9caa32c818812d46c9eb9696a7082cb37c9941

add link to stabilization PR and explain why the code has UB
---

diff --git a/personal/_posts/2024-08-14-places.md b/personal/_posts/2024-08-14-places.md
index 912f7d5..8f3532c 100644
--- a/personal/_posts/2024-08-14-places.md
+++ b/personal/_posts/2024-08-14-places.md
@@ -11,6 +11,7 @@ However, when it comes to unsafe code, a proper understanding of this dichotomy
 Consider the following [example](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=9a8802d20da16d6569510124c5827794):
 
 ```rust
+// As a "packed" struct, this type has alignment 1.
 #[repr(packed)]
 struct MyStruct {
   field: i32
@@ -21,10 +22,12 @@ let ptr = &raw const x.field;
 // This line is fine.
 let ptr_copy = &raw const *ptr;
 // But this line has UB!
+// `ptr` is a pointer to `i32` and thus requires 4-byte alignment on
+// memory accesses, but `x` is just 1-aligned.
 let val = *ptr;
 ```
 
-Here I am using the unstable but soon-to-be-stabilized "raw borrow" operator, `&raw const`.
+Here I am using the unstable but [soon-to-be-stabilized](https://github.com/rust-lang/rust/pull/127679) "raw borrow" operator, `&raw const`.
 You may know it in its stable form as a macro, `ptr::addr_of!`, but the `&` syntax makes the interplay of places and values more explicit so we will use it here.
 
 The last line has Undefined Behavior (UB) because `ptr` points to a field of a packed struct, which is not sufficiently aligned.