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

[web.git] / personal / _posts / 2024-08-14-places.md

diff --git a/personal/_posts/2024-08-14-places.md b/personal/_posts/2024-08-14-places.md
index 912f7d52d221130f7b78c41661a0e47ed97c2f5d..8f3532c3574f08881704ca4d7c2ecb395c1cc7aa 100644 (file)
--- 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
 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
 #[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!
 // 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;
 ```
 
 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.
 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.