X-Git-Url: https://git.ralfj.de/web.git/blobdiff_plain/7a9caa32c818812d46c9eb9696a7082cb37c9941..c00fca4e0107673ce66185625dadbb8c3c48ed09:/personal/_posts/2024-08-14-places.md?ds=sidebyside

diff --git a/personal/_posts/2024-08-14-places.md b/personal/_posts/2024-08-14-places.md
index 912f7d5..4b93ee5 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.
@@ -123,10 +126,16 @@ let _ = *ptr; // This is fine!
 let _val = *ptr; // This is UB.
 ```
 
-The reason for this is that the `_` pattern does *not* incur a place-to-value coercion.
+Note that the grammar above cannot represent this program: in the full grammar of Rust, the `let` syntax is something like "`let` _Pattern_ `=` _PlaceExpr_ `;`",
+and then pattern desugaring decides what to do with that place expression.
+If the pattern is a binder (the common case), a `load` gets inserted to compute the initial value for the local variable that this binder refers to.
+However, if the pattern is `_`, then the place expression still gets evaluated---but the result of that evaluation is simply discarded.
+MIR uses a `PlaceMention` statement to indicate these semantics.
+
+In particular, this means that the `_` pattern does *not* incur a place-to-value coercion!
 The desugared form of the relevant part of this code is:
 ```rust
-let _ = *(load ptr); // This is fine!
+PlaceMention(*(load ptr)); // This is fine!
 let _val = load *(load ptr); // This is UB.
 ```
 As you can see, the first line does not actually load from the pointer (the only `load` is there to load the pointer itself from the local variable that stores it).
@@ -142,6 +151,17 @@ match *ptr { _val => "not happy" } // This is UB.
 The scrutinee of a `match` expression is a place expression, and if the pattern is `_` then a value is never constructed.
 However, when an actual binder is present, this introduces a local variable and a place-to-value coercion is inserted to compute the value that will be stored in that local variable.
 
+**Note on `unsafe` blocks.**
+Note that wrapping an expression in a block forces it to be a value expression.
+This means that `unsafe { *ptr }` always loads from the pointer!
+In other words:
+```rust
+let ptr = std::ptr::null::<i32>();
+let _ = *ptr; // This is fine!
+let _ = unsafe { *ptr }; // This is UB.
+```
+The fact that braces force a value expression can occasionally be useful, but the fact that `unsafe` blocks do that is definitely quite unfortunate.
+
 ### Are there also value-to-place coercions?
 
 So far, we have discussed what happens when a place expression is encountered in a spot where a value expression was expected.