clarify how 'let _' relates to the grammar above

[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..813bfc91fea7769306ae45e25237dc231dbb1379 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
+// 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).