X-Git-Url: https://git.ralfj.de/web.git/blobdiff_plain/2581834cd54e4c420d2a75314189a2aba93ff94e..6a09e54876b302feb4f66e055c39fa4007755a5c:/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 ac298b8..813bfc9 100644
--- a/personal/_posts/2024-08-14-places.md
+++ b/personal/_posts/2024-08-14-places.md
@@ -1,6 +1,7 @@
 ---
 title: "What is a place expression?"
 categories: programming rust
+reddit: /rust/comments/1esavn3/what_is_a_place_expression/
 ---
 
 One of the more subtle aspects of the Rust language is the fact that there are actually two kinds of expressions:
@@ -10,6 +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
@@ -20,10 +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.
@@ -34,6 +38,9 @@ That is the topic of this post.
 
 <!-- MORE -->
 
+(You might have already encountered the distinction of place expressions and value expressions in C and C++, where they are called lvalue expressions and rvalue expressions, respectively.
+While the basic syntactic concept is the same as in Rust, the exact cases that are UB are different, so we will focus entirely on Rust here.)
+
 ### Making the implicit explicit
 
 The main reason why this dichotomy of place expressions and value expressions is so elusive is that it is entirely implicit.
@@ -67,7 +74,7 @@ However, the expression `my_var` (referencing a local variable), according to th
 This is because `my_var` actually denotes a place in memory, and there's multiple things one can do with a place:
 one can load the contents of the place from memory (which produces a value), one can create a pointer to the place (which also produces a value, but does not access memory at all),
 or one can store a value into this place (which in Rust produces the `()` value, but the side-effect of changing the contents of memory is more relevant).
-Besides local variable, the other main example of a place expression is the result of the `*` operator, which takes a *value* (of pointer type) and turns it into a place.
+Besides local variables, the other main example of a place expression is the result of the `*` operator, which takes a *value* (of pointer type) and turns it into a place.
 Furthermore, given a place of struct type, we can use a field projection to obtain a place just for that field.
 
 This may sound odd, because it means that `let new_var = my_var;` is not actually a valid statement in our grammar!
@@ -119,10 +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).