add note regarding unsafe blocks and place expressions

[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 8f3532c3574f08881704ca4d7c2ecb395c1cc7aa..4b93ee5e1dd1ac1083fa935d5952de785ade3d1a 100644 (file)
--- a/personal/_posts/2024-08-14-places.md
+++ b/personal/_posts/2024-08-14-places.md
@@ -126,10 +126,16 @@ let _ = *ptr; // This is fine!
 let _val = *ptr; // This is UB.
 ```
 
 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
 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).
 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).


@@ -145,6 +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.
 
 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.
 ### 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.