Auto merge of #72269 - RalfJung:unsafe-borrow-of-packed, r=Mark-Simulacrum

move borrow-of-packed-field unsafety check out of loop

Looks like during the place refactoring, this code somehow got into this loop even though it does not actually depend on the loop variables.

Author: bors

src/librustc_mir/transform/check_unsafety.rs

@@ -184,7 +184,7 @@ impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
     }
 
     fn visit_place(&mut self, place: &Place<'tcx>, context: PlaceContext, _location: Location) {
-        // prevent
+        // On types with `scalar_valid_range`, prevent
         // * `&mut x.field`
         // * `x.field = y;`
         // * `&x.field` if `field`'s type has interior mutability
@@ -194,26 +194,26 @@ impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
             self.check_mut_borrowing_layout_constrained_field(*place, context.is_mutating_use());
         }
 
+        if context.is_borrow() {
+            if util::is_disaligned(self.tcx, self.body, self.param_env, *place) {
+                let source_info = self.source_info;
+                let lint_root = self.body.source_scopes[source_info.scope]
+                    .local_data
+                    .as_ref()
+                    .assert_crate_local()
+                    .lint_root;
+                self.require_unsafe(
+                    "borrow of packed field",
+                    "fields of packed structs might be misaligned: dereferencing a \
+                    misaligned pointer or even just creating a misaligned reference \
+                    is undefined behavior",
+                    UnsafetyViolationKind::BorrowPacked(lint_root),
+                );
+            }
+        }
+
         for (i, elem) in place.projection.iter().enumerate() {
             let proj_base = &place.projection[..i];
-
-            if context.is_borrow() {
-                if util::is_disaligned(self.tcx, self.body, self.param_env, *place) {
-                    let source_info = self.source_info;
-                    let lint_root = self.body.source_scopes[source_info.scope]
-                        .local_data
-                        .as_ref()
-                        .assert_crate_local()
-                        .lint_root;
-                    self.require_unsafe(
-                        "borrow of packed field",
-                        "fields of packed structs might be misaligned: dereferencing a \
-                        misaligned pointer or even just creating a misaligned reference \
-                        is undefined behavior",
-                        UnsafetyViolationKind::BorrowPacked(lint_root),
-                    );
-                }
-            }
             let old_source_info = self.source_info;
             if let [] = proj_base {
                 let decl = &self.body.local_decls[place.local];