Auto merge of #118602 - TaKO8Ki:rollup-njcouns, r=TaKO8Ki

Rollup of 5 pull requests

Successful merges:

 - #118495 (Restrict what symbols can be used in `#[diagnostic::on_unimplemented]` format strings)
 - #118540 (codegen, miri: fix computing the offset of an unsized field in a packed struct)
 - #118551 (more targeted errors when extern types end up in places they should not)
 - #118573 (rustc: Harmonize `DefKind` and `DefPathData`)
 - #118586 (Improve example in `slice::windows()` doc)

r? `@ghost`
`@rustbot` modify labels: rollup

1 files changed, 9 insertions, 0 deletions

diff --git a/compiler/rustc_codegen_llvm/src/builder.rs b/compiler/rustc_codegen_llvm/src/builder.rs
index 7b259055d40..acd85dd9a2d 100644
--- a/compiler/rustc_codegen_llvm/src/builder.rs
+++ b/compiler/rustc_codegen_llvm/src/builder.rs
@@ -489,6 +489,15 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
 
     #[instrument(level = "trace", skip(self))]
     fn load_operand(&mut self, place: PlaceRef<'tcx, &'ll Value>) -> OperandRef<'tcx, &'ll Value> {
+        if place.layout.is_unsized() {
+            let tail = self.tcx.struct_tail_with_normalize(place.layout.ty, |ty| ty, || {});
+            if matches!(tail.kind(), ty::Foreign(..)) {
+                // Unsized locals and, at least conceptually, even unsized arguments must be copied
+                // around, which requires dynamically determining their size. Therefore, we cannot
+                // allow `extern` types here. Consult t-opsem before removing this check.
+                panic!("unsized locals must not be `extern` types");
+            }
+        }
         assert_eq!(place.llextra.is_some(), place.layout.is_unsized());
 
         if place.layout.is_zst() {