simplify handling of valtrees for unsized types

compiler/rustc_const_eval/src/interpret/place.rs

@@ -934,14 +934,26 @@ where
         Ok(MPlaceTy { mplace, layout: place.layout, align: place.align })
     }
 
+    pub fn allocate_dyn(
+        &mut self,
+        layout: TyAndLayout<'tcx>,
+        kind: MemoryKind<M::MemoryKind>,
+        meta: MemPlaceMeta<M::Provenance>,
+    ) -> InterpResult<'tcx, MPlaceTy<'tcx, M::Provenance>> {
+        let Some((size, align)) = self.size_and_align_of(&meta, &layout)? else {
+            span_bug!(self.cur_span(), "cannot allocate space for `extern` type, size is not known")
+        };
+        let ptr = self.allocate_ptr(size, align, kind)?;
+        Ok(MPlaceTy::from_aligned_ptr_with_meta(ptr.into(), layout, meta))
+    }
+
     pub fn allocate(
         &mut self,
         layout: TyAndLayout<'tcx>,
         kind: MemoryKind<M::MemoryKind>,
     ) -> InterpResult<'tcx, MPlaceTy<'tcx, M::Provenance>> {
         assert!(layout.is_sized());
-        let ptr = self.allocate_ptr(layout.size, layout.align.abi, kind)?;
-        Ok(MPlaceTy::from_aligned_ptr(ptr.into(), layout))
+        self.allocate_dyn(layout, kind, MemPlaceMeta::None)
     }
 
     /// Returns a wide MPlace of type `&'static [mut] str` to a new 1-aligned allocation.

compiler/rustc_const_eval/src/interpret/terminator.rs

@@ -393,15 +393,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             ));
             assert_eq!(dest_offset, None);
             // Allocate enough memory to hold `src`.
-            let Some((size, align)) = self.size_and_align_of_mplace(&src)? else {
-                span_bug!(
-                    self.cur_span(),
-                    "unsized fn arg with `extern` type tail should not be allowed"
-                )
-            };
-            let ptr = self.allocate_ptr(size, align, MemoryKind::Stack)?;
-            let dest_place =
-                MPlaceTy::from_aligned_ptr_with_meta(ptr.into(), callee_arg.layout, src.meta);
+            let dest_place = self.allocate_dyn(src.layout, MemoryKind::Stack, src.meta)?;
             // Update the local to be that new place.
             *M::access_local_mut(self, dest_frame, dest_local)? = Operand::Indirect(*dest_place);
         }