fix panic on macro-expanded impls with primitive self type

The new is_macro_expanded path in get_attributes_for_automatic_derive
runs for any macro-expanded impl, not just #[automatically_derived]
ones. Unlike derive impls, macro-expanded impls can target primitive
types (e.g. vstd's std_specs/ops.rs generates impl ... for u8/usize/...
via macro_rules!), whose Res is PrimTy and has no DefId. Calling
path.res.def_id() on those panics; switch to opt_def_id() and fall
through to normal classification when there's no def_id, matching
the existing fallback arms.

Author: hz2

source/rust_verify/src/external.rs

@@ -684,8 +684,18 @@ fn get_attributes_for_automatic_derive<'tcx>(
                 };
 
                 let type_def_id = match impll.self_ty.kind {
+                    // self type may resolve to a primitive (e.g. macro-generated impls
+                    // on u8, usize, etc. in vstd), which has no def_id
                     rustc_hir::TyKind::Path(rustc_hir::QPath::Resolved(None, path)) => {
-                        path.res.def_id()
+                        match path.res.opt_def_id() {
+                            Some(def_id) => def_id,
+                            None => {
+                                if is_auto_derived {
+                                    warn_unknown();
+                                }
+                                return None;
+                            }
+                        }
                     }
                     _ => {
                         if is_auto_derived {