fix static_ptr_ty for foreign statics, and more comments in check_unsafety

Author: RalfJung

compiler/rustc_middle/src/ty/util.rs

@@ -529,8 +529,11 @@ impl<'tcx> TyCtxt<'tcx> {
         // Make sure that any constants in the static's type are evaluated.
         let static_ty = self.normalize_erasing_regions(ty::ParamEnv::empty(), self.type_of(def_id));
 
+        // Make sure that accesses to unsafe statics end up using raw pointers.
         if self.is_mutable_static(def_id) {
             self.mk_mut_ptr(static_ty)
+        } else if self.is_foreign_item(def_id) {
+            self.mk_imm_ptr(static_ty)
         } else {
             self.mk_imm_ref(self.lifetimes.re_erased, static_ty)
         }

compiler/rustc_mir/src/transform/check_unsafety.rs

@@ -204,6 +204,9 @@ impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
             if let [] = proj_base {
                 let decl = &self.body.local_decls[place.local];
                 if decl.internal {
+                    // If the projection root is an artifical local that we introduced when
+                    // desugaring `static`, give a more specific error message
+                    // (avoid the general "raw pointer" clause below, that would only be confusing).
                     if let Some(box LocalInfo::StaticRef { def_id, .. }) = decl.local_info {
                         if self.tcx.is_mutable_static(def_id) {
                             self.require_unsafe(