restore incorrectly removed code

But fix its interaction with places and GC, allocating the symbol
on startup instead of per-place.

src/racket/src/compenv.c

@@ -31,6 +31,8 @@
 READ_ONLY static Scheme_Object *scheme_local[MAX_CONST_LOCAL_POS][MAX_CONST_LOCAL_TYPES][MAX_CONST_LOCAL_FLAG_VAL + 1];
 READ_ONLY static Scheme_Object *toplevels[MAX_CONST_TOPLEVEL_DEPTH][MAX_CONST_TOPLEVEL_POS][SCHEME_TOPLEVEL_FLAGS_MASK + 1];
 
+READ_ONLY static Scheme_Object *unshadowable_symbol;
+
 /* If locked, these are probably sharable: */
 THREAD_LOCAL_DECL(static Scheme_Hash_Table *toplevels_ht);
 THREAD_LOCAL_DECL(static Scheme_Hash_Table *locals_ht[2]);
@@ -99,6 +101,12 @@ void scheme_init_compenv_places(void)
   }
 }
 
+void scheme_init_compenv_symbol(void)
+{
+  REGISTER_SO(unshadowable_symbol);
+  unshadowable_symbol = scheme_intern_symbol("unshadowable");
+}
+
 /*========================================================================*/
 /*                       compilation info management                      */
 /*========================================================================*/
@@ -2419,7 +2427,7 @@ Scheme_Object *scheme_namespace_lookup_value(Scheme_Object *sym, Scheme_Env *gen
 Scheme_Object *scheme_find_local_shadower(Scheme_Object *sym, Scheme_Object *sym_marks, Scheme_Comp_Env *env)
 {
   Scheme_Comp_Env *frame;
-  Scheme_Object *esym, *uid = NULL, *env_marks;
+  Scheme_Object *esym, *uid = NULL, *env_marks, *prop;
 
   /* Walk backward through the frames, looking for a renaming binding
      with the same marks as the given identifier, sym. Skip over
@@ -2431,6 +2439,8 @@ Scheme_Object *scheme_find_local_shadower(Scheme_Object *sym, Scheme_Object *sym
     for (i = frame->num_bindings; i--; ) {
       if (frame->values[i]) {
 	if (SAME_OBJ(SCHEME_STX_VAL(sym), SCHEME_STX_VAL(frame->values[i])))  {
+          prop = scheme_stx_property(frame->values[i], unshadowable_symbol, NULL);
+          if (SCHEME_FALSEP(prop)) {
             esym = frame->values[i];
             env_marks = scheme_stx_extract_marks(esym);
             if (scheme_equal(env_marks, sym_marks)) {
@@ -2444,6 +2454,7 @@ Scheme_Object *scheme_find_local_shadower(Scheme_Object *sym, Scheme_Object *sym
           }
 	}
       }
+    }
     if (uid)
       break;
 
@@ -2453,6 +2464,8 @@ Scheme_Object *scheme_find_local_shadower(Scheme_Object *sym, Scheme_Object *sym
           if (SAME_OBJ(SCHEME_STX_VAL(sym), 
                        SCHEME_STX_VAL(COMPILE_DATA(frame)->const_names[i]))) {
             esym = COMPILE_DATA(frame)->const_names[i];
+            prop = scheme_stx_property(esym, unshadowable_symbol, NULL);
+            if (SCHEME_FALSEP(prop)) {
               env_marks = scheme_stx_extract_marks(esym);
               if (scheme_equal(env_marks, sym_marks)) { /* This used to have 1 || --- why? */
                 sym = esym;
@@ -2466,6 +2479,7 @@ Scheme_Object *scheme_find_local_shadower(Scheme_Object *sym, Scheme_Object *sym
 	  }
 	}
       }
+    }
     if (uid)
       break;
   }

src/racket/src/env.c

@@ -282,6 +282,8 @@ Scheme_Env *scheme_engine_instance_init()
 
   scheme_init_logging_once();
 
+  scheme_init_compenv_symbol();
+
 #if defined(MZ_PLACES_WAITPID)
   scheme_places_start_child_signal_handler();
 #endif

src/racket/src/schpriv.h

@@ -307,6 +307,7 @@ void scheme_init_paramz(Scheme_Env *env);
 void scheme_init_parameterization();
 void scheme_init_getenv(void);
 void scheme_init_inspector(void);
+void scheme_init_compenv_symbol(void);
 
 #ifndef DONT_USE_FOREIGN
 void scheme_init_foreign_globals();