Freeze strings from `nil.to_s`, `true.to_s`, `false.to_s`.

This is an experimental changes in Ruby 2.7.

include/mruby.h

@@ -1022,6 +1022,12 @@ MRB_API mrb_value mrb_str_new_cstr(mrb_state*, const char*);
 MRB_API mrb_value mrb_str_new_static(mrb_state *mrb, const char *p, size_t len);
 #define mrb_str_new_lit(mrb, lit) mrb_str_new_static(mrb, (lit), mrb_strlen_lit(lit))
 
+MRB_API mrb_value mrb_obj_freeze(mrb_state*, mrb_value);
+#define mrb_str_new_frozen(mrb,p,len) mrb_obj_freeze(mrb,mrb_str_new(mrb,p,len))
+#define mrb_str_new_cstr_frozen(mrb,p) mrb_obj_freeze(mrb,mrb_str_new_cstr(mrb,p))
+#define mrb_str_new_static_frozen(mrb,p,len) mrb_obj_freeze(mrb,mrb_str_new_static(mrb,p,len))
+#define mrb_str_new_lit_frozen(mrb,lit) mrb_obj_freeze(mrb,mrb_str_new_lit(mrb,lit))
+
 #ifdef _WIN32
 MRB_API char* mrb_utf8_from_locale(const char *p, int len);
 MRB_API char* mrb_locale_from_utf8(const char *p, int len);

src/kernel.c

@@ -431,7 +431,7 @@ mrb_obj_extend_m(mrb_state *mrb, mrb_value self)
   return mrb_obj_extend(mrb, argc, argv, self);
 }
 
-static mrb_value
+MRB_API mrb_value
 mrb_obj_freeze(mrb_state *mrb, mrb_value self)
 {
   if (!mrb_immediate_p(self)) {

src/object.c

@@ -83,13 +83,13 @@ mrb_true(mrb_state *mrb, mrb_value obj)
 static mrb_value
 nil_to_s(mrb_state *mrb, mrb_value obj)
 {
-  return mrb_str_new(mrb, 0, 0);
+  return mrb_str_new_frozen(mrb, 0, 0);
 }
 
 static mrb_value
 nil_inspect(mrb_state *mrb, mrb_value obj)
 {
-  return mrb_str_new_lit(mrb, "nil");
+  return mrb_str_new_lit_frozen(mrb, "nil");
 }
 
 /***********************************************************************
@@ -150,7 +150,7 @@ true_xor(mrb_state *mrb, mrb_value obj)
 static mrb_value
 true_to_s(mrb_state *mrb, mrb_value obj)
 {
-  return mrb_str_new_lit(mrb, "true");
+  return mrb_str_new_lit_frozen(mrb, "true");
 }
 
 /* 15.2.5.3.4  */
@@ -257,7 +257,7 @@ false_or(mrb_state *mrb, mrb_value obj)
 static mrb_value
 false_to_s(mrb_state *mrb, mrb_value obj)
 {
-  return mrb_str_new_lit(mrb, "false");
+  return mrb_str_new_lit_frozen(mrb, "false");
 }
 
 void

src/string.c

@@ -235,6 +235,13 @@ mrb_str_new_static(mrb_state *mrb, const char *p, size_t len)
   return mrb_obj_value(s);
 }
 
+MRB_API mrb_value
+mrb_str_freeze(mrb_state *mrb, const char *p, size_t len)
+{
+  struct RString *s = str_new_static(mrb, p, len);
+  return mrb_obj_value(s);
+}
+
 static void
 str_decref(mrb_state *mrb, mrb_shared_string *shared)
 {