Implement Ruby2.7's frozen strings from Symbol#to_s.

mrbgems/mruby-symbol-ext/mrblib/symbol.rb

@@ -10,7 +10,7 @@ class Symbol
   # Same as <code>sym.to_s.capitalize.intern</code>.
 
   def capitalize
-    (self.to_s.capitalize! || self).to_sym
+    self.to_s.capitalize.to_sym
   end
 
   ##
@@ -20,7 +20,7 @@ class Symbol
   # Same as <code>sym.to_s.downcase.intern</code>.
 
   def downcase
-    (self.to_s.downcase! || self).to_sym
+    self.to_s.downcase.to_sym
   end
 
   ##
@@ -30,7 +30,7 @@ class Symbol
   # Same as <code>sym.to_s.upcase.intern</code>.
 
   def upcase
-    (self.to_s.upcase! || self).to_sym
+    self.to_s.upcase.to_sym
   end
 
   ##
@@ -41,7 +41,7 @@ class Symbol
 
   def casecmp(other)
     return nil unless other.kind_of?(Symbol)
-    lhs =  self.to_s; lhs.upcase!
+    lhs =  self.to_s.upcase
     rhs = other.to_s.upcase
     lhs <=> rhs
   end

src/class.c

@@ -72,6 +72,7 @@ mrb_class_name_class(mrb_state *mrb, struct RClass *outer, struct RClass *c, mrb
       }
       return;
     }
+    name = mrb_str_dup(mrb, name);
     mrb_str_cat_cstr(mrb, name, "::");
     mrb_str_cat_cstr(mrb, name, mrb_sym_name(mrb, id));
   }

src/symbol.c

@@ -517,14 +517,18 @@ mrb_sym_str(mrb_state *mrb, mrb_sym sym)
 {
   mrb_int len;
   const char *name = mrb_sym_name_len(mrb, sym, &len);
+  mrb_value str;
 
   if (!name) return mrb_undef_value(); /* can't happen */
   if (SYMBOL_INLINE_P(sym)) {
-    mrb_value str = mrb_str_new(mrb, name, len);
+    str = mrb_str_new(mrb, name, len);
     RSTR_SET_ASCII_FLAG(mrb_str_ptr(str));
-    return str;
   }
-  return mrb_str_new_static(mrb, name, len);
+  else {
+    str = mrb_str_new_static(mrb, name, len);
+  }
+  MRB_SET_FROZEN_FLAG(mrb_str_ptr(str));
+  return str;
 }
 
 static const char*