Raise TypeError if calling ENV.freeze

Previously, you could call ENV.freeze, but it would not have
the desired effect, as you could still modify ENV.

Fixes [Bug #15920]

hash.c

@@ -5699,6 +5699,20 @@ env_reject(void)
     return rb_hash_delete_if(env_to_hash());
 }
 
+/*
+ * call-seq:
+ *   ENV.freeze -> raises TypeError
+ *
+ * Ruby does not allow ENV to be frozen, so calling ENV.freeze
+ * raises TypeError.
+ */
+static VALUE
+env_freeze(VALUE self)
+{
+    rb_raise(rb_eTypeError, "cannot freeze ENV");
+    return self; /* Not reached */
+}
+
 /*
  * call-seq:
  *   ENV.shift -> Array or nil
@@ -6058,6 +6072,7 @@ Init_Hash(void)
     rb_define_singleton_method(envtbl, "filter", env_select, 0);
     rb_define_singleton_method(envtbl, "filter!", env_select_bang, 0);
     rb_define_singleton_method(envtbl, "shift", env_shift, 0);
+    rb_define_singleton_method(envtbl, "freeze", env_freeze, 0);
     rb_define_singleton_method(envtbl, "invert", env_invert, 0);
     rb_define_singleton_method(envtbl, "replace", env_replace, 1);
     rb_define_singleton_method(envtbl, "update", env_update, 1);

test/ruby/test_env.rb

@@ -496,6 +496,10 @@ class TestEnv < Test::Unit::TestCase
     end
   end
 
+  def test_frozen
+    assert_raise(TypeError) { ENV.freeze }
+  end
+
   def test_frozen
     ENV[PATH_ENV] = "/"
     ENV.each do |k, v|