hash.c: Add a feature to manipulate ruby2_keywords flag

It was found that a feature to check and add ruby2_keywords flag to an
existing Hash is needed when arguments are serialized and deserialized.
It is possible to do the same without explicit APIs, but it would be
good to provide them as a core feature.

https://github.com/rails/rails/pull/38105#discussion_r361863767

Hash.ruby2_keywords_hash?(hash) checks if hash is flagged or not.
Hash.ruby2_keywords_hash(hash) returns a duplicated hash that has a
ruby2_keywords flag,

[Bug #16486]

hash.c

@@ -1874,6 +1874,52 @@ rb_hash_s_try_convert(VALUE dummy, VALUE hash)
     return rb_check_hash_type(hash);
 }
 
+/*
+ *  call-seq:
+ *     Hash.ruby2_keywords_hash?(hash) -> true or false
+ *
+ *  Checks if a given hash is flagged by Module#ruby2_keywords (or
+ *  Proc#ruby2_keywords).
+ *  This method is not for casual use; debugging, researching, and
+ *  some truly necessary cases like serialization of arguments.
+ *
+ *     ruby2_keywords def foo(*args)
+ *       Hash.ruby2_keywords_hash?(args.last)
+ *     end
+ *     foo(k: 1)   #=> true
+ *     foo({k: 1}) #=> false
+ */
+static VALUE
+rb_hash_s_ruby2_keywords_hash_p(VALUE dummy, VALUE hash)
+{
+    Check_Type(hash, T_HASH);
+    return (RHASH(hash)->basic.flags & RHASH_PASS_AS_KEYWORDS) ? Qtrue : Qfalse;
+}
+
+/*
+ *  call-seq:
+ *     Hash.ruby2_keywords_hash(hash) -> hash
+ *
+ *  Duplicates a given hash and adds a ruby2_keywords flag.
+ *  This method is not for casual use; debugging, researching, and
+ *  some truly necessary cases like deserialization of arguments.
+ *
+ *     h = {k: 1}
+ *     h = Hash.ruby2_keywords_hash(h)
+ *     def foo(k: 42)
+ *       k
+ *     end
+ *     foo(*[h]) #=> 1 with neither a warning or an error
+ */
+static VALUE
+rb_hash_s_ruby2_keywords_hash(VALUE dummy, VALUE hash)
+{
+    Check_Type(hash, T_HASH);
+    hash = rb_hash_dup(hash);
+    RHASH(hash)->basic.flags |= RHASH_PASS_AS_KEYWORDS;
+    return hash;
+}
+
 struct rehash_arg {
     VALUE hash;
     st_table *tbl;
@@ -6415,6 +6461,9 @@ Init_Hash(void)
 
     rb_define_method(rb_cHash, "deconstruct_keys", rb_hash_deconstruct_keys, 1);
 
+    rb_define_singleton_method(rb_cHash, "ruby2_keywords_hash?", rb_hash_s_ruby2_keywords_hash_p, 1);
+    rb_define_singleton_method(rb_cHash, "ruby2_keywords_hash", rb_hash_s_ruby2_keywords_hash, 1);
+
     /* Document-class: ENV
      *
      * ENV is a hash-like accessor for environment variables.

test/ruby/test_hash.rb

@@ -1756,4 +1756,28 @@ class TestHash < Test::Unit::TestCase
       super
     end
   end
+
+  ruby2_keywords def get_flagged_hash(*args)
+    args.last
+  end
+
+  def check_flagged_hash(k: :NG)
+    k
+  end
+
+  def test_ruby2_keywords_hash?
+    flagged_hash = get_flagged_hash(k: 1)
+    assert_equal(true, Hash.ruby2_keywords_hash?(flagged_hash))
+    assert_equal(false, Hash.ruby2_keywords_hash?({}))
+    assert_raise(TypeError) { Hash.ruby2_keywords_hash?(1) }
+  end
+
+  def test_ruby2_keywords_hash!
+    hash = {k: 1}
+    assert_equal(false, Hash.ruby2_keywords_hash?(hash))
+    hash = Hash.ruby2_keywords_hash(hash)
+    assert_equal(true, Hash.ruby2_keywords_hash?(hash))
+    assert_equal(1, check_flagged_hash(*[hash]))
+    assert_raise(TypeError) { Hash.ruby2_keywords_hash(1) }
+  end
 end