Add a Module#const_added callback

[Feature #17881]

Works similarly to `method_added` but for constants.

```ruby
Foo::BAR = 42 # call Foo.const_added(:FOO)
class Foo::Baz; end # call Foo.const_added(:Baz)
Foo.autoload(:Something, "path") # call Foo.const_added(:Something)
```

2 files changed, 49 insertions, 0 deletions

diff --git a/test/ruby/test_module.rb b/test/ruby/test_module.rb
index 9368940050..0a6959c5e6 100644
--- a/test/ruby/test_module.rb
+++ b/test/ruby/test_module.rb
@@ -1675,6 +1675,45 @@ class TestModule < Test::Unit::TestCase
     assert_match(/::X\u{df}:/, c.new.to_s)
   end
 
+
+  def test_const_added
+    eval(<<~RUBY)
+      module TestConstAdded
+        @memo = []
+        class << self
+          attr_accessor :memo
+
+          def const_added(sym)
+            memo << sym
+          end
+        end
+        CONST = 1
+        module SubModule
+        end
+
+        class SubClass
+        end
+      end
+      TestConstAdded::OUTSIDE_CONST = 2
+      module TestConstAdded::OutsideSubModule; end
+      class TestConstAdded::OutsideSubClass; end
+    RUBY
+    TestConstAdded.const_set(:CONST_SET, 3)
+    assert_equal [
+      :CONST,
+      :SubModule,
+      :SubClass,
+      :OUTSIDE_CONST,
+      :OutsideSubModule,
+      :OutsideSubClass,
+      :CONST_SET,
+    ], TestConstAdded.memo
+  ensure
+    if self.class.const_defined? :TestConstAdded
+      self.class.send(:remove_const, :TestConstAdded)
+    end
+  end
+
   def test_method_added
     memo = []
     mod = Module.new do
diff --git a/test/ruby/test_settracefunc.rb b/test/ruby/test_settracefunc.rb
index e973e3a384..0524c35873 100644
--- a/test/ruby/test_settracefunc.rb
+++ b/test/ruby/test_settracefunc.rb
@@ -108,6 +108,10 @@ class TestSetTraceFunc < Test::Unit::TestCase
                  events.shift)
     assert_equal(["line", 4, __method__, self.class],
                  events.shift)
+    assert_equal(["c-call", 4, :const_added, Module],
+                 events.shift)
+    assert_equal(["c-return", 4, :const_added, Module],
+                 events.shift)
     assert_equal(["c-call", 4, :inherited, Class],
                  events.shift)
     assert_equal(["c-return", 4, :inherited, Class],
@@ -345,6 +349,8 @@ class TestSetTraceFunc < Test::Unit::TestCase
 
     [["c-return", 2, :add_trace_func, Thread],
      ["line", 3, __method__, self.class],
+     ["c-call", 3, :const_added, Module],
+     ["c-return", 3, :const_added, Module],
      ["c-call", 3, :inherited, Class],
      ["c-return", 3, :inherited, Class],
      ["class", 3, nil, nil],
@@ -487,6 +493,8 @@ class TestSetTraceFunc < Test::Unit::TestCase
      [:line,     5, 'xyzzy', self.class,  method,           self,        :inner, :nothing],
      [:c_return, 4, "xyzzy", Integer,     :times,           1,           :outer, 1],
      [:line,     7, 'xyzzy', self.class,  method,           self,        :outer, :nothing],
+     [:c_call,   7, "xyzzy", Module,      :const_added,     TestSetTraceFunc, :outer, :nothing],
+     [:c_return, 7, "xyzzy", Module,      :const_added,     TestSetTraceFunc, :outer, nil],
      [:c_call,   7, "xyzzy", Class,       :inherited,       Object,      :outer, :nothing],
      [:c_return, 7, "xyzzy", Class,       :inherited,       Object,      :outer, nil],
      [:class,    7, "xyzzy", nil,         nil,              xyzzy.class, nil,    :nothing],
@@ -620,6 +628,8 @@ CODE
      [:line,     7, 'xyzzy', self.class,  method,           self,        :outer, :nothing],
      [:c_call,   7, "xyzzy", Class,       :inherited,       Object,      :outer, :nothing],
      [:c_return, 7, "xyzzy", Class,       :inherited,       Object,      :outer, nil],
+     [:c_call,   7, "xyzzy", Class,       :const_added,     Object,      :outer, :nothing],
+     [:c_return, 7, "xyzzy", Class,       :const_added,     Object,      :outer, nil],
      [:class,    7, "xyzzy", nil,         nil,              xyzzy.class, nil,    :nothing],
      [:line,     8, "xyzzy", nil,         nil,              xyzzy.class, nil,    :nothing],
      [:line,     9, "xyzzy", nil,         nil,              xyzzy.class, :XYZZY_outer, :nothing],