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)
```

1 files changed, 125 insertions, 0 deletions

diff --git a/spec/ruby/core/module/const_added_spec.rb b/spec/ruby/core/module/const_added_spec.rb
new file mode 100644
index 0000000000..ff2ee0987f
--- /dev/null
+++ b/spec/ruby/core/module/const_added_spec.rb
@@ -0,0 +1,125 @@
+require_relative '../../spec_helper'
+require_relative 'fixtures/classes'
+
+describe "Module#const_added" do
+  ruby_version_is "3.1" do
+    it "is a private instance method" do
+      Module.should have_private_instance_method(:const_added)
+    end
+
+    it "returns nil in the default implementation" do
+      Module.new do
+        const_added(:TEST).should == nil
+      end
+    end
+
+    it "is called when a new constant is assigned on self" do
+      ScratchPad.record []
+
+      mod = Module.new do
+        def self.const_added(name)
+          ScratchPad << name
+        end
+      end
+
+      mod.module_eval(<<-RUBY, __FILE__, __LINE__ + 1)
+        TEST = 1
+      RUBY
+
+      ScratchPad.recorded.should == [:TEST]
+    end
+
+    it "is called when a new constant is assigned on self throught const_set" do
+      ScratchPad.record []
+
+      mod = Module.new do
+        def self.const_added(name)
+          ScratchPad << name
+        end
+      end
+
+      mod.const_set(:TEST, 1)
+
+      ScratchPad.recorded.should == [:TEST]
+    end
+
+    it "is called when a new module is defined under self" do
+      ScratchPad.record []
+
+      mod = Module.new do
+        def self.const_added(name)
+          ScratchPad << name
+        end
+      end
+
+      mod.module_eval(<<-RUBY, __FILE__, __LINE__ + 1)
+        module SubModule
+        end
+
+        module SubModule
+        end
+      RUBY
+
+      ScratchPad.recorded.should == [:SubModule]
+    end
+
+    it "is called when a new class is defined under self" do
+      ScratchPad.record []
+
+      mod = Module.new do
+        def self.const_added(name)
+          ScratchPad << name
+        end
+      end
+
+      mod.module_eval(<<-RUBY, __FILE__, __LINE__ + 1)
+        class SubClass
+        end
+
+        class SubClass
+        end
+      RUBY
+
+      ScratchPad.recorded.should == [:SubClass]
+    end
+
+    it "is called when an autoload is defined" do
+      ScratchPad.record []
+
+      mod = Module.new do
+        def self.const_added(name)
+          ScratchPad << name
+        end
+      end
+
+      mod.autoload :Autoload, "foo"
+      ScratchPad.recorded.should == [:Autoload]
+    end
+
+    it "is called with a precise caller location with the line of definition" do
+      ScratchPad.record []
+
+      mod = Module.new do
+        def self.const_added(name)
+          location = caller_locations(1, 1)[0]
+          ScratchPad << location.lineno
+        end
+      end
+
+      line = __LINE__
+      mod.module_eval(<<-RUBY, __FILE__, __LINE__ + 1)
+        TEST = 1
+
+        module SubModule
+        end
+
+        class SubClass
+        end
+      RUBY
+
+      mod.const_set(:CONST_SET, 1)
+
+      ScratchPad.recorded.should == [line + 2, line + 4, line + 7, line + 11]
+    end
+  end
+end