[Compatibility] Add Module#const_added

CHANGELOG.md

@@ -22,6 +22,7 @@ Compatibility:
 * Add `String#byteindex` and `String#byterindex` (#3039, @itarato).
 * Add implementations of `rb_proc_call_with_block`, `rb_proc_call_kw`, `rb_proc_call_with_block_kw` and `rb_funcall_with_block_kw` (#3068, @andrykonchin).
 * Add optional `timeout` argument to `Thread::Queue#pop` (#3039, @itarato).
+* Add `Module#const_added` (#3039,  @itarato).
 
 Performance:
 

spec/ruby/core/module/const_added_spec.rb

@@ -121,5 +121,36 @@ class SubClass
 
       ScratchPad.recorded.should == [line + 2, line + 4, line + 7, line + 11]
     end
+
+    it "is called when the constant is ready to be used" do
+      mod = Module.new do
+        def self.const_added(name)
+          ScratchPad.record const_get(name)
+        end
+      end
+
+      mod.module_eval(<<-RUBY, __FILE__, __LINE__ + 1)
+        TEST = 123
+      RUBY
+
+      ScratchPad.recorded.should == 123
+    end
+
+    it "records re-definition of existing constants" do
+      ScratchPad.record []
+
+      mod = Module.new do
+        def self.const_added(name)
+          ScratchPad << const_get(name)
+        end
+      end
+
+      mod.module_eval(<<-RUBY, __FILE__, __LINE__ + 1)
+        TEST = 123
+        TEST = 456
+      RUBY
+
+      ScratchPad.recorded.should == [123, 456]
+    end
   end
 end

spec/truffleruby.next-specs

@@ -25,3 +25,5 @@ spec/ruby/core/string/byterindex_spec.rb
 spec/ruby/core/queue/deq_spec.rb
 spec/ruby/core/queue/pop_spec.rb
 spec/ruby/core/queue/shift_spec.rb
+
+spec/ruby/core/module/const_added_spec.rb

src/main/java/org/truffleruby/core/module/ModuleFields.java

@@ -30,6 +30,7 @@
 import org.truffleruby.RubyLanguage;
 import org.truffleruby.collections.ConcurrentOperations;
 import org.truffleruby.collections.ConcurrentWeakSet;
+import org.truffleruby.core.CoreLibrary;
 import org.truffleruby.core.encoding.Encodings;
 import org.truffleruby.core.encoding.TStringUtils;
 import org.truffleruby.core.kernel.KernelNodes;
@@ -494,6 +495,12 @@ private RubyConstant setConstantInternal(RubyContext context, Node currentNode,
             invalidateConstantIncludedBy(name);
         }
 
+        final CoreLibrary coreLibrary = context.getCoreLibrary();
+        if (currentNode != null && coreLibrary != null && coreLibrary.isLoaded()) {
+            final RubySymbol constSymbol = context.getLanguageSlow().getSymbol(name);
+            RubyContext.send(currentNode, rubyModule, "const_added", constSymbol);
+        }
+
         return autoload ? newConstant : previous;
     }
 

src/main/ruby/truffleruby/core/module.rb

@@ -105,6 +105,9 @@ def prepend(*modules)
     self
   end
 
+  private def const_added(name)
+  end
+
   def const_defined?(name, inherit = true)
     Primitive.module_const_defined?(self, name, inherit, true)
   end