Warn when $= is used

spec/ruby/language/predefined_spec.rb

@@ -1378,3 +1378,29 @@ def obj.foo2; yield; end
     end
   end
 end
+
+# Some other pre-defined global variables
+
+describe "Predefined global $=" do
+  before :each do
+    @verbose, $VERBOSE = $VERBOSE, nil
+    @dollar_assign = $=
+  end
+
+  after :each do
+    $= = @dollar_assign
+    $VERBOSE = @verbose
+  end
+
+  it "warns when accessed" do
+    -> { a = $= }.should complain(/is no longer effective/)
+  end
+
+  it "warns when assigned" do
+    -> { $= = "_" }.should complain(/is no longer effective/)
+  end
+
+  it "returns the value assigned" do
+    ($= = "xyz").should == "xyz"
+  end
+end

src/main/ruby/truffleruby/core/truffle/kernel_operations.rb

@@ -133,7 +133,18 @@ def $LOAD_PATH.resolve_feature_path(file_name)
 
     $, = nil # It should be defined by the time boot has finished.
 
-    $= = false
+    define_hooked_variable(
+      :$=,
+      -> {
+        warn 'variable $= is no longer effective', uplevel: 1 if Warning[:deprecated]
+        Primitive.global_variable_get :$=
+      },
+      -> v {
+        warn 'variable $= is no longer effective', uplevel: 1 if Warning[:deprecated]
+        Primitive.global_variable_set :$=, v
+      })
+
+    Primitive.global_variable_set :$=, false
 
     define_hooked_variable(
       :$VERBOSE,