silences "possibly useless use of :: in void context" warnings

The AS utility silence_warnings does not really silence this
one, because it is issued at parse-time. It seemed to in
some places because the constant was the only expression in
the block and therefore it was its return value, that could
potentially be used by silence_warnings are return value of
the yield call.

To bypass the warning we assign to a variable. The chosen
variable is "_" because it is special-cased in parse.c not
to issue an "assigned but unused variable" warning in turn.

activesupport/test/dependencies_test.rb

@@ -923,10 +923,8 @@ def test_remove_constant_does_not_trigger_loading_autoloads
 
   def test_remove_constant_does_not_autoload_already_removed_parents_as_a_side_effect
     with_autoloading_fixtures do
-      silence_warnings do
-        ::A
-        ::A::B
-      end
+      _ = ::A    # assignment to silence parse-time warning "possibly useless use of :: in void context"
+      _ = ::A::B # assignment to silence parse-time warning "possibly useless use of :: in void context"
       ActiveSupport::Dependencies.remove_constant('A')
       ActiveSupport::Dependencies.remove_constant('A::B')
       assert !defined?(A)
@@ -936,9 +934,7 @@ def test_remove_constant_does_not_autoload_already_removed_parents_as_a_side_eff
   def test_load_once_constants_should_not_be_unloaded
     with_autoloading_fixtures do
       ActiveSupport::Dependencies.autoload_once_paths = ActiveSupport::Dependencies.autoload_paths
-      silence_warnings do
-        ::A
-      end
+      _ = ::A # assignment to silence parse-time warning "possibly useless use of :: in void context"
       assert defined?(A)
       ActiveSupport::Dependencies.clear
       assert defined?(A)