Use in built rspec warnings rather than kernel

Changelog.md

@@ -5,6 +5,7 @@ Bug Fixes:
 
 * When using null object doubles, prevent typos triggering dynamic matchers.
   (Eric Mueller, #1455)
+* Use `RSpec.warning` for an expectation warning rather than `Kernel.warn`. (Jon Rowe, #1472)
 
 Enhancements:
 

lib/rspec/expectations/handler.rb

@@ -4,11 +4,10 @@ module Expectations
     module ExpectationHelper
       def self.check_message(msg)
         unless msg.nil? || msg.respond_to?(:to_str) || msg.respond_to?(:call)
-          ::Kernel.warn [
-            "WARNING: ignoring the provided expectation message argument (",
-            msg.inspect,
-            ") since it is not a string or a proc."
-          ].join
+          RSpec.warning(
+            "ignoring the provided expectation message argument" \
+            "(#{ msg.inspect }) since it is not a string or a proc"
+          )
         end
       end
 

spec/rspec/expectations/handler_spec.rb

@@ -145,6 +145,16 @@ module Expectations
 
           expect(handle_matcher_result.error_generator.opts).to eq({ :message => "custom" })
         end
+
+        it 'warns when the expectation message is not a string or proc' do
+          matcher = double("matcher", :matches? => true)
+          actual = Object.new
+          not_a_string_or_proc = Object.new
+
+          expect(capture_warnings {
+            RSpec::Expectations::PositiveExpectationHandler.handle_matcher(actual, matcher, not_a_string_or_proc)
+          }).to contain_exactly(a_string_matching(/since it is not a string or a proc/))
+        end
       end
     end
 

spec/spec_helper.rb

@@ -25,6 +25,20 @@ def dedent(string)
     string.gsub(/^\s+\|/, '').chomp
   end
 
+  def capture_warnings(&block)
+    warning_notifier = RSpec::Support.warning_notifier
+    warnings = []
+    RSpec::Support.warning_notifier = lambda { |warning| warnings << warning }
+
+    begin
+      block.call
+    ensure
+      RSpec::Support.warning_notifier = warning_notifier
+    end
+
+    warnings
+  end
+
   # We have to use Hash#inspect in examples that have multi-entry
   # hashes because the #inspect output on 1.8.7 is non-deterministic
   # due to the fact that hashes are not ordered. So we can't simply