Add warning whitelist support for specs

Ported from ammar/regexp_parser#26

spec/spec_helper.rb

@@ -14,6 +14,10 @@ if ENV['COVERAGE'] == 'true'
   end
 end
 
+# Require warning support first in order to catch any warnings emitted during boot
+require_relative './support/warning'
+$stderr = MutantSpec::Warning::EXTRACTOR
+
 require 'tempfile'
 require 'concord'
 require 'anima'
@@ -58,4 +62,9 @@ RSpec.configure do |config|
   config.include(MessageHelper)
   config.include(ParserHelper)
   config.include(Mutant::AST::Sexp)
+
+  config.after(:suite) do
+    $stderr = STDERR
+    MutantSpec::Warning.assert_no_warnings
+  end
 end

spec/support/warning.rb

@@ -0,0 +1,64 @@
+require 'yaml'
+require 'equalizer'
+require 'memoizable'
+require 'ice_nine'
+
+module MutantSpec
+  class Warning
+    def self.assert_no_warnings
+      return if EXTRACTOR.warnings.empty?
+
+      fail UnexpectedWarnings, EXTRACTOR.warnings.to_a
+    end
+
+    class UnexpectedWarnings < StandardError
+      MSG = 'Unexpected warnings: %s'.freeze
+
+      def initialize(warnings)
+        super(MSG % warnings.join("\n"))
+      end
+    end
+
+    class Extractor < DelegateClass(IO)
+      PATTERN = /\A(?:.+):(?:\d+): warning: (?:.+)\n\z/.freeze
+
+      include Equalizer.new(:whitelist, :seen, :io), Memoizable
+
+      def initialize(io, whitelist)
+        @whitelist = whitelist
+        @seen      = Set.new
+        @io        = io
+
+        super(io)
+      end
+
+      def write(message)
+        return super if PATTERN !~ message
+
+        add(message.chomp)
+
+        self
+      end
+
+      def warnings
+        seen.dup
+      end
+      memoize :warnings
+
+    private
+
+      def add(warning)
+        return if whitelist.any?(&warning.public_method(:end_with?))
+
+        seen << warning
+      end
+
+      attr_reader :whitelist, :seen, :io
+    end
+
+    warnings  = Pathname.new(__dir__).join('warnings.yml').freeze
+    whitelist = IceNine.deep_freeze(YAML.load(warnings.read))
+
+    EXTRACTOR = Extractor.new(STDERR, whitelist)
+  end
+end

spec/support/warnings.yml

@@ -0,0 +1,11 @@
+---
+- 'lib/parser/lexer.rb:10791: warning: assigned but unused variable - testEof'
+- 'lib/parser/source/rewriter.rb:392: warning: assigned but unused variable - begin_pos'
+- 'lib/rspec/core/memoized_helpers.rb:288: warning: method redefined; discarding old output'
+- 'lib/rspec/core/memoized_helpers.rb:295: warning: method redefined; discarding old output'
+- 'lib/rspec/core/memoized_helpers.rb:295: warning: previous definition of config was here'
+- 'lib/rspec/core/memoized_helpers.rb:295: warning: previous definition of mutation_a_test_result was here'
+- 'lib/rspec/core/memoized_helpers.rb:295: warning: previous definition of output was here'
+- 'spec/support/shared_context.rb:35: warning: previous definition of output was here'
+- 'spec/support/shared_context.rb:4: warning: method redefined; discarding old config'
+- 'spec/support/shared_context.rb:4: warning: method redefined; discarding old mutation_a_test_result'