Add -Werror flag to turn Ruby warnings into errors

lib/minitest.rb

@@ -148,6 +148,10 @@ def self.run args = []
     Minitest.seed = options[:seed]
     srand Minitest.seed
 
+    if options[:error_on_warning]
+      require "minitest/error_on_warning"
+    end
+
     reporter = CompositeReporter.new
     reporter << SummaryReporter.new(options[:io], options)
     reporter << ProgressReporter.new(options[:io], options) unless options[:quiet]
@@ -253,6 +257,19 @@ def self.process_args args = [] # :nodoc:
         options[:skip] = s.chars.to_a
       end
 
+      opts.on "-Werror", String, "Turn Ruby warnings into errors" do |s|
+        case s
+        when "error"
+          options[:error_on_warning] = true
+          $VERBOSE = true
+          ::Warning[:deprecated] if ::Warning.respond_to?(:[]=) # Ruby 2.7+
+        else
+          if ::Warning.respond_to?(:[]=) # Ruby 2.7+
+            ::Warning[s.to_sym] = true
+          end
+        end
+      end
+
       unless extensions.empty?
         opts.separator ""
         opts.separator "Known extensions: #{extensions.join(", ")}"
@@ -995,6 +1012,15 @@ def result_label # :nodoc:
     end
   end
 
+  ##
+  # Assertion raised on warning when running in -Werror mode.
+
+  class UnexpectedWarning < Assertion
+    def result_label # :nodoc:
+      "Warning"
+    end
+  end
+
   ##
   # Assertion wrapping an unexpected error that was raised during a run.
 

lib/minitest/error_on_warning.rb

@@ -0,0 +1,11 @@
+module Minitest
+
+  module ErrorOnWarning
+    def warn(message, category: nil)
+      message = "[#{category}] #{message}" if category
+      raise UnexpectedWarning, message
+    end
+  end
+
+  ::Warning.singleton_class.prepend(ErrorOnWarning)
+end