[rubygems/rubygems] Add build warning when rake based extension is present, but rake is not specified as dependency.

https://github.com/rubygems/rubygems/commit/75fe5475b6

lib/rubygems/ext/build_error.rb

@@ -2,5 +2,7 @@
 ##
 # Raised when there is an error while building extensions.
 
+require 'rubygems/exceptions'
+
 class Gem::Ext::BuildError < Gem::InstallError
 end

lib/rubygems/specification_policy.rb

@@ -1,3 +1,4 @@
+require 'rubygems/ext'
 require 'rubygems/user_interaction'
 
 class Gem::SpecificationPolicy
@@ -76,6 +77,8 @@ class Gem::SpecificationPolicy
 
     validate_dependencies
 
+    validate_extensions
+
     validate_removed_attributes
 
     if @warnings > 0
@@ -417,6 +420,17 @@ http://spdx.org/licenses or '#{Gem::Licenses::NONSTANDARD}' for a nonstandard li
     end
   end
 
+  def validate_extensions # :nodoc:
+    builder = Gem::Ext::Builder.new(@specification)
+
+    rake_extension = @specification.extensions.any? {|s| builder.builder_for(s) == Gem::Ext::RakeBuilder }
+    rake_dependency = @specification.dependencies.any? {|d| d.name == 'rake'}
+
+    warning <<-WARNING if rake_extension && !rake_dependency
+You have specified rake based extension, but rake is not added as dependency. It is recommended to add rake as a dependency since there's no guarantee rake will be already installed.
+    WARNING
+  end
+
   def warning(statement) # :nodoc:
     @warnings += 1
 

test/rubygems/test_gem_specification.rb

@@ -2838,6 +2838,37 @@ duplicate dependency on c (>= 1.2.3, development), (~> 1.2) use:
     end
   end
 
+  def test_validate_rake_extension_have_rake_dependency_warning
+    util_setup_validate
+
+    Dir.chdir @tempdir do
+      @a1.extensions = ['Rakefile']
+      File.write File.join(@tempdir, 'Rakefile'), ''
+
+      use_ui @ui do
+        @a1.validate
+      end
+
+      assert_match(/add rake as a dependency/, @ui.error)
+    end
+  end
+
+  def test_validate_rake_extension_have_rake_dependency_no_warning
+    util_setup_validate
+
+    Dir.chdir @tempdir do
+      @a1.extensions = ['Rakefile']
+      @a1.add_runtime_dependency 'rake'
+      File.write File.join(@tempdir, 'Rakefile'), ''
+
+      use_ui @ui do
+        @a1.validate
+      end
+
+      refute_match(/add rake as a dependency/, @ui.error)
+    end
+  end
+
   def test_validate_description
     util_setup_validate