show-source: if the current context is a C object, display a warning

Fixes #691 (some adjustments to show-source behaviour)

lib/pry/code_object.rb

@@ -37,6 +37,28 @@ class Pry
       def command?
         is_a?(Module) && self <= Pry::Command
       end
+
+      # @return [Boolean] `true` if this module was defined by means of the C API,
+      #   `false` if it's a Ruby module.
+      # @note If a module defined by C was extended with a lot of methods written
+      #   in Ruby, this method would fail.
+      def c_module?
+        if is_a?(WrappedModule)
+
+          method_locations = wrapped.methods(false).map do |m|
+            wrapped.method(m).source_location
+          end
+
+          method_locations.concat(wrapped.instance_methods(false).map do |m|
+                                    wrapped.instance_method(m).source_location
+                                  end)
+
+          c_methods = method_locations.grep(nil).count
+          ruby_methods = method_locations.count - c_methods
+
+          c_methods > ruby_methods
+        end
+      end
     end
 
     include Pry::Helpers::CommandHelpers

lib/pry/commands/show_info.rb

@@ -22,7 +22,15 @@ class Pry
       raise CommandError, no_definition_message if !code_object
       @original_code_object = code_object
 
-      if show_all_modules?(code_object)
+      if !obj_name && code_object.c_module? && !opts[:all]
+        result = "Warning: You're inside an object, whose class is defined by means\n" +
+                 "         of the C Ruby API. Pry cannot display the information for\n" +
+                 "         this class."
+        if code_object.candidates.any?
+          result += "\n         However, you can view monkey-patches applied to this class.\n" +
+                    "         Just execute the same command with the '--all' switch."
+        end
+      elsif show_all_modules?(code_object)
         # show all monkey patches for a module
 
         result = content_and_headers_for_all_module_candidates(code_object)

spec/commands/show_source_spec.rb

@@ -603,6 +603,39 @@ describe "show-source" do
               out.should_not =~ /def a; end/
             end
           end
+
+          describe "monkey-patched C modules" do
+            # Monkey-patch Array and add 15 methods, so its internal rank is
+            # high enough to make this definition primary.
+            class Array
+              15.times do |i|
+                define_method(:"doge#{i}") do
+                  :"doge#{i}"
+                end
+              end
+            end
+
+            describe "when current context is a C object" do
+              it "should display a warning, and not monkey-patched definition" do
+                out = pry_eval([1, 2, 3], 'show-source')
+                expect(out).not_to match(/doge/)
+                expect(out).to match(/warning/i)
+              end
+
+              it "recommends to use the --all switch when other candidates are found" do
+                out = pry_eval([], 'show-source')
+                expect(out).to match(/'--all' switch/i)
+              end
+            end
+
+            describe "when current context is something other than a C object" do
+              it "should display a candidate, not a warning" do
+                out = pry_eval('show-source Array')
+                expect(out).to match(/doge/)
+                expect(out).not_to match(/warning/i)
+              end
+            end
+          end
         end
       end
     end