Fail fast when asked to config.include something that is not a module.

Previously, if you did `config.include :not_a_module, :some_meta`,
and defined an example with `:some_meta` metadata, the ruby type
error would be raised when the example was being prepared to run,
and would lead to other errors due to RSpec’s example setup and
teardown not completing. It was quite confusing.

Instead, we fail fast at the point of the mistake.

Also, we apply this to `config.extend` and `config.prepend`
for consistency.

Author: myronmarston

lib/rspec/core/configuration.rb

@@ -1175,9 +1175,9 @@ def exclusion_filter
       # @see #extend
       # @see #prepend
       def include(mod, *filters)
-        meta = Metadata.build_hash_from(filters, :warn_about_example_group_filtering)
-        @include_modules.append(mod, meta)
-        on_existing_matching_groups(meta) { |group| safe_include(mod, group) }
+        define_mixed_in_module(mod, filters, @include_modules, :include) do |group|
+          safe_include(mod, group)
+        end
       end
 
       # Tells RSpec to extend example groups with `mod`. Methods defined in
@@ -1211,9 +1211,9 @@ def include(mod, *filters)
       # @see #include
       # @see #prepend
       def extend(mod, *filters)
-        meta = Metadata.build_hash_from(filters, :warn_about_example_group_filtering)
-        @extend_modules.append(mod, meta)
-        on_existing_matching_groups(meta) { |group| safe_extend(mod, group) }
+        define_mixed_in_module(mod, filters, @extend_modules, :extend) do |group|
+          safe_extend(mod, group)
+        end
       end
 
       if RSpec::Support::RubyFeatures.module_prepends_supported?
@@ -1250,9 +1250,9 @@ def extend(mod, *filters)
         # @see #include
         # @see #extend
         def prepend(mod, *filters)
-          meta = Metadata.build_hash_from(filters, :warn_about_example_group_filtering)
-          @prepend_modules.append(mod, meta)
-          on_existing_matching_groups(meta) { |group| safe_prepend(mod, group) }
+          define_mixed_in_module(mod, filters, @prepend_modules, :prepend) do |group|
+            safe_prepend(mod, group)
+          end
         end
       end
 
@@ -1956,6 +1956,16 @@ def safe_extend(mod, host)
         end
         # :nocov:
       end
+
+      def define_mixed_in_module(mod, filters, mod_list, config_method, &block)
+        unless Module === mod
+          raise TypeError, "`RSpec.configuration.#{config_method}` expects a module but got: #{mod.inspect}"
+        end
+
+        meta = Metadata.build_hash_from(filters, :warn_about_example_group_filtering)
+        mod_list.append(mod, meta)
+        on_existing_matching_groups(meta, &block)
+      end
     end
     # rubocop:enable Metrics/ClassLength
   end

spec/rspec/core/configuration_spec.rb

@@ -901,6 +901,19 @@ def file_at(relative_path)
       end
     end
 
+    config_methods = %w[ include extend ]
+    config_methods << "prepend" if RSpec::Support::RubyFeatures.module_prepends_supported?
+    config_methods.each do |config_method|
+      it "raises an immediate `TypeError` when you attempt to `config.#{config_method}` with something besides a module" do
+        expect {
+          config.send(config_method, :not_a_module)
+        }.to raise_error(TypeError, a_string_including(
+          "configuration.#{config_method}",
+          "expects a module but got", "not_a_module"
+        ))
+      end
+    end
+
     describe "#include" do
       include_examples "warning of deprecated `:example_group` during filtering configuration", :include, Enumerable