Filter prepended-into-ancestors modules in ActionControllerHelpers DSL compiler

lib/tapioca/dsl/compilers/action_controller_helpers.rb

@@ -150,8 +150,30 @@ def create_unknown_proxy_method(helper_methods, method_name)
 
         #: (Module[top] mod) -> Array[String]
         def gather_includes(mod)
-          mod.ancestors
-            .reject { |ancestor| ancestor.is_a?(Class) || ancestor == mod || name_of(ancestor).nil? }
+          ancestors = mod.ancestors
+
+          # Exclude modules that were prepended into another ancestor in the chain
+          # rather than explicitly included by the user. Otherwise, modules like
+          # `DEBUGGER__::TrapInterceptor` (prepended into `::Kernel` by the `debug`
+          # gem when loaded in-process by, e.g., the Ruby LSP Tapioca add-on) leak
+          # into generated RBIs.
+          prepended_into_ancestors = ancestors.each_with_object(Set.new) do |ancestor, set|
+            next unless ancestor.is_a?(Module)
+
+            ancestor.ancestors.each do |sub|
+              break if sub == ancestor
+
+              set << sub
+            end
+          end
+
+          ancestors
+            .reject do |ancestor|
+              ancestor.is_a?(Class) ||
+                ancestor == mod ||
+                name_of(ancestor).nil? ||
+                prepended_into_ancestors.include?(ancestor)
+            end
             .map { |ancestor| T.must(qualified_name_of(ancestor)) }
             .reverse
         end

spec/tapioca/dsl/compilers/action_controller_helpers_spec.rb

@@ -383,6 +383,36 @@ class HelperProxy < ::ActionView::Base
               RBI
               assert_equal expected, rbi_for(:UserController)
             end
+
+            it "does not include modules prepended into an ancestor (e.g. debug.gem's TrapInterceptor)" do
+              # `debug/session` prepends `DEBUGGER__::TrapInterceptor` (and related
+              # modules) into `::Kernel`. The Ruby LSP Tapioca add-on loads
+              # `debug/session` in-process, so when it triggers DSL regeneration,
+              # modules prepended into `::Kernel` leak into any helper whose ancestor
+              # chain traverses `::Kernel`.
+              #
+              # These modules were not explicitly included by the user and should not
+              # appear in generated RBIs.
+              require "debug/session"
+
+              add_ruby_file("kernel_helper.rb", <<~RUBY)
+                module KernelHelper
+                  include Kernel
+                end
+              RUBY
+
+              add_ruby_file("controller.rb", <<~RUBY)
+                class UserController < ActionController::Base
+                  helper KernelHelper
+                  helper_method :foo
+                  def foo
+                    "bar"
+                  end
+                end
+              RUBY
+
+              refute_includes(rbi_for(:UserController), "DEBUGGER__")
+            end
           end
         end
       end