From 60197b3db191d300a33464ff89ccb8a14e35abc1 Mon Sep 17 00:00:00 2001
From: Adam Traver <adam.traver@shopify.com>
Date: Fri, 17 Apr 2026 02:47:08 +0000
Subject: [PATCH] Filter prepended-into-ancestors modules in
 ActionControllerHelpers

When a module is prepended into another ancestor in the helpers chain
(e.g. `DEBUGGER__::TrapInterceptor` prepended into `::Kernel` by
debug.gem, or `ActiveSupport::CoreExt::ERBUtil*` prepended into
`::ERB::Util` by ActiveSupport), it appears in `mod.ancestors` even
though the user never explicitly included it. Emitting `include ::X`
for those modules pollutes generated RBIs and, in the DEBUGGER case,
causes Sorbet to fail with "Unable to resolve constant DEBUGGER__"
because debug.gem is not typechecked.

This shows up in practice when the Ruby LSP Tapioca add-on regenerates
DSL RBIs inside a ruby-lsp process that has `debug/session` loaded --
the chain is visible in the LSP but not in plain-shell invocations of
`bin/tapioca dsl`, which is very confusing to diagnose.

Fix: in `gather_includes`, compute the set of modules that appear in
any ancestor's prepend chain and exclude them from the output.
---
 .../compilers/action_controller_helpers.rb    | 26 ++++++++++++++--
 .../action_controller_helpers_spec.rb         | 30 +++++++++++++++++++
 2 files changed, 54 insertions(+), 2 deletions(-)

diff --git a/lib/tapioca/dsl/compilers/action_controller_helpers.rb b/lib/tapioca/dsl/compilers/action_controller_helpers.rb
index 3350255ef..cf7908eb1 100644
--- a/lib/tapioca/dsl/compilers/action_controller_helpers.rb
+++ b/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
diff --git a/spec/tapioca/dsl/compilers/action_controller_helpers_spec.rb b/spec/tapioca/dsl/compilers/action_controller_helpers_spec.rb
index f73d31717..f00c8283a 100644
--- a/spec/tapioca/dsl/compilers/action_controller_helpers_spec.rb
+++ b/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