style: root the RuboCop project index at the tap

Library/Homebrew/style.rb

@@ -236,6 +236,15 @@ def self.run_rubocop(files, output_type,
         args << "--config" << (HOMEBREW_REPOSITORY/"docs/docs_rubocop_style.yml")
       elsif files.any? { |f| f.to_s.start_with? HOMEBREW_LIBRARY_PATH }
         base_dir = HOMEBREW_LIBRARY_PATH
+      elsif (tap = single_tap(files))
+        # RuboCop roots its project index at the working directory for this
+        # config (see `tap_rubocop_style.yml`). A tap has to be indexed on its
+        # own: rooted at `Library`, the index spans every installed tap and the
+        # cross-file cops pair one tap's constants and methods with another's as
+        # reassignments and duplicates. The paths stay as given, so the shared
+        # config's `Taps/...` exclusions keep matching a symlinked tap.
+        args << "--config" << (HOMEBREW_LIBRARY/"tap_rubocop_style.yml")
+        base_dir = tap.path
       else
         args << "--config" << (HOMEBREW_LIBRARY/".rubocop.yml")
         base_dir = HOMEBREW_LIBRARY if files.any? { |f| f.to_s.start_with? HOMEBREW_LIBRARY }
@@ -282,6 +291,15 @@ def self.run_rubocop(files, output_type,
       end
     end
 
+    sig { params(files: T::Array[Pathname]).returns(T.nilable(Tap)) }
+    def self.single_tap(files)
+      taps = files.filter_map { |file| Tap.from_path(file) }
+      return if taps.empty? || taps.length != files.length
+      return unless taps.map(&:name).uniq.one?
+
+      taps.first
+    end
+
     sig {
       params(
         files:           T::Array[Pathname],

Library/Homebrew/test/style_spec.rb

@@ -256,5 +256,37 @@ class Test
 
       described_class.run_rubocop([ruby_file], :json, fix: true, todo: true)
     end
+
+    it "roots RuboCop at the tap when every file is in one tap" do
+      tap_path = HOMEBREW_TAP_DIRECTORY/"homebrew/homebrew-foo"
+      script = tap_path/"cmd/foo.rb"
+      script.dirname.mkpath
+      script.write "# frozen_string_literal: true\n"
+      result = double(status: double(exitstatus: 0), stdout: '{"files":[]}')
+
+      expect(described_class).to receive(:system_command).with(
+        anything,
+        hash_including(args: include("--config", HOMEBREW_LIBRARY/"tap_rubocop_style.yml", script), chdir: tap_path),
+      ).and_return(result)
+
+      described_class.run_rubocop([script], :json)
+    end
+
+    it "uses the shared config when files span multiple taps" do
+      scripts = %w[foo bar].map do |name|
+        script = HOMEBREW_TAP_DIRECTORY/"homebrew/homebrew-#{name}/cmd/#{name}.rb"
+        script.dirname.mkpath
+        script.write "# frozen_string_literal: true\n"
+        script
+      end
+      result = double(status: double(exitstatus: 0), stdout: '{"files":[]}')
+
+      expect(described_class).to receive(:system_command).with(
+        anything,
+        hash_including(args: include("--config", HOMEBREW_LIBRARY/".rubocop.yml"), chdir: HOMEBREW_LIBRARY),
+      ).and_return(result)
+
+      described_class.run_rubocop(scripts, :json)
+    end
   end
 end

Library/tap_rubocop_style.yml

@@ -0,0 +1,7 @@
+---
+# `brew style` passes this file as `--config` when every target is inside one
+# tap. RuboCop roots its project index at the directory of a `.rubocop*` config
+# file and at the working directory for any other name, so this file lets
+# `brew style` root the index at the tap it is checking instead of at every
+# tap under `Library/Taps`.
+inherit_from: .rubocop.yml