Add cop to police os checks

The methods `OS.linux?` and `OS.mac?` should only be used
in `extend/os` and this cop makes sure of that.

Library/.rubocop.yml

@@ -55,6 +55,11 @@ FormulaAudit:
 FormulaAuditStrict:
   Enabled: true
 
+Homebrew/MoveToExtendOS:
+  Exclude:
+    - "Homebrew/{extend,test}/**/*"
+    - "Taps/**/*"
+
 # enable all Homebrew custom cops
 Homebrew:
   Enabled: true

Library/Homebrew/rubocops/all.rb

@@ -14,6 +14,7 @@ end
 
 require_relative "io_read"
 require_relative "shell_commands"
+require_relative "platform"
 
 require_relative "formula_desc"
 require_relative "components_order"

Library/Homebrew/rubocops/platform.rb

@@ -0,0 +1,25 @@
+# typed: false
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Homebrew
+      # This cop ensures that platform specific code ends up in `extend/os`.
+      #
+      # @api private
+      class MoveToExtendOS < Base
+        MSG = "Move calls to `OS.linux?` and `OS.mac?` to `extend/os`."
+
+        def_node_matcher :os_check?, <<~PATTERN
+          (send (const nil? :OS) {:mac? | :linux?})
+        PATTERN
+
+        def on_send(node)
+          return unless os_check?(node)
+
+          add_offense(node)
+        end
+      end
+    end
+  end
+end

Library/Homebrew/test/rubocops/platform_spec.rb

@@ -0,0 +1,22 @@
+# typed: false
+# frozen_string_literal: true
+
+require "rubocops/platform"
+
+describe RuboCop::Cop::Homebrew::MoveToExtendOS do
+  subject(:cop) { described_class.new }
+
+  it "registers an offense when using `OS.linux?`" do
+    expect_offense(<<~RUBY)
+      OS.linux?
+      ^^^^^^^^^ Move calls to `OS.linux?` and `OS.mac?` to `extend/os`.
+    RUBY
+  end
+
+  it "registers an offense when using `OS.mac?`" do
+    expect_offense(<<~RUBY)
+      OS.mac?
+      ^^^^^^^ Move calls to `OS.linux?` and `OS.mac?` to `extend/os`.
+    RUBY
+  end
+end