Update deprecation tracking for newer Rails versions

Since Rails 7.1 the preferred way to track deprecations is to use the
deprecation trackers via `Rails.application.deprecators`.

Here we change the implementation to use that if it is available,
otherwise fallback to tracking deprecations via the ActiveSupport
singleton object for older Rails versions.

Author: daugaard

lib/deprecation_tracker.rb

@@ -73,7 +73,17 @@ def self.init_tracker(opts = {})
     mode = opts[:mode]
     transform_message = opts[:transform_message]
     deprecation_tracker = DeprecationTracker.new(shitlist_path, transform_message, mode)
-    if defined?(ActiveSupport)
+    # Since Rails 7.1 the preferred way to track deprecations is to use the deprecation trackers via
+    # `Rails.application.deprecators`.
+    # We fallback to tracking deprecations via the ActiveSupport singleton object if Rails.application.deprecators is
+    # not defined for older Rails versions.
+    if defined?(Rails) && defined?(Rails.application) && defined?(Rails.application.deprecators)
+      Rails.application.deprecators.each do |deprecator|
+        deprecator.behavior << -> (message, _callstack = nil, _deprecation_horizon = nil, _gem_name = nil) {
+          deprecation_tracker.add(message)
+        }
+      end
+    elsif defined?(ActiveSupport)
       ActiveSupport::Deprecation.behavior << -> (message, _callstack = nil, _deprecation_horizon = nil, _gem_name = nil) {
         deprecation_tracker.add(message)
       }

spec/deprecation_tracker_spec.rb

@@ -251,6 +251,48 @@
     end
   end
 
+  describe "#init_tracker" do
+    it "returns a new instance of DeprecationTracker" do
+      tracker = DeprecationTracker.init_tracker({mode: "save"})
+      expect(tracker).to be_a(DeprecationTracker)
+    end
+
+    it "subscribes to KernelWarnTracker deprecation events" do
+      expect do
+        DeprecationTracker.init_tracker({mode: "save"})
+      end.to change(DeprecationTracker::KernelWarnTracker.callbacks, :size).by(1)
+    end
+
+    context "when Rails.application.deprecation is not defined and ActiveSupport is defined" do
+      it "sets the ActiveSupport::Deprecation behavior" do
+        # Stub ActiveSupport::Deprecation with a simple behavior array
+        stub_const("ActiveSupport::Deprecation", Class.new {
+          def self.behavior
+            @behavior ||= []
+          end
+        })
+
+        expect do
+          DeprecationTracker.init_tracker({mode: "save"})
+        end.to change(ActiveSupport::Deprecation.behavior, :size).by(1)
+      end
+    end
+
+    context "when Rails.application.deprecation is defined" do
+      it "sets the behavior of each Rails.application.deprecators" do
+        # Stub Rails.application.deprecators with an array of mock deprecators
+        fake_deprecator_1 = double("Deprecator", behavior: [])
+        fake_deprecator_2 = double("Deprecator", behavior: [])
+        stub_const("Rails", Module.new)
+        allow(Rails).to receive_message_chain(:application, :deprecators).and_return([fake_deprecator_1, fake_deprecator_2])
+
+        expect do
+          DeprecationTracker.init_tracker({ mode: "save" })
+        end.to change(fake_deprecator_1.behavior, :size).by(1).and change(fake_deprecator_2.behavior, :size).by(1)
+      end
+    end
+  end
+
   describe DeprecationTracker::KernelWarnTracker do
     it "captures Kernel#warn" do
       warn_messages = []