Improve how we handle deprecation warnings

Shift to not logging deprecation warnings on production environments.
Also provides examples of deprecation warnings. We can uncomment these
messages to disable the usage entirely — will raise an exception if
used once added to the resolved list.

config/initializers/gitlab_experiment.rb

 # frozen_string_literal: true
 
-module Gitlab
-  class Experiment
-    class Configuration
-      def self.deprecated(*args, version:, stack: 0)
-        # do nothing here
-      end
-    end
-  end
-end
-
 Gitlab::Experiment.configure do |config|
   # The base experiment class that will be instantiated when using the
   # `experiment` DSL, is our ApplicationExperiment. If a custom experiment
@@ -83,4 +73,31 @@ Gitlab::Experiment.configure do |config|
       )
     ))
   end
+
+  # Deprecation warnings resolution for 0.7.0
+  #
+  # We're working through deprecation warnings one by one in:
+  # https://gitlab.com/gitlab-org/gitlab/-/issues/350944
+  #
+  config.singleton_class.prepend(Module.new do
+    # Disable all deprecations in non dev/test environments.
+    #
+    def deprecated(*args, version:, stack: 0)
+      super if Gitlab.dev_or_test_env?
+    end
+
+    # Maintain a list of resolved deprecations to ensure that no new uses appear.
+    #
+    # Once a resolved deprecation warning has been added here, any future use will
+    # raise an exception.
+    #
+    ActiveSupport::Deprecation.disallowed_warnings += [
+      # 'Gitlab::Experiment 0.8 (instead use `control`)', # don't use `use`
+      # 'Gitlab::Experiment 0.8 (instead use `candidate`)', # don't use `try`
+      # 'Gitlab::Experiment 0.8 (instead use `variant(:variant_name)`)', # don't use `try(:variant_name)`
+      # 'Gitlab::Experiment 0.8 (instead use `assigned(:candidate)`)', # don't use variant(:variant_name) to assign
+      # 'Gitlab::Experiment 0.8 (instead use `assigned`)', # don't use variant.name to get the assigned variant
+      # 'Gitlab::Experiment 0.8, instead register variants using:', # don't use public `*_behavior` methods
+    ]
+  end)
 end

spec/experiments/application_experiment_spec.rb

@@ -451,4 +451,29 @@ RSpec.describe ApplicationExperiment, :experiment do
       end
     end
   end
+
+  context "with deprecation warnings" do
+    before do
+      Gitlab::Experiment::Configuration.instance_variable_set(:@__dep_versions, nil) # clear the internal memoization
+
+      allow(ActiveSupport::Deprecation).to receive(:new).and_call_original
+    end
+
+    it "doesn't warn on non dev/test environments" do
+      allow(Gitlab).to receive(:dev_or_test_env?).and_return(false)
+
+      expect { experiment(:example) { |e| e.use { } } }.not_to raise_error
+      expect(ActiveSupport::Deprecation).not_to have_received(:new).with(anything, 'Gitlab::Experiment')
+    end
+
+    it "warns on dev and test environments" do
+      allow(Gitlab).to receive(:dev_or_test_env?).and_return(true)
+
+      # This will eventually raise an ActiveSupport::Deprecation exception,
+      # it's ok to change it when that happens.
+      expect { experiment(:example) { |e| e.use { } } }.not_to raise_error
+
+      expect(ActiveSupport::Deprecation).to have_received(:new).with(anything, 'Gitlab::Experiment')
+    end
+  end
 end

spec/support/gitlab_experiment.rb

@@ -26,7 +26,3 @@ RSpec.configure do |config|
     stub_snowplow
   end
 end
-
-# Once you've resolved a given deprecation, you can disallow it here, which
-# will raise an exception if it's used anywhere.
-ActiveSupport::Deprecation.disallowed_warnings << "`experiment_group?` is deprecated"