Deprecate implementing #discount on benefits

Instead, follow the deprecation message's instructions.

Author: mamhoff

promotions/app/models/solidus_promotions/benefit.rb

@@ -113,6 +113,53 @@ def discount(adjustable, ...)
       end
     end
 
+    def self.inherited(klass)
+      def klass.method_added(method_added)
+        if method_added == :discount
+          Spree.deprecator.warn <<~MSG
+            Please refactor `#{name}`. You're defining `#discount`. Instead, define a method for each type of discountable
+            that your benefit can discount. For example:
+            ```
+            class MyBenefit < SolidusPromotions::Benefit
+              def can_discount?(discountable)
+                discountable.is_a?(Spree::LineItem)
+              end
+
+              def discount(order, _options = {})
+                amount = compute_amount(line_item, ...)
+                return if amount.zero?
+
+                ItemDiscount.new(
+                  item: line_item,
+                  label: adjustment_label(line_item),
+                  amount: amount,
+                  source: self
+                )
+              end
+            ```
+            can now become
+            ```
+            class MyBenefit < SolidusPromotions::Benefit
+              def discount_line_item?(order, ...)
+                amount = compute_amount(line_item, ...)
+                return if amount.zero?
+
+                ItemDiscount.new(
+                  item: line_item,
+                  label: adjustment_label(line_item),
+                  amount: amount,
+                  source: self
+                )
+              end
+            end
+            ```
+          MSG
+        end
+        super
+      end
+      super
+    end
+
     # Computes the discount amount for the given adjustable.
     #
     # Ensures the returned amount is negative and does not exceed the

promotions/spec/models/solidus_promotions/benefit_spec.rb

@@ -148,6 +148,79 @@ def compute_line_item(_line_item, _options) = 1
     end
   end
 
+  describe "inherited hook" do
+    context "for a well-formed benefit" do
+      subject(:benefit) do
+        Class.new(described_class) do
+          def discount_line_item(_line_item, _options = {})
+            true
+          end
+        end
+      end
+
+      it "does not emit a deprecation warning" do
+        expect(Spree.deprecator).not_to receive(:warn)
+        benefit
+      end
+    end
+
+    context "for a legacy benefit" do
+      subject(:benefit) do
+        Class.new(described_class) do
+          def self.name
+            "LegacyBenefit"
+          end
+
+          def discount(_line_item, _options = {})
+            true
+          end
+        end
+      end
+
+      it "emits a deprecation warning" do
+        expect(Spree.deprecator).to receive(:warn).with(<<~MSG)
+          Please refactor `LegacyBenefit`. You're defining `#discount`. Instead, define a method for each type of discountable
+          that your benefit can discount. For example:
+          ```
+          class MyBenefit < SolidusPromotions::Benefit
+            def can_discount?(discountable)
+              discountable.is_a?(Spree::LineItem)
+            end
+
+            def discount(order, _options = {})
+              amount = compute_amount(line_item, ...)
+              return if amount.zero?
+
+              ItemDiscount.new(
+                item: line_item,
+                label: adjustment_label(line_item),
+                amount: amount,
+                source: self
+              )
+            end
+          ```
+          can now become
+          ```
+          class MyBenefit < SolidusPromotions::Benefit
+            def discount_line_item?(order, ...)
+              amount = compute_amount(line_item, ...)
+              return if amount.zero?
+
+              ItemDiscount.new(
+                item: line_item,
+                label: adjustment_label(line_item),
+                amount: amount,
+                source: self
+              )
+            end
+          end
+          ```
+        MSG
+        benefit
+      end
+    end
+  end
+
   describe ".original_promotion_action" do
     let(:spree_promotion) { create :promotion, :with_adjustable_action }
     let(:spree_promotion_action) { spree_promotion.actions.first }