[Fix #909] Fix a false positive for Rails/ActionControllerFlashBeforeRender

Fixes #909.

This PR fixes a false positive for `Rails/ActionControllerFlashBeforeRender`
when using `flash` before `redirect_to` in `if` branch.

Author: koic

changelog/fix_a_false_positive_for_rails_action_controller_flash_before_render.md

@@ -0,0 +1 @@
+* [#909](https://github.com/rubocop/rubocop-rails/issues/909): Fix a false positive for `Rails/ActionControllerFlashBeforeRender` when using `flash` before `redirect_to` in `if` branch. ([@koic][])

lib/rubocop/cop/rails/action_controller_flash_before_render.rb

@@ -70,6 +70,8 @@ def followed_by_render?(flash_node)
           flash_assignment_node = find_ancestor(flash_node, type: :send)
           context = flash_assignment_node
           if (node = context.each_ancestor(:if, :rescue).first)
+            return false if use_redirect_to?(context)
+
             context = node
           elsif context.right_siblings.empty?
             return true
@@ -95,6 +97,12 @@ def instance_method_or_block?(node)
           def_node || block_node
         end
 
+        def use_redirect_to?(context)
+          context.right_siblings.compact.any? do |sibling|
+            sibling.send_type? && sibling.method?(:redirect_to)
+          end
+        end
+
         def find_ancestor(node, type:)
           node.each_ancestor(type).first
         end

spec/rubocop/cop/rails/action_controller_flash_before_render_spec.rb

@@ -202,6 +202,23 @@ def create
             RUBY
           end
 
+          it 'does not register an offense when using `flash` before `redirect_to` in `if` branch' do
+            expect_no_offenses(<<~RUBY)
+              class HomeController < #{parent_class}
+                def create
+                  if condition
+                    flash[:alert] = "msg"
+                    redirect_to :index
+
+                    return
+                  end
+
+                  render :index
+                end
+              end
+            RUBY
+          end
+
           it 'does not register an offense when using `flash` in multiline `rescue` branch before `redirect_to`' do
             expect_no_offenses(<<~RUBY)
               class HomeController < #{parent_class}