Merge pull request #1956 from Shopify/uk-short-circuit-rails-lookalikes

Short circuit Rails app loading if Rails constant isn't defined

lib/tapioca/loaders/loader.rb

@@ -47,12 +47,23 @@ def load_bundle(gemfile, initialize_file, require_file, halt_upon_load_error)
         ).void
       end
       def load_rails_application(environment_load: false, eager_load: false, app_root: ".", halt_upon_load_error: true)
-        return unless File.exist?("#{app_root}/config/application.rb")
+        return unless File.exist?(File.expand_path("config/application.rb", app_root))
 
-        if environment_load
-          require "./#{app_root}/config/environment"
+        load_path = if environment_load
+          "config/environment"
         else
-          require "./#{app_root}/config/application"
+          "config/application"
+        end
+
+        require File.expand_path(load_path, app_root)
+
+        unless defined?(Rails)
+          say(
+            "\nTried to load the app from `#{load_path}` as a Rails application " \
+              "but the `Rails` constant wasn't defined after loading the file.",
+            :yellow,
+          )
+          return
         end
 
         eager_load_rails_app if eager_load

spec/tapioca/cli/dsl_spec.rb

@@ -2801,6 +2801,56 @@ class PostCompiler < Tapioca::Dsl::Compiler
         end
       end
 
+      describe "rails-like-applications" do
+        before(:all) do
+          @project.write!("config/environment.rb", <<~RB)
+            require_relative "application.rb"
+          RB
+
+          @project.write!("config/application.rb", <<~RB)
+            # Don't define any constant named `Rails`.
+            require "smart_properties"
+
+            class Post
+              include SmartProperties
+              property :title, accepts: String
+            end
+          RB
+
+          @project.require_real_gem("smart_properties", "1.15.0")
+          @project.bundle_install!
+        end
+
+        after(:all) do
+          @project.remove!("config/application.rb")
+        end
+
+        it "shows a warning about the Rails constant not having been loaded" do
+          res = @project.tapioca("dsl")
+
+          assert_stdout_equals(<<~OUT, res)
+            Loading DSL extension classes... Done
+            Loading Rails application...\u0020
+            Tried to load the app from `config/environment` as a Rails application but the `Rails` constant wasn't defined after loading the file.
+            Done
+            Loading DSL compiler classes... Done
+            Compiling DSL RBI files...
+
+                  create  sorbet/rbi/dsl/post.rbi
+
+            Done
+
+            Checking generated RBI files...  Done
+              No errors found
+
+            All operations performed in working directory.
+            Please review changes and commit them.
+          OUT
+          assert_empty_stderr(res)
+          assert_success_status(res)
+        end
+      end
+
       describe "halt-upon-load-error" do
         before(:all) do
           @project.write!("config/environment.rb", <<~RB)