Warn when --only-bootsnap-rbs-cache runs without TAPIOCA_RBS_CACHE=1

Without the env var set, `bootsnap/setup` isn't required at boot and no
RBS cache gets populated. Previously the command would still print
"Bootsnap RBS cache populated", misleading users into thinking it had
done useful work.

Addresses #2617 (comment)

Author: KaanOzkan

lib/tapioca/commands/dsl_generate.rb

@@ -18,7 +18,11 @@ def execute
         load_application
 
         if @only_bootsnap_rbs_cache
-          say("Bootsnap RBS cache populated, exiting before RBI generation.", :green)
+          if ENV["TAPIOCA_RBS_CACHE"] == "1"
+            say("Bootsnap RBS cache populated, exiting before RBI generation.", :green)
+          else
+            say_error("Warning: --only-bootsnap-rbs-cache requires TAPIOCA_RBS_CACHE=1 to populate the cache", :yellow)
+          end
           return
         end
 

spec/tapioca/cli/dsl_spec.rb

@@ -669,13 +669,34 @@ class Post
             end
           RB
 
-          result = @project.tapioca("dsl --only-bootsnap-rbs-cache Post")
+          result = @project.tapioca("dsl --only-bootsnap-rbs-cache Post", env: { "TAPIOCA_RBS_CACHE" => "1" })
 
           assert_stdout_includes(result, <<~OUT)
             Bootsnap RBS cache populated, exiting before RBI generation.
           OUT
 
-          assert_empty_stderr(result)
+          assert_stderr_includes(result, "bootsnap miss:")
+          refute_project_file_exist("sorbet/rbi/dsl/post.rbi")
+          assert_success_status(result)
+        end
+
+        it "warns when --only-bootsnap-rbs-cache is set without TAPIOCA_RBS_CACHE=1" do
+          @project.write!("lib/post.rb", <<~RB)
+            require "smart_properties"
+
+            class Post
+              include SmartProperties
+              property :title, accepts: String
+            end
+          RB
+
+          result = @project.tapioca("dsl --only-bootsnap-rbs-cache Post")
+
+          assert_stderr_includes(
+            result,
+            "Warning: --only-bootsnap-rbs-cache requires TAPIOCA_RBS_CACHE=1 to populate the cache",
+          )
+          refute_includes(result.out, "Bootsnap RBS cache populated")
           refute_project_file_exist("sorbet/rbi/dsl/post.rbi")
           assert_success_status(result)
         end