[czj]

What are you trying to accomplish?

Fixes #2717

Rails 8.2 can compile HTML+ERB templates through Herb (config.action_view.erb_implementation = :herb) and ships bin/rails herb:check to find templates that won't compile before you adopt it. Component templates compile through Herb too: Template#compiled_source hands the ERB handler an :html template. But herb:check reads the view paths, which don't include sidecar templates, and inline templates aren't files at all. So an app can pass herb:check and still raise Herb::Engine::CompilationError the first time a component renders.

What approach did you choose and why?

• ViewComponent::HerbChecker works like ActionView::HerbChecker. It compiles each component's HTML+ERB templates with ActionView::Template::Handlers::ERB, using implementation: ERB::Herb, validate_ruby: true and the SecurityValidator. It returns Failures for Herb::Engine::CompilationError and SecurityError. It checks each component's own sidecar and inline templates. It skips #call methods and inherited templates, which the parent's check already covers.
• bin/rails view_component:herb:check eager loads the app, runs the checker and prints failures in the same format as Rails' task.
• herb:check now checks components too. A prerequisite prints component failures without stopping, so Rails still checks the views, and an at_exit makes the command fail once it has finished. People already run that command before switching to Herb. API-only apps don't get herb:check from Rails, so the hook is skipped there.
• Both tasks are only defined when Action View has ERB::Herb, which means Rails 8.2 and later. supported? checks this without loading the herb gem.
• To feed the handler exactly what the compiler feeds it, I pulled handler_template and handler_source out of Template#compiled_source, and added Compiler#source_templates. Compilation itself doesn't change.

Alternatives I considered:

• Hooking into Rails' checker: it only accepts view-path resolvers, and adding a registration API would need a change in Rails.
• A docs-only note: it wouldn't actually catch anything.

Anything you want to highlight for special attention from reviewers?

• Tests that need Herb are skipped in CI for now. Both rails_main and rails_main_head are locked to Rails revisions from before Herb support (9cc62ec, 11417ba), so HerbChecker.supported? is false everywhere CI runs. The tests for source_templates, handler_template/handler_source and the rake wiring run on every Rails version. The Herb tests will start running once the rails_main lock is bumped. I couldn't run the suite locally, so I'd appreciate a run against current Rails main.
• The at_exit in the prerequisite is a little unusual. I couldn't find another way for a prerequisite to report failures and still let Rails check the views. Happy to drop the hook and keep view_component:herb:check on its own if you'd rather.
• It checks every loaded component, including those that come from gems (as Rails checks engine view paths). Should it be limited to components under Rails.root?

[joelhawksley]

@czj thank you for the PR, but I think we should just do this in Rails: rails/rails#59137

[czj]

That's better, indeed ! Thanks :)