Rails 8.2: `herb:check` does not reach sidecar templates

[activefx]

Rails 8.2 compiles templates with the HTML format through Herb when an application runs the 8.2 framework defaults (rails/rails#58721, config.action_view.erb_implementation = :herb). It also ships bin/rails herb:check, "the check to run before adopting the default".

Sidecar templates follow the new default, and the check does not reach them.

Why sidecars are affected

ViewComponent::Template#compiled_source calls the registered handler with a DataWithSource whose format is :html:

handler.call(DataWithSource.new(format:, identifier: @path, short_identifier:, type:), this_source)

ActionView::Template::Handlers::ERB#call picks the implementation from template.format, so under :herb every .html.erb sidecar and inline ERB template compiles through Herb::Engine.

Why the check misses them

ActionView::HerbChecker.check(ActionController::Base.view_paths) asks each resolver for all_unbound_templates. Sidecar files under app/components are not in the view paths, and inline templates are not files at all. An application can pass herb:check and still raise at the first render of a component whose template has an unclosed tag or ERB output in an attribute name.

Reproduction

Rails main at 2cc9c08 (8.2.0.alpha), herb 0.10.4, view_component 4.15.0, with ActionView::Template::Handlers::ERB::Herb set as the ERB implementation:

<%# app/components/broken_component.html.erb %>
<div><%= title %>

ActionView::HerbChecker.check(ActionController::Base.view_paths)
# => []
ActionController::Base.renderer.render(BrokenComponent.new)
# raises Herb::Engine::CompilationError

The checker returns no failures, so bin/rails herb:check reports that every template compiles.

Possible directions

• A task shipped by ViewComponent, for example view_component:herb_check, that compiles the templates of every ViewComponent::Base descendant the way HerbChecker does: the ERB handler with implementation: ActionView::Template::Handlers::ERB::Herb, validate_ruby: true and the SecurityValidator.
• Or a hook into herb:check, if Rails is open to checkers registering extra template sources.
• At least a note in the Rails 8.2 upgrade section of the ViewComponent docs.

#2503 added compatibility tests with Herb, so rendering is covered. This is about the check that runs before anything renders. I run this compile in the CI of a component library built on ViewComponent and can share the code.