Resolve declared component dependencies that never opted in

docs/CHANGELOG.md

@@ -10,6 +10,12 @@ nav_order: 6
 
 ## main
 
+* Resolve `# Template Dependency: SomeComponent` declarations naming a component that hasn't included `ViewComponent::ExperimentallyCacheable`.
+
+    The declaration was previously dropped: the class name was emitted to the Digestor verbatim, which looked for a template at that path, found none, and logged `Couldn't find template for digesting: SomeComponent`. That made the escape hatch unusable for exactly the dependencies it exists for, since a component reached through a local variable or a helper module is often one you don't control and can't add the module to. Naming a component in a declaration now digests it, opted in or not.
+
+    *Erik Axel Nielsen*
+
 ## 4.15.0
 
 * Add experimental caching support, opt-in per component via `include ViewComponent::ExperimentallyCacheable`.

docs/guide/caching.md

@@ -187,7 +187,7 @@ The same works in a template, where the branch is often the more natural place f
 <%= render component.new(post: @post) %>
 ```
 
-Declared components must include `ViewComponent::ExperimentallyCacheable` themselves, since a component that hasn't opted in has no digest to depend on.
+A declared component doesn't have to include `ViewComponent::ExperimentallyCacheable` itself. The declaration alone gets its template, Ruby class, sidecar files, and superclasses digested, which matters because the components this escape hatch exists for are often ones you don't control.
 
 ## Caveats
 

lib/view_component/cache_digest.rb

@@ -97,7 +97,7 @@ def component_for(virtual_path)
         name = registry[virtual_path.delete_prefix("#{VIRTUAL_PATH_PREFIX}/")]
         return unless name
 
-        constantize_component(name)
+        constantize_view_component(name)
       end
 
       # Scan a template's source for renders of cacheable components.
@@ -168,6 +168,10 @@ def resolve_render_parser(parser)
       # that detail out of application code, so `SomeComponent` is translated
       # to the path the Digestor can resolve.
       #
+      # Any component resolves here, not just ones that opted into caching. The
+      # hatch exists for dependencies the source scanner can't see, which are
+      # exactly the ones whose target the application may not control.
+      #
       # @return [Array<Array(String, String)>] pairs of declared name and
       #   synthetic virtual path
       def explicit_component_dependencies(source)
@@ -176,8 +180,18 @@ def explicit_component_dependencies(source)
         source.scan(EXPLICIT_DEPENDENCY).flatten.uniq.filter_map do |declared|
           next unless /\A(?:::)?[A-Z]/.match?(declared)
 
-          component = constantize_component(declared)
-          [declared, virtual_path_for(component)] if component
+          component = constantize_view_component(declared)
+          next unless component
+
+          virtual_path = virtual_path_for(component)
+          next unless virtual_path
+
+          # Being named by a declaration is what makes a component resolvable:
+          # the Resolver synthesizes templates from the registry, so a component
+          # that never opted in has to be added to it before the Digestor asks.
+          register(component)
+
+          [declared, virtual_path]
         end
       end
 
@@ -238,6 +252,20 @@ def constantize_component(constant_name)
         # Never let digest computation break rendering.
         nil
       end
+
+      # Resolve a constant name to a component, whether or not it opted into
+      # caching.
+      #
+      # @return [Class, nil]
+      def constantize_view_component(constant_name)
+        component = constant_name.safe_constantize
+        return unless component.is_a?(Class) && component < ViewComponent::Base
+
+        component
+      rescue
+        # Never let digest computation break rendering.
+        nil
+      end
     end
 
     # Resolved once at load time rather than memoized, so no class-level state

test/sandbox/app/components/cacheable_plain_dependency_component.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+# Renders a component that static analysis can't see and that hasn't opted into
+# caching, declared with the `# Template Dependency:` escape hatch.
+class CacheablePlainDependencyComponent < ViewComponent::Base
+  include ViewComponent::ExperimentallyCacheable
+
+  # Template Dependency: ErbComponent
+
+  def initialize(component: ErbComponent)
+    @component = component
+  end
+
+  def call
+    render @component.new(message: "plain")
+  end
+end

test/sandbox/test/experimentally_cacheable_test.rb

@@ -121,13 +121,35 @@ def test_declared_template_paths_are_left_alone
     assert_includes dependencies, "integration_examples/erb_partial"
   end
 
-  def test_declared_names_that_are_not_cacheable_components_are_left_alone
+  def test_declared_names_that_are_not_components_are_left_alone
     assert_empty ViewComponent::CacheDigest.explicit_component_dependencies(
-      "# Template Dependency: ErbComponent"
+      "# Template Dependency: NotAConstantAnywhere"
+    )
+    assert_empty ViewComponent::CacheDigest.explicit_component_dependencies(
+      "# Template Dependency: ActiveSupport::Digest"
     )
     assert_empty ViewComponent::CacheDigest.explicit_component_dependencies("no declarations here")
   end
 
+  # The hatch exists for dependencies static analysis can't see, which are
+  # exactly the ones whose target an application may not control. Declaring a
+  # component is the opt-in, so the target needn't include the module itself.
+  def test_declared_components_resolve_without_opting_into_caching
+    refute_respond_to ErbComponent, :__vc_cacheable?
+
+    assert_equal(
+      [["ErbComponent", "view_component/cache_digest/erb_component"]],
+      ViewComponent::CacheDigest.explicit_component_dependencies("# Template Dependency: ErbComponent")
+    )
+  end
+
+  def test_cache_digest_changes_when_a_component_declared_without_opting_in_changes
+    assert_digest_changes(
+      "app/components/erb_component.html.erb",
+      "<div>changed</div>\n"
+    ) { CacheablePlainDependencyComponent.cache_digest }
+  end
+
   # Components rendered from an inline template are invisible to Action View's
   # trackers, which only read template files.
   def test_cache_digest_changes_when_a_child_of_an_inline_template_changes