Add configuration to treat content as a slot

Like discussed many times before, this finally adds content as a slot to
ViewComponent so that we can begin separating the block provided when
rendering from content for performance and ergonomic reasons.

This adds two new methods:

* `content_is_a_slot!` - This makes the component _and its descendants_
  treat `content` as a slot, and the block provided to render calls _is
  always executed_.
* `do_not_use_content_as_a_slot!` - This keeps the existing behavior
  where `content` is lazily evaluated and is used to return content.

There's likely more implications from this change that need to be walked
through (like how slots are used, and easily setting content) before
this can be merged.

lib/view_component/base.rb

@@ -103,6 +103,13 @@ def render_in(view_context, &block)
       @__vc_content_evaluated = false
       @__vc_render_in_block = block
 
+      if self.class.send(:__vc_content_is_a_slot?)
+        @__vc_content_evaluated = true
+        if __vc_render_in_block_provided?
+          view_context.capture(self, &@__vc_render_in_block)
+        end
+      end
+
       before_render
 
       if render?
@@ -549,6 +556,10 @@ def render_template_for(variant = nil, format = nil)
           child.instance_variable_set(:@__vc_ancestor_calls, vc_ancestor_calls)
         end
 
+        if defined?(@__vc_content_is_a_slot)
+          child.instance_variable_set(:@__vc_content_is_a_slot, @__vc_content_is_a_slot)
+        end
+
         super
       end
 
@@ -687,6 +698,19 @@ def initialize_parameters
       def provided_collection_parameter
         @provided_collection_parameter ||= nil
       end
+
+      def __vc_content_is_a_slot?
+        defined?(@__vc_content_is_a_slot) && @__vc_content_is_a_slot
+      end
+
+      def content_is_a_slot!
+        @__vc_content_is_a_slot = true
+        renders_one :content
+      end
+
+      def do_not_use_content_as_a_slot!
+        @__vc_content_is_a_slot = false
+      end
     end
 
     ActiveSupport.run_load_hooks(:view_component, self)

lib/view_component/slotable.rb

@@ -298,8 +298,10 @@ def define_slot(slot_name, collection:, callable:)
       end
 
       def validate_plural_slot_name(slot_name)
-        if RESERVED_NAMES[:plural].include?(slot_name.to_sym)
-          raise ReservedPluralSlotNameError.new(name, slot_name)
+        if slot_name.to_sym == :contents && !__vc_content_is_a_slot?
+          if RESERVED_NAMES[:plural].include?(slot_name.to_sym)
+            raise ReservedPluralSlotNameError.new(name, slot_name)
+          end
         end
 
         raise_if_slot_name_uncountable(slot_name)
@@ -309,12 +311,12 @@ def validate_plural_slot_name(slot_name)
       end
 
       def validate_singular_slot_name(slot_name)
-        if slot_name.to_sym == :content
+        if slot_name.to_sym == :content && !__vc_content_is_a_slot?
           raise ContentSlotNameError.new(name)
-        end
-
-        if RESERVED_NAMES[:singular].include?(slot_name.to_sym)
-          raise ReservedSingularSlotNameError.new(name, slot_name)
+        elsif !__vc_content_is_a_slot?
+          if RESERVED_NAMES[:singular].include?(slot_name.to_sym)
+            raise ReservedSingularSlotNameError.new(name, slot_name)
+          end
         end
 
         raise_if_slot_conflicts_with_call(slot_name)

test/sandbox/app/components/content_as_slot_component.html.erb

@@ -0,0 +1,5 @@
+<div>
+  <% if content? %>
+    <%= content %>
+  <% end %>
+</div>

test/sandbox/app/components/content_as_slot_component.rb

@@ -0,0 +1,3 @@
+class ContentAsSlotComponent < ViewComponent::Base
+  content_is_a_slot!
+end

test/sandbox/app/components/content_not_a_slot_component.rb

@@ -0,0 +1,3 @@
+class ContentNotASlotComponent < ViewComponent::Base
+  do_not_use_content_as_a_slot!
+end

test/sandbox/test/slotable_test.rb

@@ -819,4 +819,62 @@ def test_overridden_slot_name_can_be_inherited
   def test_slot_name_methods_are_not_shared_accross_components
     assert_not_equal SlotsComponent.instance_method(:title).owner, SlotNameOverrideComponent::OtherComponent.instance_method(:title).owner
   end
+
+  def test_content_as_a_slot_component
+    component = ContentAsSlotComponent.new
+    render_inline(component) do |c|
+      c.with_content do
+        "The truth is out there"
+      end
+    end
+
+    assert component.content?
+    assert_selector "div", text: "The truth is out there"
+  end
+
+  def test_content_as_a_slot_component_with_content
+    component = ContentAsSlotComponent.new
+    component.with_content do
+      "The truth is out there"
+    end
+    render_inline(component)
+
+    assert component.content?
+    assert_selector "div", text: "The truth is out there"
+  end
+
+  def test_content_as_a_slot_inheritance
+    new_component_class = Class.new(ContentAsSlotComponent)
+    assert new_component_class.send(:__vc_content_is_a_slot?)
+  end
+
+  def test_content_is_not_a_slot
+    new_component_class = Class.new(SlotsComponent) do
+      do_not_use_content_as_a_slot!
+    end
+    refute new_component_class.send(:__vc_content_is_a_slot?)
+
+    render_inline(SlotsComponent.new) do |component|
+      component.with_title do
+        "This is my title!"
+      end
+
+      component.with_subtitle do
+        "This is my subtitle!"
+      end
+
+      component.with_footer do
+        "This is the footer"
+      end
+    end
+
+    assert_text "No tabs provided"
+    assert_text "No items provided"
+  end
+
+  def test_content_is_not_a_slot_inheritance
+    refute ContentNotASlotComponent.send(:__vc_content_is_a_slot?)
+    new_component_class = Class.new(ContentNotASlotComponent)
+    refute new_component_class.send(:__vc_content_is_a_slot?)
+  end
 end