turbo_stream tag builder: support :partial with block

[seanpdoyle]

The Background

Consider an application with a shared application/flash partial that accepts its message as a partial-local assignment:

<%# app/views/application/_flash.html.erb %>

<p role="alert"><%= message %></p>

Also consider an application/layout.turbo_stream.erb template for appending the Flash messages to a element with [id="flashes"]:

<%# app/views/layouts/application.turbo_stream.erb %>

<% flash.each do |name, message| %>
  <%= turbo_stream.append "flashes", partial: "application/flash", locals: { message: message } %>
<% end %>

This works as you'd expect, since the :partial and :locals keyword arguments are forwarded along to the underlying render call.

The Scenario

Now consider that the application/flash message changed its interface to expect the message as block content yielded to the partial instead of an assignment to the :locals options:

 <%# app/views/application/_flash.html.erb %>

-<p role="alert"><%= message %></p>
+<p role="alert"><%= yield %></p>

The layouts/application.turbo_stream.erb template would need to change as well:

 <%# app/views/layouts/application.turbo_stream.erb %>

 <% flash.each do |name, message| %>
-  <%= turbo_stream.append "flashes", partial: "application/flash", locals: { message: message } %>
+  <%= turbo_stream.append "flashes", partial: "application/flash" do %>
+    <span style="color: red"><%= message %></span>
+  <%= end %>
 <% end %>

The Problem

This style of invocation of turbo_stream.append does not work the same as if it were passed a block of content generated by calling render with the same keywords.

The presence of a &block argument triggers an entirely separate code path than the presence of the **rendering keywords.

To work around this issue, you'd have to capture the rendering separately:

 <%# app/views/layouts/application.turbo_stream.erb %>

 <% flash.each do |name, message| %>
-  <%= turbo_stream.append "flashes", partial: "application/flash", locals: { message: message } %>
+  <% content = capture do %>
+    <%= render partial: "application/flash" do %>
+      <span style="color: red"><%= message %></span>
+    <% end %>
+  <% end %>
+
+  <%= turbo_stream.append "flashes", content %>
 <% end %>

The Proposal

This commit alters the tag builder's decision making process to incorporate a check for a combination of both a &block and :partial or :layout keyword arguments.