Optimized nested navigation rendering: don't cross template boundary (#2213)

The number of calls to sub-templates when rendering the navigation is O(n^2) relative to the number of pages. This is the only such instance, which is why I think this is worth optimizing.

The optimization here doesn't improve the complexity, it just removes the overhead of instantiating a new Jinja template for each of these calls.

E.g. for 710 pages on the site, the number of calls to Jinja's `get_template` (implying `new_context` and others) crosses a million ==(710**2)*2. They're not expensive but also not super cheap, and add up to a big percentage of overall site build times.

material/partials/nav-item.html

@@ -2,6 +2,7 @@
   This file was automatically generated - do not edit
 -#}
 {% set features = config.theme.features or [] %}
+{% macro do_nav_item(nav_item, path, level) %}
 {% set class = "md-nav__item" %}
 {% if nav_item.active %}
   {% set class = class ~ " md-nav__item--active" %}
@@ -31,9 +32,7 @@
       <ul class="md-nav__list" data-md-scrollfix>
         {% set base = path %}
         {% for nav_item in nav_item.children %}
-          {% set path = base ~ "-" ~ loop.index %}
+          {{ do_nav_item(nav_item, path = base ~ "-" ~ loop.index, level = level + 1) }}
-          {% set level = level + 1 %}
-          {% include "partials/nav-item.html"  %}
         {% endfor %}
       </ul>
     </nav>
@@ -65,3 +64,5 @@
     </a>
   </li>
 {% endif %}
+{% endmacro %}
+{{ do_nav_item(nav_item, path, level) }}

src/partials/nav-item.html

@@ -23,6 +23,9 @@
 <!-- Retrieve features from configuration -->
 {% set features = config.theme.features or [] %}
 
+<!-- Wrap everything into a macro to reduce file roundtrips -->
+{% macro do_nav_item(nav_item, path, level) %}
+
 <!-- Determine class according to state -->
 {% set class = "md-nav__item" %}
 {% if nav_item.active %}
@@ -82,9 +85,7 @@
         <!-- Render nested item list -->
         {% set base = path %}
         {% for nav_item in nav_item.children %}
-          {% set path = base ~ "-" ~ loop.index %}
+          {{ do_nav_item(nav_item, path = base ~ "-" ~ loop.index, level = level + 1) }}
-          {% set level = level + 1 %}
-          {% include "partials/nav-item.html"  %}
         {% endfor %}
       </ul>
     </nav>
@@ -136,3 +137,7 @@
     </a>
   </li>
 {% endif %}
+
+{% endmacro %}
+
+{{ do_nav_item(nav_item, path, level) }}