feat: Allowing post_url tag to receive liquid variables (#9776)

Merge pull request 9776

docs/_docs/liquid/tags.md

@@ -188,3 +188,25 @@ You can also use this tag to create a link to a post in Markdown as follows:
 [Name of Link]({% post_url 2010-07-21-name-of-post %})
 ```
 {% endraw %}
+
+Now lets say you have a [datafile]({{ '/docs/datafiles/' | relative_url }}) `_data/cool_posts.yaml` used to keep track
+of certain posts that you intend to be listed as say *Cool Posts*:
+
+```yaml
+- title: "An Awesome Post"
+  slug: "2010-07-21-name-of-post"
+- title: "Another Awesome Post"
+  slug: "2016-07-26-name-of-post"
+```
+
+You may list such posts using the `post_url` tag as well (from {%- include docs_version_badge.html version="4.5.0" -%}):
+
+{% raw %}
+```liquid
+Cool posts:
+
+{%- for cool_post in site.data.cool_posts %}
+- [{{ cool_post.title }}]({% post_url {{ cool_post.slug }} %})
+{%- endfor %}
+```
+{% endraw %}

features/post_url_tag.feature

@@ -157,3 +157,37 @@ Feature: PostUrl Tag
     But the _site directory should exist
     And I should see "<p><a href=\"/cats%20and%20dogs/2019/02/04/hello-world.html\">Post 1</a></p>" in "_site/index.html"
     And I should see "<p><a href=\"/2019/02/05/hello-again.html\">Post 2</a></p>" in "_site/index.html"
+
+  Scenario: Calling for a post via a liquid variable
+    Given I have a _posts directory
+    And I have the following post:
+      | title       | date       | content           |
+      | Hello World | 2019-02-04 | Lorem ipsum dolor |
+    And I have an "index.md" page with content:
+      """
+      {% assign value='2019-02-04-hello-world' %}
+      [Welcome]({% post_url {{ value }} %})
+      """
+    When I run jekyll build
+    Then I should get a zero exit status
+    And the _site directory should exist
+    And I should see "<p><a href=\"/2019/02/04/hello-world.html\">Welcome</a></p>" in "_site/index.html"
+
+  Scenario: Calling for posts via a liquid variable in a for tag
+    Given I have a _posts directory
+    And I have the following post:
+      | title         | date       | content           |
+      | Hello World   | 2019-02-04 | Lorem ipsum dolor |
+      | We Meet Again | 2019-02-05 | Alpha beta gamma  |
+    And I have an "index.md" page with content:
+      """
+      {% assign posts = '2019-02-04-hello-world;2019-02-05-we-meet-again' | split: ';' %}
+      {%- for slug in posts -%}
+        [{{ slug }}]({% post_url {{ slug }} %})
+      {%- endfor %}
+      """
+    When I run jekyll build
+    Then I should get a zero exit status
+    And the _site directory should exist
+    And I should see "<a href=\"/2019/02/04/hello-world.html\">2019-02-04-hello-world</a>" in "_site/index.html"
+    And I should see "<a href=\"/2019/02/05/we-meet-again.html\">2019-02-05-we-meet-again</a>" in "_site/index.html"

lib/jekyll/tags/post_url.rb

@@ -66,23 +66,28 @@ module Jekyll
       def initialize(tag_name, markup, tokens)
         super
         @markup = markup.strip
-
-        begin
-          @post_comparer = PostComparer.new(@markup)
-        rescue StandardError
-          raise_markup_parse_error
-        end
+        @template = Liquid::Template.parse(@markup) if @markup.include?("{{")
 
         # Deprecated instance_variables.
         # To be removed in Jekyll v5.0.
         @orig_post = @markup
-        @post = @post_comparer
+        @post = nil
       end
 
       def render(context)
         @context = context
+        @resolved_markup = @template&.render(@context) || @markup
         site = context.registers[:site]
 
+        begin
+          @post_comparer = PostComparer.new(@resolved_markup)
+        rescue StandardError
+          raise_markup_parse_error
+        end
+        # For backwards compatibility only; deprecated instance_variable.
+        # To be removed in Jekyll v5.0.
+        @post = @post_comparer
+
         # First pass-through.
         site.posts.docs.each do |post|
           return relative_url(post) if @post_comparer == post
@@ -104,23 +109,23 @@ module Jekyll
 
       def raise_markup_parse_error
         raise Jekyll::Errors::PostURLError, <<~MSG
-          Could not parse name of post #{@markup.inspect} in tag 'post_url'.
+          Could not parse name of post #{@resolved_markup.inspect} in tag 'post_url'.
           Make sure the correct name is given to the tag.
         MSG
       end
 
       def raise_post_not_found_error
         raise Jekyll::Errors::PostURLError, <<~MSG
-          Could not find post #{@markup.inspect} in tag 'post_url'.
+          Could not find post #{@resolved_markup.inspect} in tag 'post_url'.
           Make sure the post exists and the correct name is given to the tag.
         MSG
       end
 
       def log_legacy_usage_deprecation
         Jekyll::Deprecator.deprecation_message(
-          "A call to '{% post_url #{@markup} %}' did not match a post using the new matching " \
-          "method of checking name (path-date-slug) equality. Please make sure that you change " \
-          "this tag to match the post's name exactly."
+          "A call to '{% post_url #{@resolved_markup} %}' did not match a post using the new " \
+          "matching method of checking name (path-date-slug) equality. Please make sure that " \
+          "you change this tag to match the post's name exactly."
         )
       end
     end