Liquefied link tag (#6269)

Merge pull request 6269

docs/_docs/templates.md

@@ -588,6 +588,23 @@ One major benefit of using the `link` or `post_url` tag is link validation. If t
 
 Note you cannot add filters to `link` tags. For example, you cannot append a string using Liquid filters, such as `{% raw %}{% link mypage.html | append: "#section1" %} {% endraw %}`. To link to sections on a page, you will need to use regular HTML or Markdown linking techniques.
 
+The name of the file you want to link can be specified as a variable instead of an actual file name. For example, suppose you defined a variable in your page's front matter like this:
+
+```yaml
+---
+title: My page
+my_variable: footer_company_a.html
+---
+```
+
+You could then reference that variable in your link:
+
+```liquid
+{% raw %}{% link {{ page.my_variable }} %}{% endraw %}
+```
+
+In this example, the link would add link to the file `footer_company_a.html`.
+
 ### Linking to posts
 
 If you want to include a link to a post on your site, the `post_url` tag will generate the correct permalink URL for the post you specify.

lib/jekyll/tags/link.rb

@@ -18,14 +18,17 @@ module Jekyll
       def render(context)
         site = context.registers[:site]
 
+        liquid = site.liquid_renderer.file("(jekyll:link)")
+        relative_path = liquid.parse(@relative_path).render(context)
+
         site.each_site_file do |item|
-          return item.url if item.relative_path == @relative_path
+          return item.url if item.relative_path == relative_path
           # This takes care of the case for static files that have a leading /
-          return item.url if item.relative_path == "/#{@relative_path}"
+          return item.url if item.relative_path == "/#{relative_path}"
         end
 
         raise ArgumentError, <<-MSG
-Could not find document '#{@relative_path}' in tag '#{self.class.tag_name}'.
+Could not find document '#{relative_path}' in tag '#{self.class.tag_name}'.
 
 Make sure the document exists and the path is correct.
 MSG

test/test_tags.rb

@@ -789,6 +789,45 @@ CONTENT
     end
   end
 
+  context "simple page with dynamic linking to a page" do
+    setup do
+      content = <<CONTENT
+---
+title: linking
+---
+
+{% assign contacts_filename = 'contacts' %}
+{% assign contacts_ext = 'html' %}
+{% link {{contacts_filename}}.{{contacts_ext}} %}
+{% assign info_path = 'info.md' %}
+{% link {{\ info_path\ }} %}
+{% assign screen_css_path = '/css' %}
+{% link {{ screen_css_path }}/screen.css %}
+CONTENT
+      create_post(content, {
+        "source"      => source_dir,
+        "destination" => dest_dir,
+        "read_all"    => true,
+      })
+    end
+
+    should "not cause an error" do
+      refute_match(%r!markdown\-html\-error!, @result)
+    end
+
+    should "have the URL to the 'contacts' item" do
+      assert_match(%r!/contacts\.html!, @result)
+    end
+
+    should "have the URL to the 'info' item" do
+      assert_match(%r!/info\.html!, @result)
+    end
+
+    should "have the URL to the 'screen.css' item" do
+      assert_match(%r!/css/screen\.css!, @result)
+    end
+  end
+
   context "simple page with linking" do
     setup do
       content = <<CONTENT
@@ -815,6 +854,33 @@ CONTENT
     end
   end
 
+  context "simple page with dynamic linking" do
+    setup do
+      content = <<CONTENT
+---
+title: linking
+---
+
+{% assign yaml_with_dots_path = '_methods/yaml_with_dots.md' %}
+{% link {{yaml_with_dots_path}} %}
+CONTENT
+      create_post(content, {
+        "source"           => source_dir,
+        "destination"      => dest_dir,
+        "collections"      => { "methods" => { "output" => true } },
+        "read_collections" => true,
+      })
+    end
+
+    should "not cause an error" do
+      refute_match(%r!markdown\-html\-error!, @result)
+    end
+
+    should "have the URL to the 'yaml_with_dots' item" do
+      assert_match(%r!/methods/yaml_with_dots\.html!, @result)
+    end
+  end
+
   context "simple page with nested linking" do
     setup do
       content = <<CONTENT
@@ -867,6 +933,28 @@ CONTENT
     end
   end
 
+  context "simple page with invalid dynamic linking" do
+    should "cause an error" do
+      content = <<CONTENT
+---
+title: Invalid linking
+---
+
+{% assign non_existent_path = 'non-existent-collection-item' %}
+{% link {{\ non_existent_path\ }} %}
+CONTENT
+
+      assert_raises ArgumentError do
+        create_post(content, {
+          "source"           => source_dir,
+          "destination"      => dest_dir,
+          "collections"      => { "methods" => { "output" => true } },
+          "read_collections" => true,
+        })
+      end
+    end
+  end
+
   context "include tag with parameters" do
     context "with symlink'd include" do
       should "not allow symlink includes" do