Configure default language for syntax-highlighting

features/collections.feature

@@ -391,7 +391,7 @@ Feature: Collections
     When I run jekyll build
     Then I should get a zero exit status
     Then the _site directory should exist
-    And I should see "Second document's output: <p>Use <code class=\"highlighter-rouge\">Jekyll.configuration</code> to build a full configuration for use w/Jekyll.</p>\n\n<p>Whatever: foo.bar</p>" in "_site/index.html"
+    And I should see "Second document's output: <p>Use <code class=\"language-plaintext highlighter-rouge\">Jekyll.configuration</code> to build a full configuration for use w/Jekyll.</p>\n\n<p>Whatever: foo.bar</p>" in "_site/index.html"
 
   Scenario: Documents have an output attribute, which is the converted HTML based on site.config
     Given I have an "index.html" page that contains "Second document's output: {{ site.documents[2].output }}"

lib/jekyll/converters/markdown/kramdown_parser.rb

@@ -99,6 +99,7 @@ def initialize(config)
         def setup
           @config["syntax_highlighter"] ||= highlighter
           @config["syntax_highlighter_opts"] ||= {}
+          @config["syntax_highlighter_opts"]["default_lang"] ||= "plaintext"
           @config["syntax_highlighter_opts"]["guess_lang"] = @config["guess_lang"]
           @config["coderay"] ||= {} # XXX: Legacy.
           modernize_coderay_config

test/test_kramdown.rb

@@ -75,6 +75,44 @@ def fixture_converter(config)
       refute(result.css(selector).empty?, result.to_html)
     end
 
+    context "when configured" do
+      setup do
+        @source = <<~TEXT
+          ## Code Sample
+
+              def ruby_fu
+                "Hello"
+              end
+        TEXT
+      end
+
+      should "have 'plaintext' as the default syntax_highlighter language" do
+        converter = fixture_converter(@config)
+        parser = converter.setup && converter.instance_variable_get(:@parser)
+        parser_config = parser.instance_variable_get(:@config)
+
+        assert_equal "plaintext", parser_config.dig("syntax_highlighter_opts", "default_lang")
+      end
+
+      should "accept the specified default syntax_highlighter language" do
+        override = {
+          "kramdown" => {
+            "syntax_highlighter_opts" => {
+              "default_lang" => "yaml",
+            },
+          },
+        }
+        converter = fixture_converter(Utils.deep_merge_hashes(@config, override))
+        parser = converter.setup && converter.instance_variable_get(:@parser)
+        parser_config = parser.instance_variable_get(:@config)
+
+        assert_equal "yaml", parser_config.dig("syntax_highlighter_opts", "default_lang")
+        refute_match %r!<div class="language-plaintext!, converter.convert(@source)
+        refute_match %r!<div class="language-html!, converter.convert(@source)
+        assert_match %r!<div class="language-yaml!, converter.convert(@source)
+      end
+    end
+
     context "when asked to convert smart quotes" do
       should "convert" do
         converter = fixture_converter(@config)