fix(parser) fixes sublanguage with no rule matches

Resolves highlightjs#2504.

CHANGES.md

@@ -1,3 +1,12 @@
+## Version 10.0.1
+
+Parser Engine Changes:
+
+- (bug) Fix sublanguage with no relevance score (#) [Josh Goebel][]
+
+[Josh Goebel]: https://github.com/yyyc514
+
+
 ## Version 10.0.0
 
 New languages:

src/highlight.js

@@ -443,6 +443,22 @@ const HLJS = function(hljs) {
     }
   }
 
+  // returns a valid highlight result, without actually
+  // doing any actual work, auto highlight starts with
+  // this and it's possible for small snippets that
+  // auto-detection may not find a better match
+  function justTextHighlightResult(code) {
+    const result = {
+      relevance: 0,
+      emitter: new options.__emitter(options),
+      value: escape(code),
+      illegal: false,
+      top: PLAINTEXT_LANGUAGE
+    };
+    result.emitter.addText(code)
+    return result;
+  }
+
   /*
   Highlighting with language detection. Accepts a string with the code to
   highlight. Returns an object with the following properties:
@@ -456,11 +472,7 @@ const HLJS = function(hljs) {
   */
   function highlightAuto(code, languageSubset) {
     languageSubset = languageSubset || options.languages || Object.keys(languages);
-    var result = {
-      relevance: 0,
-      emitter: new options.__emitter(options),
-      value: escape(code)
-    };
+    var result = justTextHighlightResult(code)
     var second_best = result;
     languageSubset.filter(getLanguage).filter(autoDetection).forEach(function(name) {
       var current = _highlight(name, code, false);
@@ -589,7 +601,7 @@ const HLJS = function(hljs) {
     window.addEventListener('DOMContentLoaded', initHighlighting, false);
   }
 
-  var PLAINTEXT_LANGUAGE = { disableAutodetect: true };
+  const PLAINTEXT_LANGUAGE = { disableAutodetect: true, name: 'Plain text' };
 
   function registerLanguage(name, language) {
     var lang;