Make SemanticHighlighter::run() cancelable

In case the task needs to be canceled, stop doing its job
and return as soon as possible.

Change-Id: Id5a3462d9f0a19eda782aebdd4c25407318118db
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/qmljseditor/qmljssemantichighlighter.cpp

@@ -224,12 +224,16 @@ public:
 protected:
     void accept(Node *ast)
     {
+        if (m_futureInterface.isCanceled())
+            return;
         if (ast)
             ast->accept(this);
     }
 
     void scopedAccept(Node *ast, Node *child)
     {
+        if (m_futureInterface.isCanceled())
+            return;
         m_scopeBuilder.push(ast);
         accept(child);
         m_scopeBuilder.pop();
@@ -553,6 +557,7 @@ SemanticHighlighter::SemanticHighlighter(QmlJSEditorDocument *document)
             this, &SemanticHighlighter::applyResults);
     connect(&m_watcher, &QFutureWatcherBase::finished,
             this, &SemanticHighlighter::finished);
+    m_futureSynchronizer.setCancelOnWait(true);
 }
 
 void SemanticHighlighter::rerun(const QmlJSTools::SemanticInfo &semanticInfo)