start: function start()

in views/vue.js [9500:9615]
45 lines of code
23 McCabe index (conditional complexity)

      start: function start(tag, attrs, unary) {
        // check namespace.
        // inherit parent ns if there is one
        const ns =
          (currentParent && currentParent.ns) || platformGetTagNamespace(tag);

        // handle IE svg bug
        /* istanbul ignore if */
        if (isIE && ns === "svg") {
          attrs = guardIESVGBug(attrs);
        }

        let element = createASTElement(tag, attrs, currentParent);
        if (ns) {
          element.ns = ns;
        }

        if (isForbiddenTag(element) && !isServerRendering()) {
          element.forbidden = true;
          "development" !== "production" &&
            warn$2(
              "Templates should only be responsible for mapping the state to the " +
                "UI. Avoid placing tags with side-effects in your templates, such as " +
                "<" +
                tag +
                ">" +
                ", as they will not be parsed."
            );
        }

        // apply pre-transforms
        for (let i = 0; i < preTransforms.length; i++) {
          element = preTransforms[i](element, options) || element;
        }

        if (!inVPre) {
          processPre(element);
          if (element.pre) {
            inVPre = true;
          }
        }
        if (platformIsPreTag(element.tag)) {
          inPre = true;
        }
        if (inVPre) {
          processRawAttrs(element);
        } else if (!element.processed) {
          // structural directives
          processFor(element);
          processIf(element);
          processOnce(element);
          // element-scope stuff
          processElement(element, options);
        }

        function checkRootConstraints(el) {
          {
            if (el.tag === "slot" || el.tag === "template") {
              warnOnce(
                "Cannot use <" +
                  el.tag +
                  "> as component root element because it may " +
                  "contain multiple nodes."
              );
            }
            if (el.attrsMap.hasOwnProperty("v-for")) {
              warnOnce(
                "Cannot use v-for on stateful component root element because " +
                  "it renders multiple elements."
              );
            }
          }
        }

        // tree management
        if (!root) {
          root = element;
          checkRootConstraints(root);
        } else if (!stack.length) {
          // allow root elements with v-if, v-else-if and v-else
          if (root.if && (element.elseif || element.else)) {
            checkRootConstraints(element);
            addIfCondition(root, {
              exp: element.elseif,
              block: element
            });
          } else {
            warnOnce(
              "Component template should contain exactly one root element. " +
                "If you are using v-if on multiple elements, " +
                "use v-else-if to chain them instead."
            );
          }
        }
        if (currentParent && !element.forbidden) {
          if (element.elseif || element.else) {
            processIfConditions(element, currentParent);
          } else if (element.slotScope) {
            // scoped slot
            currentParent.plain = false;
            const name = element.slotTarget || '"default"';
            (currentParent.scopedSlots || (currentParent.scopedSlots = {}))[
              name
            ] = element;
          } else {
            currentParent.children.push(element);
            element.parent = currentParent;
          }
        }
        if (!unary) {
          currentParent = element;
          stack.push(element);
        } else {
          closeElement(element);
        }
      },