code: function checkKey (element) {
        if (element.name === 'template' || element.name === 'slot') {
          for (const child of element.children) {
            if (child.type === 'VElement') {
              checkKey(child)
            }
          }
        } else if (!utils.isCustomComponent(element) && !utils.hasDirective(element, 'bind', 'key')) {
          context.report({
            node: element.startTag,
            loc: element.startTag.loc,
            message: "Elements in iteration expect to have 'v-bind:key' directives."
          })
        }
      }
docs: Check the given element about `v-bind:key` attributes.
  @param {ASTNode} element The element node to check.