code: function checkChildKey (context, vFor, child) {
    const childFor = utils.getDirective(child, 'for')
    // if child has v-for, check if parent iterator is used in v-for
    if (childFor != null) {
      const childForRefs = childFor.value.references
      const variables = vFor.parent.parent.variables
      const usedInFor = childForRefs.some(cref =>
        variables.some(variable =>
          cref.id.name === variable.id.name &&
          variable.kind === 'v-for'
        )
      )
      // if parent iterator is used, skip other checks
      // iterator usage will be checked later by child v-for
      if (usedInFor) {
        return
      }
    }
    // otherwise, check if parent iterator is directly used in child's key
    checkKey(context, vFor, child)
  }
docs: Check the child element in tempalte v-for about `v-bind:key` attributes.
  @param {RuleContext} context The rule context to report.
  @param {ASTNode} vFor The attribute node of `v-for` to check.
  @param {ASTNode} child The child node to check.