New Post: Add error to collection if items invalid

It would be useful to be able to add an error for a collection property if any of the items fail validation.

Fantasy code below:

RuleFor(x => x.Guests)
    .SetCollectionValidator(x => new GuestValidator(dataSource, x.Guests))
    .AllItemsValid()
    .WithMessage("Please select a pickup point for each guest");

If the first guest element were invalid, the validation result would include errors on the Guests property and also on Guests[0].

Has anybody solved this issue with a simple extension? Is this worth considering as a core library feature?