I am serious when I say, that's awesome, as well as the FnOnce approach, but it has the problem of it needing to desugar to as many fn foos as optional fields are elided, and one of the big advantages of the feature is that ordering doesn't matter for the struct init syntax. For tuples you can either elide the start, the end or use _ in the middle, but that's limiting to the point where I don't know if it is even worth it to include it as part of the initial RFC.