Vector manifesto

[TellowKrinkle]

One drawback to fixed-size arrays not conforming to RandomAccessCollection is that such a conformance is required for significant compiler optimizations for operations like flatMap(_:) (discussion). Operations like makeIterator() and reversed() will also be difficult to implement efficiently due to fixed-size arrays being (trivial) value types. The optimizer will probably need to be modified to understand fixed-size arrays and emit efficient code for them.

The linked discussion says that [[T]].flatMap { $0 } was more efficient than [(T, T, T, T)].flatMap { [$0.0, $0.1, $0.2, $0.3] }, AKA passing arrays directly is more efficient than creating arrays from tuples and returning them. It also said

@Ben_Cohen

But it appears you can do even better if you conform the struct to itself be a collection, allowing it to be flat mapped directly:

Which is the case we would be dealing with here.