This single API (and the associated ContainerValues) is the #1 most important addition to #SwiftUI in years. It makes building custom components that behave like system ones almost trivially easy. It is downright magical and I'm thrilled to finally rip out all of my old _VariadicView hacks:
https://developer.apple.com/documentation/swiftui/group/init(subviews:transform:)
A huge thanks to the SwiftUI team for making this public.