SwiftUI: How any `Layout`-conforming type becomes a container view when it's used with a `@ViewBuilder` closure.

LayoutCallAsFunction.swift

// Excerpt from `SwiftUI.framework/Modules/SwiftUI.swiftmodule/arm64-apple-ios.swiftinterface`
// in Xcode 14.0b1
//
// This is how any `Layout`-conforming type becomes a container view
// when it's used with a `@ViewBuilder` closure.
@available(iOS 16.0, macOS 13.0, tvOS 16.0, watchOS 9.0, *)
extension SwiftUI.Layout {
  @_alwaysEmitIntoClient public func callAsFunction<V>(@SwiftUI.ViewBuilder _ content: () -> V) -> some SwiftUI.View where V : SwiftUI.View {
        return _VariadicView.Tree(
            root: _LayoutRoot(self), content: content())
    }
  
}