SwiftUI @ViewBuilder Result is a TupleView, How is Apple Using It And Able to Avoid Turning Things Into AnyVIew?

Using Swift
4

ViewBuilder is a function builder.

ViewBuilder-based closure content may produce any kind of views. If there are two or more subviews, it produces a TupleView:

HStack {
    Text("a")
    Text("b")
}

If there are no subviews, it produces an EmptyView:

HStack {
}

The best place to read about function builders is the original proposal. But there are also many articles about function builders including this answer on stackoverflow.com.

The implementation of ViewBuilder looks like this:

@_functionBuilder
public struct ViewBuilder {
    @_alwaysEmitIntoClient
    public static func buildBlock() -> EmptyView {
        .init()
    }

    @_alwaysEmitIntoClient
    public static func buildBlock<Content>(_ content: Content) -> Content where Content: View {
        content
    }

    @_alwaysEmitIntoClient
    public static func buildIf<Content>( _ content: Content?) -> Content? where Content: View {
        content
    }

    @_alwaysEmitIntoClient
    public static func buildEither<TrueContent, FalseContent>(
        first: TrueContent) -> _ConditionalContent<TrueContent, FalseContent>
        where TrueContent: View, FalseContent: View
    {
        .init(storage: .trueContent(first))
    }

    @_alwaysEmitIntoClient
    public static func buildEither<TrueContent, FalseContent>(
        second: FalseContent) -> _ConditionalContent<TrueContent, FalseContent>
        where TrueContent: View, FalseContent: View
    {
        .init(storage: .falseContent(second))
    }

    @_alwaysEmitIntoClient
    public static func buildBlock<C0, C1>(
        _ c0: C0,
        _ c1: C1) -> TupleView<(C0, C1)>
        where C0: View, C1: View
    {
        .init((c0, c1))
    }

    @_alwaysEmitIntoClient
    public static func buildBlock<C0, C1, C2>(
        _ c0: C0,
        _ c1: C1,
        _ c2: C2) -> TupleView<(C0, C1, C2)>
        where C0: View, C1: View, C2: View
    {
        .init((c0, c1, c2))
    }

    // ...
}
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