Automatic builder for any Swift type

configurator.swift

@dynamicMemberLookup
struct Configurator<T> {
    var configure: (inout T) -> Void = { _ in }

    subscript<V>(dynamicMember keyPath: WritableKeyPath<T, V>) -> Builder<V> {
        .init(configure: configure, keyPath: keyPath)
    }

    static subscript<V>(dynamicMember keyPath: WritableKeyPath<T, V>) -> Builder<V> {
        .init(configure: { _ in }, keyPath: keyPath)
    }

    @dynamicMemberLookup
    struct Builder<V> {
        var configure: (inout T) -> Void
        var keyPath: WritableKeyPath<T, V>

        func callAsFunction(_ value: V) -> Configurator {
            return .init { [configure, keyPath] object in
                configure(&object)
                object[keyPath: keyPath] = value
            }
        }

        subscript<C>(dynamicMember childKeyPath: WritableKeyPath<V, C>) -> Builder<C> {
            .init(configure: configure, keyPath: keyPath.appending(path: childKeyPath))
        }
    }

    func apply(_ value: inout T) {
        configure(&value)
    }
}

protocol DefaultConstructible {
    init()
}

extension Configurator where T: DefaultConstructible {
    func build() -> T {
        var result = T()
        apply(&result)
        return result
    }
}

example.swift

struct Whatever: DefaultConstructible {
    struct ChildData {
        var foo: Int = 0
    }

    var name: String = ""
    var age: Int = 0
    var child = ChildData()

    typealias Config = Configurator<Whatever>
}

let me = Whatever.Config
    .name("Me")
    .age(-1)
    .child.foo(12)
    .build()