Simple Swift function builder

ArrayBuilder.swift

@resultBuilder
enum ArrayBuilder {

    @inlinable
    static func buildExpression<T>(_ expression: T) -> [T] {
        [expression]
    }

    @inlinable
    static func buildBlock<T>(_ components: [T]...) -> [T] {
        components.flatMap { $0 }
    }

    @inlinable
    static func buildOptional<T>(_ component: [T]?) -> [T] {
        component ?? []
    }

    @inlinable
    static func buildArray<T>(_ components: [[T]]) -> [T] {
        components.flatMap { $0 }
    }
}

let flag = true

@ArrayBuilder
var numbers: [Int] {
    1
    2
    if flag {
        3
        4
    }
    for i in 5..<10 {
        i
        0
    }
    10
    11
}

print(numbers)