ConstraintBuilder.swift

import Foundation
import UIKit

@_functionBuilder struct ConstraintBuilder {
    static func buildBlock<T, U>(_ from: NSLayoutAnchor<T>,
                                 _ relation: Relation<U>,
                                 _ to: NSLayoutAnchor<T>) -> NSLayoutConstraint {
        return U.constraint(from: from, to: to, constant: relation.constant)
    }
}

func constraint(@ConstraintBuilder block: () -> NSLayoutConstraint) -> NSLayoutConstraint {
    block()
}

protocol RelationTrait {
    static func constraint<T>(from: NSLayoutAnchor<T>,
                              to: NSLayoutAnchor<T>,
                              constant: CGFloat) -> NSLayoutConstraint
}

enum Relatons {
    enum Equal: RelationTrait {
        static func constraint<T>(from: NSLayoutAnchor<T>,
                                  to: NSLayoutAnchor<T>,
                                  constant: CGFloat) -> NSLayoutConstraint {
            from.constraint(equalTo: to, constant: constant)
        }
    }

    enum LessThanOrEqual: RelationTrait {
        static func constraint<T>(from: NSLayoutAnchor<T>,
                                  to: NSLayoutAnchor<T>,
                                  constant: CGFloat) -> NSLayoutConstraint {
            from.constraint(lessThanOrEqualTo: to, constant: constant)
        }
    }

    enum GreaterThanOrEqual: RelationTrait {
        static func constraint<T>(from: NSLayoutAnchor<T>,
                                  to: NSLayoutAnchor<T>,
                                  constant: CGFloat) -> NSLayoutConstraint {
            from.constraint(greaterThanOrEqualTo: to, constant: constant)
        }
    }
}

struct Relation<Trait: RelationTrait> {
    let constant: CGFloat

    init(_ constant: CGFloat = 0) {
        self.constant = constant
    }
}

typealias Equal = Relation<Relatons.Equal>
typealias LessThanOrEqual = Relation<Relatons.LessThanOrEqual>
typealias GreaterThanOrEqual = Relation<Relatons.GreaterThanOrEqual>

let view1 = UIView()
let view2 = UIView()

let c1: NSLayoutConstraint = constraint {
    view1.bottomAnchor
    Equal(10)
    view2.topAnchor
}

let c2: NSLayoutConstraint = constraint {
    view1.leadingAnchor
    GreaterThanOrEqual(-20)
    view2.trailingAnchor
}

let c3: NSLayoutConstraint = constraint {
    view1.leadingAnchor
    GreaterThanOrEqual()
    view2.trailingAnchor
}