UIFont+Features.swift

import UIKit
import CoreText

extension UIFont {
    
    typealias Feature = (type: Int, selector: Int)
    struct Features {
        static var ProportionalNumbers: Feature = (kNumberSpacingType, kProportionalNumbersSelector)
        static var AlternatePunctuation: Feature = (kCharacterAlternativesType, 1) // Magic!
        // Add more...
    }
    
    func fontByAddingFeatures(features: [Feature]) -> UIFont {
        // Using Objective-C classes because of some compiler errors...
        let attributes = self.fontDescriptor().fontAttributes()
        
        var featureDisctionaries: Array<NSObject> = [];
        
        let attr: AnyObject! = attributes[UIFontDescriptorFeatureSettingsAttribute]
        if let existingFeatureDisctionaries = attr as? Array<NSObject> {
            featureDisctionaries = existingFeatureDisctionaries;
        }
        
        for feature in features {
            featureDisctionaries.append([
                UIFontFeatureTypeIdentifierKey: feature.type,
                UIFontFeatureSelectorIdentifierKey: feature.selector,
                ])
        }
        
        let descriptor = self.fontDescriptor().fontDescriptorByAddingAttributes([
            UIFontDescriptorFeatureSettingsAttribute: featureDisctionaries,
            ])
        return UIFont(descriptor: descriptor, size: 0)
    }
}

Usage.swift

label.font.fontByAddingFeatures([
    UIFont.Features.ProportionalNumbers,
    UIFont.Features.AlternatePunctuation,
    ])