A (simpler) protocol to implement than _ObjectiveCBridgeable which implicitly implements it. Tested with Swift 2.2, Xcode 7.3.1. SourceKit tends to have problems parsing this (it will cause crashes) -- edits welcome, although I haven't taken a look in a bit.

ObjCBridgeable.swift

import protocol Foundation.NSObjectProtocol

/// The minimum requirements for bridging conforming Swift "named types"
/// into a representable Objective-C type, i.e. a subclass of NSObject.
///
/// Note that implementing the direct descendents of `ObjCBridgeable`
/// provides automatic conformance to `_ObjectiveCBridgeable`.
protocol ObjCBridgeable: _ObjectiveCBridgeable {
    
    // Provides code-completion since the latter is hidden, and provides
    // an extremely reasonable restriction which the standard library lacks.
    //
    // Note: the reason this is missing is possibly due to the ability to
    // convert to C structs and related types, a feature which was vastly
    // improved with Swift 2.3/3.0.
    associatedtype _ObjectiveCType: NSObjectProtocol

    static func _objc_metatype() -> _ObjectiveCType.Type

    init?(_objc_bridge: _ObjectiveCType)
    var _objc_bridge: _ObjectiveCType { get }
}

extension ObjCBridgeable // _ObjectiveCBridgeable
{
    static func _isBridgedToObjectiveC() -> Bool { return true }

    static func _getObjectiveCType() -> Any.Type {
        return _objc_metatype()
    }

    func _bridgeToObjectiveC() -> Self._ObjectiveCType {
        return self._objc_bridge
    }
    
    static func _forceBridgeFromObjectiveC(
        source: Self._ObjectiveCType,
        inout result: Self?
    ) {
        result = Self.init(_objc_bridge: source)
    }
    
    static func _conditionallyBridgeFromObjectiveC(
        source: Self._ObjectiveCType,
        inout result: Self?
    ) -> Bool {
        _forceBridgeFromObjectiveC(source, result: &result)
        return true
    }
}