Bug 17691 – core.mutex.Mutex : {lock,unlock} should be @nogc

Their underlying implementations are already @nogc.

Since those functions are virtual, adding any attribute to them is breaking change, because it adds additional requirements to derived classes. New code that doesn't need to use derived classes, should prefer the `final` alternatives (lock_nothrow, tryLock_nothrow and unlock_nothrow). This violates the "open" part of OCP, so a valid, though probably unrelated to your problem, enhancement request is to create a class derived from Mutex that has all the attributes, but allows those methods to be overridden.