Thin Lock Implementation

[Bartosz Milewski]

Sean Kelly wrote:
> If the "val = null" assignment is forgotten in makeLocal then an attempt 
> to synchronize on c will throw a SharingException (or whatever it's 
> called).

I would like to make the nulling of the original reference part of the 
cast. In other words, makeLocal would take its argument by non-const 
reference and null it. That still doesn't solve the problem of multiple 
aliases, hence the need for SharingException.

> The benefit of being able to cast away the shared label is clearly for 
> performance... if you know that you're the sole owner of a shared object 
> then you don't need to lock for otherwise synchronized accesses (so 
> long), much like assertUnique for const/invariant data.

There is one complication though--the type system. A function that takes 
a non-shared argument can't be called with a shared object. But it's 
true that a sharing cast is an optimization. The safe way to do it is to 
make a thread-local deep copy of the object and pass that to a library 
function. (Object cloning is a hot topic in D.)