Challenge: write a reference counted slice that works as much as possible like a built-in slice

[deadalnix]

On Monday, 8 November 2021 at 21:42:12 UTC, Andrei Alexandrescu 
wrote:
> 1. We could not make reference counting work properly in pure 
> functions (a pure function does not mutate data, which goes 
> against manipulating the reference count).
>

I don't think this one is a real problem, as one can cast a 
function call to pure and go through this. Dirty as hell, but 
done properly it should work even in the face of compiler 
optimization based on purity. GCC or LLVM would have no problem 
optimizing this scafolding away.

> 2. Qualifiers compound problems with interlocking: mutable data 
> is known to be single-threaded, so no need for interlocking. 
> Immutable data may be multi-threaded, meaning reference 
> counting needs atomic operations. Const data has unknown 
> origin, which means the information of how data originated 
> (mutable or not) must be saved at runtime.
>

shared_ptr does atomic operation all the time. The reality is 
that on modern machines, atomic operation are cheap *granted 
there is no contention*. It will certainly limit what the 
optimizer can do, but all in all, it's almost certainly better 
than keeping the info around and doing a runtime check.

> 3. Safety forced the use of trusted blocks all over. Not a 
> showstopper, but a complicating factor.
>
> So if there are any takers on getting RCSlice off the ground, 
> it would be very interesting.

Yes, similar to pure, a bit anoying, but workable.

I think however, you missed several massive problems:
4. Type qualifier transitivity. const RCSlice!T -> const 
RCSlice!(const T) conversion needs to happen transparently.
5. Head mutability. const RCSlice!(const T) -> RCSlice!(const T) 
conversion needs to happen transparently.
6. Safety is impossible to ensure without a runtime check at 
every step, because it is not possible to enforce construction in 
D at the moment, so destrcutors and copy constructor/postblit 
always need to do a runtime check for .init .

I believe 4 and 5 to be impossible in D right now, 6 can be 
solved using a ton of runtime checks.