[pjmlp]

The cognitive load is higher on Rust, not only do we have to annotate the code, to have the same semantics as Swift we have to write Rc<RefCell<T>> everywhere, which isn't that ergonomic.

On other side, as discussed recently at CCC regarding writing drivers in safe languages, Swift's GC as reference counting generates even less performant code than Go or .NET tracing GC, so there is also room to improvement there.

[giornogiovanna]

I always assumed that RC has much better worst-case latency than tracing. Given that Swift is a language whose primary target is user interfaces, doesn't it make sense to optimize for that?

[pjmlp]

RC is the easiest GC algorithm to implement, that is all.

It is quite bad for shared data, because handling lock count requires a lock and also trashes the cache, both bad ideas in today's modern hardware architectures.

Also contrary to common belief, it also has stop-the-world issues, because releasing a graph based data structure can originate a cascade of count == 0, thus having a similar behaviour.

Which when coupled with a naive implementation of destructors can even cause stack overflows, due to the nested calls of the data being released.

So when you start adding optimizations for dealing with delayed destruction, non recursive destruction calls, lock free counting, a cycle collector, you end up with a machinery similar to a tracing GC anyway.

Finally what many seem to forget, just because a language has a tracing GC, it doesn't mean that every single memory allocation has to go through the GC.

When a programming language additionally offers the support for value types, stack allocation, global segment static allocation and manual allocation in unsafe code, it is possible to enjoy the productivity of a tracing GC, while having the tooling to optimize the memory usage and responsiveness when required to do so.

Having said all of this, RC makes sense in Swift because it simplifies interoperability with the Objective-C runtime. If Swift had a tracing GC, they would need a similar optimization like .NET has to deal with COM (see Runtime Callable Wrapper).