You’ve now degraded the conversation into a very very weird direction. You made a claim that concurrency required locks. It simply does not and I have an existence proof of Dashmap as a hashmap that doesn’t have any locks anywhere.

The strengths and weaknesses of the standard library aren’t relevant. But if we’re going there, the reason they’re not in the Rust standard library is likely in practice concurrent data structures are an anti pattern - putting a lock around a data structure doesn’t suddenly solve higher order race conditions which is something a lot of Java programmers seem to believe because the standard library encourages this kind of thinking.

As for “my comment” about “global lock” (your words not mine), it’s that the implicit lock that’s available on every object is a bad idea for highly concurrent code (not to mention the implicit overhead that implies for every part of the object graph regardless of it being needed anywhere). Don’t get me wrong - Java took a valiant effort to define a solid memory model for concurrency when the field was still young. Many of the ideas didn’t pan out and are antipatterns these days for high performing code. Of course none of that pertains to the original point of the conversation - tracing GCs have significantly more overhead in practice because they’re very difficult to be opt in, carry quite a penalty if not, Rc/Arc is much better as it’s possible to do opt-in when you need shared ownership (which isn’t always), and in practice loops don’t come up often enough to matter and when they do there’s still solutions. In other words tracing GCs drop huge amounts of performance on the floor and you can read all the comments to see how the claims are “it’s more efficient than Rc”, or “performance is free” or even “it doesn’t matter because the programmer is more efficient”. I’d buy the efficiency argument when the only alternative was C/C++ and came with serious memory safety baggage, but not any of the others and memory safety without sacrificing performance of C++ in my view is a solved problem with Rust.