Comment by sirwhinesalot

The amount of reference-counted pointers in most Rust code is a tiny fraction compared to boxes or compiler-tracked-lifetime references.

Yes in theory it would be more efficient to store all the reference counts together, but that's in theory. In practice most Rust apps will not call clone on a shared pointer on a hot path and if they do it's usually 1 such pointer and they do something with the data as well (so it's all 1 cache line anyway)

You can't compare Rust/C++ with Swift/Nim when it comes to RC, there just aren't enough reference count operations for it to matter much (unless you're in a shitty OO C++ codebase like me that pretends it is java with std::shared_ptr everywhere)

Apps where heap compaction would be relevant in a low-level language like Rust or C++ will typically use a bump allocator which will trounce any kind of GC.