[jandrewrogers]

Rust improves on Java in that it doesn't have a GC, so no pauses, and the code gen is similar to C++ in terms of performance.

Rust is worse than C++ in that it struggles to express some idiomatic high-performance code architectures, such as using DMA for I/O, because they violate axioms of Rust's memory safety model, or in other cases because Rust currently lacks the language features. To make it work in Rust requires writing more code and disabling safety features.

Rust fixes many language design and performance issues with Java, while offering some similar types of safety, but (like Java) it is missing some elements of expressiveness of C++ that are important for high-performance architectures.

[aldanor]

Hm... unsafe {} + use pointers and you're back to c++-style? You've had unsound code in one language, now you have unsound code in other language, how come it's suddenly worse other than having to type unsafe explicitly?

I've done my fair share of C++, including low latency stuff, and in the grand scheme of things I'd say "expressiveness of C++" is completely overshadowed by its complexity and occasional ambiguity, lack of proper type system and proper generics, lack of proper module system, lack of dependency tracking, lack of a unified build systems, etc. I'm not exactly sure what you mean by expressiveness though.

[jandrewrogers]

The issue, beyond having no visible references at compile-time, is that DMA has no concept of objects and has its own rules for how and when it interacts with regions of memory. It is oblivious to ownership semantics. In things like databases, most of your address space is DMA-able memory and therefore "unsafe". In C++, you can completely hide these mechanics behind constructs that look like unique_ptr/shared_ptr but which are guaranteed to be DMA-safe. In this context, all references are treated as mutable because mutability is not a property of references per se. Conflicts can only be resolved at runtime.

Instead of using an ownership-based memory safety model, which is clearly broken for systems that use DMA, they can use schedule-based memory safety models, which are formally verifiable. These don't rely on immutable references for safety, and also eliminate most need for locking -- many of the concepts originate in research on automated lock-graph conflict resolution in databases. The evolution away from ownership-based to schedule-based models is evident in database kernels over the decades. It was originally motivated by improved concurrency; suitability for later DMA-based designs was a happy accident and C++ allows it to be expressed idiomatically.

As for expressiveness, beyond the ability construct alternative safety models, modern C++ metaprogramming facilities enable not only compile-time optimization but also verification of code correctness that would be impractical in languages like Rust.