Rust is a great language, and Scala is a great language, but they are different. They optimize for a different thing. Rust definitely rocks at high-performance, resource management, fine grained control over all the aspects of the program, at the expense of developer's time. Scala rocks at developer productivity and building abstractions, sacrificing a bit of performance.
However, I must say that Rust is also a very productive and quite powerful language (albeit IMHO not as powerful as Scala) and Scala is actually not that bad at performance either - probably better than a vast majority of other languages out there (albeit not as powerful as Rust or C++).
As far as ecosystems are considered - this is really hard to say. Rust has the whole C (and most of C++) ecosystem at hand plus a few really amazing Rust solutions like Cargo. Scala is limited mostly to JVM and JS, which are great ecosystems, but it is not true they offer everything. And there are still some caveats when using Java tools with Scala (e.g. profilers, debuggers or build systems).
Of course, general purpose languages have their limits and I'm not arguing that Scala can do everything. But it certainly can do a lot more than Rust (and its C/C++ interop) at the moment.
And I have no problem with the JVM interop. As long as you don't try to write Scala libraries that need to be consumed by Java applications, everything just works really.
A lot more than Rust? Do you count the whole embedded, low latency and scientific computing? As far as I like Scala, I don't think it is better in these areas than Rust. Even the async I/O libraries are more advanced in Rust than in Java.
I'd say Scala's support for scientific computing is significantly ahead of Rust; there are good libraries for ML, linear algebra and what have you (including relatively good bindings for native LAPACK), and there's the whole Spark ecosystem.
Scala on a well-tuned JVM can reach much lower latency than you might think. Yes, in theory you'll hit a limit that Rust could help you go beyond. In practice, are you ever going to reach that limit?
Rust's embedded support is mostly theoretical at this point, and the use cases for old-school embedded are increasingly limited. Want to write some code for that cheap off-the-shelf SoC you bought? Scala is probably a perfectly good option, because it's probably an ARM with at least 1Gb of RAM and the JVM will run there just fine. If you're actually writing something that has to run for months off a single charge then yeah, Scala won't cut it whereas Rust may one day be able to. But most "embedded" these days isn't like that.
Of course you can call C functions, but it is neither ergonomic nor performant. And good luck exchanging more complex data structures. Java can't use C structures directly, Rust can.
Out of context yes. I should have said that with respect to which language pick up next and which language has a bigger future potential. Rust is so omnivore and hits on so many critical aspects that it is hard to deny it. Given it had rich and mature ecosystem now how many would pick Scala? Those who prefer GC pauses and have a hard dependency on JVM? But even then you are pitted against Kotlin and Java itself. I would say quite tough times are awaiting Scala 3 and not only on individual level but on company level. It is still unclear how much the migration will take and whether it is worth it in the first place.
I don't exactly see how Rust can replace Scala where Scala is mostly used today. Or even Swift, despite Apple being more serious on the server-side roadmap lately and hiring a bunch of people from the JVM world.
Kotlin doesn't bring anything new and is full of ad-hoc design decisions, and Java itself is catching up (and even potentially surpassing it, see pattern matching for instance). I don't think it's a huge threat. Sure you'll see people who didn't really understand the point of Scala move to Kotlin because they just wanted a better Java really, and Kotlin does that well. But I think it's been clear that Scala shouldn't try to pursue this goal, for the past 5 years at least, after the hype around Spark cooled off.
I'm happy that Scala 3 is focusing on the biggest pain points. I'm also quite happy with the resources that have been added behind the tooling and ecosystem lately. The Scala center wouldn't exist if there wasn't a real demand for improvements in the mid to long-term future.
However, I must say that Rust is also a very productive and quite powerful language (albeit IMHO not as powerful as Scala) and Scala is actually not that bad at performance either - probably better than a vast majority of other languages out there (albeit not as powerful as Rust or C++).
As far as ecosystems are considered - this is really hard to say. Rust has the whole C (and most of C++) ecosystem at hand plus a few really amazing Rust solutions like Cargo. Scala is limited mostly to JVM and JS, which are great ecosystems, but it is not true they offer everything. And there are still some caveats when using Java tools with Scala (e.g. profilers, debuggers or build systems).