So why do we need Rust at all? What's the use case for it?
Anything that I'm missing?
In all C# codebases I've seen, you have threads and tasks, and you often run into multiple threads or tasks holding a mutable reference to the same data. That's not legal in Rust without synchronization/locking.
If you don't believe me, just spin up a new main.rs file and write code that has a data race.
While it is true that Rust is a strictly superior option for highly concurrent systems code, it still leaves areas where you can make a mistake regarding lock management and other advanced forms of synchronization.
In addition to that, .NET as platform is fairly tolerant to misuse and calling the code that is not thread-safe from multiple threads concurrently usually leads to logic bugs or "stop modifying this collection concurrently, please" exceptions but not to catastrophic memory safety issues like it happens in C/C++.
You can read more on its low-level memory model here: https://github.com/dotnet/runtime/blob/main/docs/design/spec...
> The same is true for async, which in C# is also a problem.
Now, this one is strictly not true. Async primitives are thread-safe. In Rust, you must synchronize because at the very least you must deterministically deallocate memory used by shared state between the tasks. In C#, this complexity is handled for you by a GC (ironically, you get negative sentiment towards async from people having experienced Python's async or Rust's async complexity, assuming the same applies to C#). In some scenarios, it is also a throughput optimization since it reduces memory contention by not modifying the cachelines shared between the cores, lending itself into better performance on many-core systems - the memory is modified/reclaimed when it's no longer in use, while the actively shared data is placed elsewhere.