I agree, with the possible exception of perplexing async stuff.
What do you want Rust to do differently?
What language does async right?
How did Rust not reach its async goals?
Rust even lets you choose the runtime you want. And most big libraries work with several runtimes.
Lean into being synchronous. Why should I have to manually schedule my context switches as a programmer?
As an example: Call N functions to see which one finishes first. With async this is trivial and cheap, without it it’s extremely expensive and error-prone.
Async makes everything so much harder to reason about and introduces so many warts in the languages that use it that I probably think it should be considered an anti-pattern. And I was writing asynchronous code in C in the 90's so it's not like I haven't done it but it is just plain ugly, no matter what syntactic sugar you add to make the pill easier to swallow.
The fact that it hasn't been done?
Or that you can't do it in a systems programming language whose main intent is to replace 'C'?
I don't want to start off with a strawman but in the interest of efficiency:
Because C is far from the only systems programming language and I don't see any pre-requisites in the actor model itself that would stop you from using that in a systems programming language at all. On the contrary, I think it is eminently suitable for systems programming tasks. Message passing is just another core construct and once you have that you can build on top of it without restrictions in terms of what you might be able to achieve.
Even Erlang - not your typical first choice for low level work - is used for bare metal systems programming ('GRiSP').
Maybe this should start with a definition of what you consider to be a systems programming language? Something that can work entirely without a runtime?
Yes, I think "entirely without a runtime" in the colloquial sense is what I mean. Or "replace C" if you want.
'replace C' is a much narrower brief and effectively forces you to accept a lot of the warts that C exposes to the world. This results in friction between what you wanted to do and end up doing as well as being stuck with some decisions made in the 1970's. It revisits a subset of those decisions whilst keeping the remainder. And Rust's ambitions now seem to have grown beyond 'replace C', it is trying very hard to be everything to everybody and includes a package manager and language features that a systems language does not need. In that sense it is becoming more like C++ than like C. C is small. Rust is now large.
Async/Await is a mental model that makes code (much) harder to reason about than synchronous code, in spite of all of the claims to the contrary (and I'm not even sure if all of the people making those claims really believe them, it may be hard to admit that reasoning about code you wrote yourself can be difficult). It obfuscates the thread of execution as well as the state and that's an important support to hold on to while attempting to understand what a chunk of code does. It effectively turns all of your code into a soft equivalent of interrupt driven code, and that is probably the most difficult kind of code you could try to write.
The actor model recognizes this fact and creates an abstraction that - for once - is not leaky, the code is extremely easy to reason about whilst under the hood the complexity of the implementation is hidden from the application programmer. This means that relative novices (which probably describes the bulk of all programmers alive today) can safely and predictably implement complex systems with multiple moving parts because it does not require them to have a mental model akin to a scheduler with multiple processes in flight all of which are at different stages of their execution. Reasoning about the state of a program suddenly becomes a global exercise rather than a local one and locality of state is an important tool if you want to write code that is predictable, the smaller the scope the better you will understand what you are doing.
It is funny because this would suggest that the likes of Erlang and other languages that implement the actor model are beginners languages because most experienced programmers would balk at the barrier to entry. But that barrier is mostly about a lot of the superstructure built on top of Erlang, and probably about the fact that Erlang has its roots in Prolog which was already an odd duck.
But you've made me wonder: could you write Erlang in Erlang entirely without a runtime other than a language bootstrap (which even C needs) and if not to what degree would you have to extend Erlang to be able to do so. And I think here you mean 'the Erlang virtual machine that are not written in Erlang' because Erlang the language is written in Erlang as is the vast bulk of the runtime.
The fact that the BEAM is written in another language is because it is effectively a HAL, an idealized (or not so idealized, see https://www.erlang.org/blog/beam-compiler-history/) machine to run Erlang on, not because you could not write the BEAM itself entirely in Erlang. That's mostly an optimization issue, which to me is in principal evaluations like this a matter of degree rather than a qualitative difference, though if the inefficiency is large enough it could easily become one as early versions of Erlang proved.
Maybe it is the use of a VM that should disqualify a language from being a 'systems language' by your definition?
But personally I don't care about that enough to sacrifice code readability to the point that you add entirely new footguns to a language that aims for safety because for code with long term staying power readability and ability to reason about the code is a very important property. Just as I would rather have memory safety than not (but there are many ways to achieve that particular goal).
What is amusing is that the Async/Await anti-pattern is now prevalent and just about the only 'systems languages' (using your definition) that have not adopted it are C and Go.