> You're describing things COW is designed to prevent as things COW causes.

not quite see below

> You may call COW a "magic trick"

I didn't call it a magic trick, I stated rust decided against using "magic tricks" of internally implicitly not visible doing things below the hood. Which might involve using a Cow or similar. Cow is just the tool the trick could use not the trick by itself.

> Maybe not familiar with how COW

I'm very familiar with it.

But there is a difference between using CoW manually carefully in some places and using it everywhere all the time implicitly as part of the language design.

COW prevents you from always copying everything everywhere

at the cost of on-the-fly copying things when needed

if everything is COW all the time this will mean most of the copies are not expensive because most parts of the thing you need to copy will not need to be copied because they are also cow

but there are some gotchas

first if you have it implicitly everywhere it is very easily to overlook how many copies you make, or that you make way to many and should change your code

and even with "everything COW" the cost still can be very expensive for all kinds of reasons

e.g. in some hypothetical code like `a[10] = 4` you might now have accidentally cloned a 4GiB byte buffer, COW doesn't prevent this because that 4GiB byte buffer is a single thing. Now you might come up with ways to internally split the buffer into chunks or only use lists etc. But that isn't really working for many low level system language use-cases (but totally working for many other cases, including many server programming cases).

Similar it means if you have nested types they always need an indirection to the sub-types so that you can just copy the GC or RC references (or whatever you use for memory management) when needing to make a copy of the outer type (except where an optimizer can optimize it away) which is fine, except that for a system language you often need C layout compatible types. So you would need to severely special case C-layout types.

And as far I can tell you either special case a lot of thing in a way which isolates them away from your language (things like C-layout, fundamental non-clonable types, etc.) or you need to have a implementation of the nested CoW which no longer guarantees that it's efficient at all.

Isolating things away which doesn't fit well with your language is totally an option for a general purpose language but can become very unpleasant and/or hard to use for a low level system language.

> rejecting clear improvements

I'm not rejecting anything, I'm saying that many of the reasons design decisions you called defects are fundamentally the reason why rust has such a huge success. And calling this aspects defects isn't right. It's like saying city bike without a suspension is defect or a poor decisions. They are not. But I still would prefer other approaches (like a suspension you can switch off).

Theoretical improvements are nice, but turning them into practical improvements outside of pet projects is hard.

Which is why rust mostly went with decisions where there had been a bunch of successful research and prototyping done before. And even in that aspect with rather conservative choices. I mean that is partially where the name "rust" comes from, it mostly used stuff which was so old (in research) it could have been rusted.

Because what rust pursed wasn't being a perfect language which overhauls everything. Instead it targeted a solid improvement over C/C++ for some specific set of use-cases which very intentionally does not do anything too unusual. E.g. they directly support the C-API/ABI so that it reasonably easy to include in C/C++ projects and include C/C++ projects into it, so that you can replace some components of larger C++ projects with rust.

> As for C compatibility without copying the memory model flaws, it's possible, but it requires more thought than Rust put into it.

If you combine it with higher performance requirements and requirements like wide spread platform support etc. it's very very hard. There had been research on this for years mostly for GC languages and for a long time success was rather limited. I think in "recent" years there had been some nice progress on this topic. But then this design decisions where made for rust nearly 9 years ago... I mean the first stable release of rust was 8 years ago. So not including research which didn't yet exist isn't really a poor decision.

Furthermore even that success might not be practically appalling for all use-cases. Not for technical reasons. But sadly programming language design isn't just about technical aspects.

Anyway I hop you all success with creating such a language or motivating people to create such a language. IMHO most software today doesn't need to be written in any system language at all. This even includes most "system" components. I mean a huge part of systemd could probably be written in WASM and it still would work fine for a lot of use cases as crazy as it sounds. So good luck.