This is neat and I wish C3 well. But using Nim has shown me the light on maybe the most important innovation I've seen in a native-compiled systems language: Everything, even heap-allocated data, having value semantics by default. In Nim, strings and seqs exist on the heap, but are managed by simple value-semantic wrappers on the stack, where the pointer's lifetime is easy to statically analyze. Moves and destroys ca…
Those implicit copies have downsides that make them a bad fit for various reasons.
Swift doesn't enforce value semantics, but most types in the standard library do follow them (even dictionaries and such), and those types go out of their way to use copy-on-write to try and avoid unnecessary copying as much as possible. Even with that optimization there are too many implicit copies! (it could be argued the copy-on-write makes it worse since it makes it harder to predict when they happen).
Implicit copies of very large datastructures are almost always unwanted, effectively a bug, and having the compiler check this (as in Rust or a C++ type without a copy constructor) can help detect said bugs. It's not all that dissimilar to NULL checking. NULL checking requires lots of extra annoying machinery but it avoids so many bugs it is worthwhile doing.
So you have to have a plan on how to avoid unnecessary copying. "Move-only" types is one way, but then the question is which types do you make move-only? Copying a small vector is usually fine, but a huge one probably not. You have to make the decision for each heap-allocated type if you want it move-only or implicitly copyable (with the caveats above) which is not trivial. You can also add "view" types like slices, but now you need to worry about tracking lifetimes.
For these new C alternative languages, implicit heap copies are a big nono. They have very few implicit calls. There are no destructors, allocators are explicit. Implicit copies could be supported with a default temp allocator that follows a stack discipline, but now you are imposing a specific structure to the temp allocator.
It's not something that can just be added to any language.