.NET follows platform calling convention. GC reference assignment to a memory location does involve going through a write barrier (main user of which is concurrent mark phase of GC) but otherwise it's just plain Windows or System V ABI for the respective ISA.
Practically speaking, you cannot call .NET methods directly unless they are annotated with [UnmanagedCallersOnly] which is necessary to ensure GC is in consistent state, module initializers have ran, etc. This is a concern for NativeAOT libraries as you don't have to explicitly call their entrypoint before calling them AFAIK.
This, however, is true for most languages that are not C. This is also a constraint for both Swift, which has its own reference counting and ABI (Library Evolution ABI) and likely Vala assuming it is reference counted.
The runtime vs runtime-less arguments are not exactly helpful given the context - there are """runtime"""-heavy interpreted languages like Python, Elixir or JS, there is Java which assumes JVM, but is already lower level, and then there's .NET, which under the hood looks a lot like a strange flavour of C++ when you e.g. inspect the AOT binaries with Ghidra.
Fun fact, native profilers work with NAOT applications transparently on all platforms. You can hit "sample" in activity monitor on macOS and it will show you fully symbolicated trace if symbols are available. Just recently, I used Samply to perform multi-threaded profiling and it worked just as well as it would for something written in Rust, if not better.
Swift and Vala and any other eagerly reference counted language don’t have to worry about native C code squirreling away a reference to a GC’s object and not responding to a GC marking callback (either because the mechanism doesn’t exist or because it isn’t used correctly).
How would C track references for heap-allocated data originating from (A)RC-based language?
I don't think what you say on .NET correlates with reality - you are not supposed to pass managed objects across FFI (you just can't, it won't let you unless you do unsafe cast shenanigans) - the correct way is to either marshal the data or use blittable representation (plain structs can be passed as is, either by reference or value, same goes for arrays/pointers of primitives). On the rare occasion where unmanaged code needs to keep a GC object alive, it is passed via special GCHandle but that's an exception not the rule.
Swift has its own high level representation for such objects which are very much not FFI compatible. ARC in Swift is a feature that requires compiler involvement to work correctly, and its structs are not FFI compatible unless they are "trivial". Swift also has its own more advanced Swift Library Evolution ABI, which is strictly not C ABI you expect out of your regular library and has its own calling convention and semantics.
Overall, there seem to be strange misconceptions about how these languages work so it would be best to check with the documentation first:
(I don't actually know if any other other platform, besides Swift itself, implements Swift ABI support, but .NET is going to have it in .NET 9 - likely the first non-Swift platform to do so)
Oh neat I’ve never heard of NativeAOT. I’ve only used Unity’s AOT compiler “Burst” for C# (I assume that is something different?).
Cool stuff.
edit: Actually I meant to say Unity’s IL2CPP, which transpiles IR to C++. Burst is a different tool with similar goals—it compiles IR straight down to native via LLVM.
- APIs to introspect type system properties / reflection
Swift very much has all these. And so does .NET.
As for Unity, it has diverged and lags behind "vanilla" .NET in features, language versions and performance significantly, so the experience of using it won't translate to what is expected to be "normal" C#/.NET of today.