Nim's new ARC gc, which will be default soon-ish in 2.0, does not have a "runtime" as commonly understood.
It is fully deterministic, so it simply injects alloc/free calls in the generated code at compile time. It even has an option that can show you where the calls are injected.
Everything has a runtime nowadays. Even C, even if it isn't all that large (including for example the memory structures necessary to keep malloc and free working).
I think that’s too expansive of a definition of “runtime.” The context required by C and Rust programs is generally internal, meaning that interacting with the dynamic allocator doesn’t require the programmer to be aware of the implementation’s invariants. This is in contrast to a managed language like Java, where using a managed object from C via JNI requires coordination with the runtime (which in turn preserves the invariants).
But you have to know that as well for C - there are for example several Linux system calls you really should not call manually because that would fuck up the internal state in LibC. Yes, the runtime is really small, comparatively speaking, but it is still a runtime.
Which system calls are you thinking of? The only ones that I can think of would be brk(2) and sbrk(2), but any recent libc should be resilient to calls to those.
That being said, I'll grant that the distinction between "tiny runtime" and "no runtime" isn't useful and that C can be said to have a runtime insofar as most systems provide a `crt0` or equivalent.
