To over explain, if you just need to make N forks or kick of N threads then it’s very easy to do this correctly in C. The cases where I’m going to carefully maintain shared mutable state with locking are cases where the parallelism is less efficient (Ahmdal’s law).
Java style apps that just haphazardly start threads are what rust makes safer. But that’s a category of program design I find brittle and painful.
This is a huge limitation of C's compilation model, and basically every other language since then does it differently, so not sure if that's a good example. You do want some "interconnection" between translation units, or at least less fine-grained units.
What’s better? Rust? Haskell? Swift?
It’s very hard to do multithreading at a more granular level without hitting amdahl’s law and synchronization traps.
Sure, it's not a trivial problem, but why wouldn't we want better compilation results/developer ergonomics at the price of more compiler complexity and some minimal performance penalty?
And it's not like the performance doesn't have its own set of negatives, like header-only libraries are a hack directly manifested from this compilation model.