Back in the day of Rust having a runtime, you couldn't call C functions, or functions potentially calling C functions, without worrying that it might indefinite

Back in the day of Rust having a runtime, you couldn't call C functions, or functions potentially calling C functions, without worrying that it might indefinitely block one of the limited OS threads used by the runtime. With weirdly typed coroutines it's at least part of the type system. I think Haskell happily spawns a new OS thread when one looks like it might be going to spend a while in C code?