Yeah for sure. In Java/C# I see people do this all the damn time. Use async method for REST endpoints then make a blocking DB call. Or even worse, make a non-async REST call to another service from inside an async handler.

As soon as you do that, your code isn't async anymore. And if you're using a framework like Vert.X or node that only runs one thread per core you're in big trouble.

The most reasonable answer I've seen to all this is Java's Project Loom. An attempt to make fibers transparently act like threads, so you can use regular threaded libraries as async code.

Rust is going to have the same problem Java does with async. A lot of code was written way before async was available, and it not always obvious whether something blocks.