>>> from trio.lowlevel import enable_ki_protection
>>> async def foo():
...     return
... 
>>> @enable_ki_protection
... async def bar():
...     return
... 
>>> import inspect
>>> inspect.iscoroutinefunction(foo)
True
>>> inspect.iscoroutinefunction(bar)
False

Found when I tried to update coroutine_or_error in trio/_util.py to use inspect.iscoroutinefunction in #2668

There are also a ton of wrapper/helper methods in tests that un-coroutinefunction-ify functions, which'd need to be modified in order to actually use iscoroutinefunction in couroutine_or_error. This means it'd probably also break a bunch of downstream code if coroutine_or_error was rewritten, but the upside of not having to call the function might mean that it's worth it (after a deprecation period)? That's probably for another issue though.