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Does spin-on-poll imply actual spinning (100% CPU) or actual blocking (waiting in a syscall)?

That would have to be determined at the discretion of the implementer of the library. Rust is after all capable of being used to write code that will run in a context where there is no kernel and thus no syscalls.

I don't get it - what library? I may have an `async` function and this is part of the Rust language, not tied to any library. Can I make it into a blocking function without 100% CPU?

"async/await" is just syntactic sugar for a function that returns a Future plus a state machine at yield points. You need a library (executor) to run that Future (execute that function).

The Rust standard library's block_on uses a global ThreadPool, and the docs recommend using a LocalPool if you need finer grained control.

So, to answer your question it depends on the executor (the thing that implements block_on).

(To be clear, block_on is not in the Rust standard library.)

ah, darn, thanks!

So for anyone reading for the correct details: the block_on I was thinking of is part of the futures crate, which is not in std, it's not an "official library".

It's all good! It was proposed, but decided to have an RFC first. Your point is still correct, just wanted to make sure to clarify this detail :)

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