The Size of Rust Futures

I think I might have been the first person who reported this in 2019: https://github.com/rust-lang/rust/issues/59087

It’s a pretty interesting problem. While everyone assumed that futures are lightweight and thereby more efficient than threads, there’s a couple interesting performance gotchas like this one that apply for certain libraries and applications.

Due to those I’m still m curious for how many applications the async model is actually a net win or a net loss. Too bad we don’t have time to implement all applications in both ways to gather more data.

Yup, I've ran into this when my entire program ended up being a single massive Future struct, which caused a stack overflow :)

I wish there was a Clippy lint for this (https://github.com/rust-lang/rust-clippy/issues/5263).

My current practice is to sprinkle `Box::pin` on code paths that are less common (fallbacks, error handling) or expensive anyway. This cuts down the inlined state considerably without actually causing that many allocations.

Feels like the list of Rust async gotchas is quite long. It would be good to see them all in one place.

That’s probably why C++ couroutines allocate by default.