How is the cost in terms of binary size nowadays? Last time I checked using Futures meant adding roughly 5MB to the binary. To me at least that is unacceptable, so I just rolled my own stuff on top of rayon's thread pools.

You're probably thinking of the Tokio library in regards to the size, though I can't right now validate the size.

Futures themselves are very lightweight.

Ah, yes, that was probably it. I stopped looking into Rust's Futures pretty much as soon as I noticed how heavyweight the required runtime is, which seemed ridiculous to me, since Futures are so simple conceptually. I have to look into this again.

In general, Rust's big binaries are one of the things that puts me off the most. Even dynamic linking often doesn't help much. Then again, if a few MB, which is a rounding error nowadays, is among the worst I can think of, that's actually pretty amazing ;).

When Rust switched to the system allocator by default, that slimmed down the minimum size quite a bit. In addition to that, where size matters, there are some articles about how to slim it down even more, here's an example: https://github.com/johnthagen/min-sized-rust