Using async in Rust can lead to bad surprises. I recently came across a particularly gnarly one, and I thought it was interesting enough to share a little discussion. I think that we are too used to the burden of separating async from blocking being on the programmer, and Rust can and should do better, and so can operating system APIs, especially in subtle situations like the one I describe here.

Finally in 2019, Rust stabilized the async feature, which supports asynchronous operations in a way that doesn’t require multiple operating system threads. This feature was so anticipated and hyped and in demand that there was a website whose sole purpose was to announce its stabilization.

async was controversial from its inception; it’s still controversial today; and in this post I am throwing my own 2 cents into this controversy, in defense of the feature. I am only going to try to counter one particular line of criticism here, and I don’t anticipate I’ll cover all the nuance of it – this is a multifaceted issue, and I have a day job. I am also going to assume for this post that you have some understanding of how async works, but if you don’t, or just want a refresher I heartily recommend the Tokio tutorial.