Why is `async` a keyword?

whatisaphone · December 10, 2018, 4:54pm

I was reading the async RFC and I see that async used to be an attribute, but today it is a keyword. Why was that changed? I like the symmetry between #[async] and await!(), because it hints at a world where coroutines could have been implemented as a proc macro. With async being a keyword, that symmetry is gone. Why was await left behind? Is there somewhere I can read more about this change?

rpjohnst · December 10, 2018, 6:25pm

await! is not going to be a macro forever; it is only that way today as a stopgap while the final syntax is decided.

17cupsofcoffee · December 10, 2018, 6:28pm

Note that await actually is a keyword in Rust 2018. However, enabling the async_await feature flag on nightly currently un-keywords it, for compatibility with the experimental syntax.

steveklabnik · December 10, 2018, 6:31pm

https://github.com/rust-lang/rfcs/blob/master/text/2394-async_await.md is the actual async/await RFC, and it says https://github.com/rust-lang/rfcs/blob/master/text/2394-async_await.md#built-in-syntax-instead-of-using-macros-in-generators

In the long run, we believe we will want dedicated syntax for async functions, because it is more ergonomic & the use case is compelling and significant enough to justify it (similar to - for example - having built in for loops and if statements rather than having macros which compile to loops and match statements). Given that, the only question is whether or not we could have a more expedited stability by using generators for the time being than by introducing async functions now.

It seems unlikely that using macros which expand to generators will result in a faster stabilization. Generators can express a wider range of possibilities, and have a wider range of open questions - both syntactic and semantic. This does not even address the open questions of stabilizing more procedural macros. For this reason, we believe it is more expedient to stabilize the minimal built-in async/await functionality than to attempt to stabilize generators and proc macros.

whatisaphone · December 10, 2018, 7:30pm

Looks like I was reading the wrong RFC, thanks for the links!