Lint idea: find known-blocking constructs in async contexts

[skade]

With async/await coming up, we will probably see an uptake of such code:

async hello() {
    println!("hello").
}

Users will most likely not be aware that the call to println are blocking. The same goes to any use of the stdlib io apis, most famously to_socket_addr() calls.

I think there is a chance for clippy to at least find obvious cases of this, for a list of known functions.

[hwchen]

thread::sleep seems to be a pretty common mistake:

https://old.reddit.com/r/rust/comments/e1gxf8/not_understanding_asyncawait_properly/
https://old.reddit.com/r/rust/comments/dtp6z7/what_can_i_actually_do_with_the_new_async_fn/
https://old.reddit.com/r/rust/comments/dt0ruy/how_to_await_futures_concurrently/

[kornelski]

thread::sleep is a major offender. I've found that mistake done several times in a real-world codebase.

[estebank]

I'm currently working on a linter that does exhaustive call-graph analysis to detect indirect usages of blocking calls in async functions. Right now this is working well, and requires either annotations (#[redpen::blocking]) or adding a fully-qualified path to a list ::std::thread::sleep).

The exhaustive check might never land on rustc itself due to perf considerations (although I would love if it could be upstreamed to clippy once its more mature) but we can lint on the "simple"/direct case (async fn foo(t: Duration) { sleep(t) }) relatively easily. In order to do that, it'd be a good idea to add an annotation for blocking to the language.

Adding a top-level attribute feels like a waste, and restricting it to the clippy feels wrong, but we now have the #[diagnostic] namespace, so we could annotate std::thread::sleep and the like with #[diagnostic::blocking] for the benefit of both rustc and clippy to lint against direct uses (and my tool can get those attributes itself for its analysis and lint against indirect uses) and it provides crate authors a way to specify that their functions are blocking and their incorrect direct usage will also get linted.

CC @weiznich

Edit: created rust-lang/rfcs#3639 proposing this.