Explain why borrows can't be held across yield point in async blocks/functions

[jyn514]

Consider the following code:

use std::sync::Arc;
use tokio::runtime::Runtime; // 0.3.1

async fn f() {
    let room_ref = Arc::new(Vec::new());

    let gameloop_handle = Runtime::new().unwrap().spawn(async {
        game_loop(Arc::clone(&room_ref))
    });
    gameloop_handle.await;
}

fn game_loop(v: Arc<Vec<usize>>) {}

The error message has a helpful hint:

help: to force the async block to take ownership of `room_ref` (and any other referenced variables), use the `move` keyword
  |
7 |     let gameloop_handle = Runtime::new().unwrap().spawn(async move {
8 |         game_loop(Arc::clone(&room_ref))
9 |     });

But it doesn't explain very well why move is necessary:

error[E0373]: async block may outlive the current function, but it borrows `room_ref`, which is owned by the current function
 --> src/lib.rs:7:63
  |
7 |       let gameloop_handle = Runtime::new().unwrap().spawn(async {
  |  _______________________________________________________________^
8 | |         game_loop(Arc::clone(&room_ref))
  | |                               -------- `room_ref` is borrowed here
9 | |     });
  | |_____^ may outlive borrowed value `room_ref`
  |
note: function requires argument type to outlive `'static`
 --> src/lib.rs:7:57
  |
7 |       let gameloop_handle = Runtime::new().unwrap().spawn(async {
  |  _________________________________________________________^
8 | |         game_loop(Arc::clone(&room_ref))
9 | |     });
  | |_____^

In particular, 'async block may outlive the current function' makes no sense without a very good mental model of async: the future is awaited within the current function, so of course it can't outlive it. The thing to realize here is that even though it's written as one function, it's actually two: one executed before the yield point, and one after, and the stack space for the first function goes away when you call await.

It would be nice to instead say something like

note: borrows cannot be held across a yield point, because the stack space of the current function is not preserved
help: see https://rust-lang.github.io/async-book/03_async_await/01_chapter.html#awaiting-on-a-multithreaded-executor for more information

[jyn514]

cc @estebank

[jyn514]

Hmm, reading that link, it looks like this issue is specific to multi-threaded executors and this would be fine without a 'static bound on spawn? I'm not sure how much help the compiler can be then ...

[SkiFire13]

This isn't specific to async, it also happens with std::thread::spawn requiring the closure to be 'static even if we later call JoinHandle::join in the same function.

[tmandry]

What about something like

note: async blocks are not executed immediately and either must take a reference or ownership of outside variables they use
help: to force ...

We could substitute "closures" in the non-async case as well, if it'd be helpful.

[tmandry]

Happy to help anyone who wants to implement this.

[sledgehammervampire]

Hi, I want to start contributing to Rust. Would this be a good first issue?

[sledgehammervampire]

@rustbot claim

[jyn514]

@1000teslas that'd be great! See https://rustc-dev-guide.rust-lang.org/ for instructions on getting started, and for your change I'd start by finding where the current error is emitted (it will be somewhere in compiler/) and looking around there.

[sledgehammervampire]

I think the current error message is emitted from report_escaping_closure_capture in compiler/rustc_mir/src/borrow_check/diagnostics/conflict_errors.rs. However, I'm not sure what the appropriate method for adding those new help/note messages. Would the help and note messages from DiagnosticBuilder be the right ones?

[jyn514]

@1000teslas yes, those are right (or at least close enough that you can fix any issues during review).

[sledgehammervampire]

I have tried implementing the error message for async block here. I am not sure what errors an async function would have. For example, I tried messing around here with borrowing past an await point in an async function, but it seems to compile.

I am not sure how to keep lines under 100 characters when mentioning urls in an error message. Also, for some reason, ./x.py fmt doesn't seem to fix up the formatting of my files.

[sledgehammervampire]

Should this issue be closed?