Does not build with `time` feature and `--cfg loom`

[faern]

Version

0.2.20 and latest git

Platform

macOS

Description

I'm using loom as recommended in the loom docs: by setting RUSTFLAGS="--cfg loom". This causes tokio to not compile if the time feature is activated. Can be reproduced by just creating an empty crate and adding tokio as a dependency:

[dependencies]
tokio = { version = "0.2.20", features = ["time"] }

$ RUSTFLAGS="--cfg loom" cargo build
...
error[E0432]: unresolved import `crate::sync::AtomicWaker`
 --> /Users/faern/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.2.20/src/time/driver/entry.rs:2:5
  |
2 | use crate::sync::AtomicWaker;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^ no `AtomicWaker` in `sync`

I assume crates depending on tokio should be able to use loom so I guess this is not intentional.

[carllerche]

Tokio is not currently meant to be depended on w/ the loom cfg.

Could you provide more info on what you are trying to do?

[faern]

I'm using loom to test the correctness of a channel implementation I'm writing. This channel implementation also happens to implement Future. so I write integration tests that check if the channel works with tokio. But the crate does not compile when tested with loom as described above.

I could of course just #[cfg(not(loom))] all my tests that involve tokio and not pull in the dependency for dev-dependencies if that cfg is set. But I would then not be able to have loom evaluate the correctness of my impl Future for Receiver which is what I was hoping I could do.

This is one problem of using/recommending --cfg loom instead of having a public feature or recommending that the cfg flag be named --cfg {crate_name}_loom or something. The cfg currently propagates to all dependencies and no one knows which of your dependencies might be loom aware or how they might behave with that cfg.

[faern]

I can understand if it would be really hard to make all of tokio work the way I expected it (given loom is a tokio project). But then, as an alternative solution: Is there any super simple async runtime that is loom aware that I can pull in?

Maybe I could write something simple myself...

[faern]

How do you test the memory correctness of all Future types inside tokio if not with loom?