Setting -Dwarnings breaks feature detection in build.rs

[piodul]

I'm not sure whether this is considered a bug due to custom RUSTFLAGS, but treating warnings as errors in CI doesn't sound uncommon.

Version

cargo tree | grep tokio

test-tokio-fail v0.1.0 (/tmp/test-tokio-fail)
└── tokio v1.24.1

Platform

Linux <cut> 6.0.17-200.fc36.x86_64 #1 SMP PREEMPT_DYNAMIC Wed Jan 4 16:00:03 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux

Description

Steps to reproduce:

rustup toolchain install 1.59.0
cargo new --bin test-tokio-fail
cd ./test-tokio-fail/
echo "tokio = \"=1.24.0\"" >> Cargo.toml # or 1.24.1, same issue

# We are treating warnings as errors in the CI
export RUSTFLAGS="-Dwarnings"
# Alternatively:
# export RUSTFLAGS="--cfg tokio_no_atomic_u64"

cargo +1.59.0 check

I get the following error:

   Compiling tokio v1.24.0
error[E0432]: unresolved import `crate::util::once_cell`
 --> /home/piodul/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.24.0/src/loom/std/atomic_u64_static_once_cell.rs:3:18
  |
3 | use crate::util::once_cell::OnceCell;
  |                  ^^^^^^^^^ could not find `once_cell` in `util`

For more information about this error, try `rustc --explain E0432`.
error: could not compile `tokio` due to previous error

My short investigation led me to the tokio's build.rs file. It looks like -Dwarnings affects the autocfg probe here, seen when running cargo check with high verbosity (-vv):

     Running `/tmp/test-tokio-fail/target/debug/build/tokio-0b45242da01fd5fb/build-script-build`
[tokio 1.24.0] error: unused import: `std::sync::atomic::AtomicU64 as _`
[tokio 1.24.0]  --> <anon>:3:9
[tokio 1.24.0]   |
[tokio 1.24.0] 3 |     use std::sync::atomic::AtomicU64 as _;
[tokio 1.24.0]   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[tokio 1.24.0]   |
[tokio 1.24.0]   = note: `-D unused-imports` implied by `-D warnings`
[tokio 1.24.0] 
[tokio 1.24.0] error: aborting due to previous error
[tokio 1.24.0] 
[tokio 1.24.0] cargo:rustc-cfg=tokio_no_target_has_atomic
[tokio 1.24.0] cargo:rustc-cfg=tokio_no_const_mutex_new
[tokio 1.24.0] cargo:rustc-cfg=tokio_no_atomic_u64

Because of this, cargo:rustc-cfg=tokio_no_atomic_u64 is emitted in addition to others (I get the same effect with RUSTFLAGS="--cfg tokio_no_atomic_u64"). This cfg seems to be key to trigger the error.

[Darksonn]

Thank you for reporting this. Setting -Dwarnings shouldn't break the feature detection in our build.rs file.

The specific compilation failure you ran into was fixed in 1.24.1 (I hope so), but the problem persists — it will incorrectly detect features even if you upgrade, leading to less efficient code.

[piodul]

The specific compilation failure you ran into was fixed in 1.24.1 (I hope so)

I'm afraid it wasn't... If I specify =1.24.1 in the reproducer then I get exactly the same failure:

    Checking tokio v1.24.1
error[E0432]: unresolved import `crate::util::once_cell`
 --> /home/piodul/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.24.1/src/loom/std/atomic_u64_static_once_cell.rs:3:18
  |
3 | use crate::util::once_cell::OnceCell;
  |                  ^^^^^^^^^ could not find `once_cell` in `util`

For more information about this error, try `rustc --explain E0432`.
error: could not compile `tokio` due to previous error

[taiki-e]

This is because util::once_cell is available only when rt, signal, or process is enabled, but atomic_u64_static_once_cell.rs is compiled unconditionally.

tokio/tokio/src/util/mod.rs

Lines 9 to 10 in c390a62

	#[cfg(any(feature = "rt", feature = "signal", feature = "process"))]
	pub(crate) mod once_cell;

tokio/tokio/src/loom/std/mod.rs

Lines 4 to 5 in c390a62

	mod atomic_u32;
	mod atomic_u64;

Can be reproduced with:

RUSTFLAGS='--cfg tokio_no_atomic_u64 --cfg tokio_no_const_mutex_new' cargo hack build -p tokio --no-dev-deps