extern library not resolved when specifying `--cfg` rust flag in `[target.<cfg>]`

[BugenZhao]

Problem

There are some APIs of tokio requiring a special --cfg tokio_unstable, like the tokio console with the feature tracing. By putting this rustflag in [build] or [target.<triple>] session, it works well. However, when I put the flag in the [target.<cfg>] (like [target.'cfg(all())']), tokio cannot compile.

i.e., this works,

[target.'cfg(all())']
rustflags = [
    "--cfg", "always",
  # "--cfg", "tokio_unstable"
]

[target.aarch64-apple-darwin]
rustflags = [
    "--cfg", "apple",
    "--cfg", "tokio_unstable"
]

but this doesn't.

[target.'cfg(all())']
rustflags = [
    "--cfg", "always",
    "--cfg", "tokio_unstable"
]

[target.aarch64-apple-darwin]
rustflags = [
    "--cfg", "apple",
  # "--cfg", "tokio_unstable"
]

By comparing the cargo check --verbose, I found that the --extern option for tracing passing to rustc is missing. This caused the compiler to fail to resolve the tracing crate.

- --extern tracing=/Users/bugenzhao/Developer/Works/profile-rustflags/target/debug/deps/libtracing-5ca025473bc6506d.rmeta --cap-lints allow --cfg mac --cfg tokio_unstable
+ --cap-lints allow --cfg mac --cfg tokio_unstable

error[E0433]: failed to resolve: use of undeclared crate or module `tracing`
   --> /Users/bugenzhao/.cargo/registry/src/gitproxy.zycloud.tk-1ecc6299db9ec823/tokio-1.21.1/src/macros/trace.rs:4:13
    |
4   |             tracing::trace!(
    |             ^^^^^^^ use of undeclared crate or module `tracing`

Steps

1. Initialize an empty crate.
2. Add the tokio dependency.

tokio = { version = "*", features = ["full", "tracing"] }

3. In .cargo/config.toml:

[target.'cfg(all())']
rustflags = [
    "--cfg", "tokio_unstable"
]

4. Run cargo check --verbose.

Possible Solution(s)

No response

Notes

I've also tried to specify it in the profile.<profile>.rustflags, and it does not work either. :(

Version

cargo 1.64.0-nightly (85b500cca 2022-07-24)
release: 1.64.0-nightly
commit-hash: 85b500ccad8cd0b63995fd94a03ddd4b83f7905b
commit-date: 2022-07-24
host: aarch64-apple-darwin
libgit2: 1.4.2 (sys:0.14.2 vendored)
libcurl: 7.79.1 (sys:0.4.55+curl-7.83.1 system ssl:(SecureTransport) LibreSSL/3.3.6)
os: Mac OS 12.5.0 [64-bit]

[ehuss]

Hm, I can't seem to reproduce your issue. The "Steps" section contains a different config from the one described at the top. I tried both and they seemed to work. Can you include the entire output from cargo check --verbose? Can you also run cargo config get -Zunstable-options (and remove any sensitive information like token before posting).

[BugenZhao]

I've retried it and found that it works with nightly-2022-10-01 while not with nightly-2022-07-29. The crate to reproduce and the output of cargo check --verbose is here. The output of cargo config get -Zunstable-options is the same:

target."cfg(all())".rustflags = ["--cfg", "tokio_unstable"]
# The following environment variables may affect the loaded values.
# CARGO_HOME=/Users/bugenzhao/.cargo

So I guess it's #11114 that fixed this issue.

Then I also tried to specify --cfg tokio_unstable in profile rustflags (the diff here), but both of the toolchains don't work. Is this a similar issue to #11114?

[ehuss]

The issue with profile rustflags is a little different. The profile information is not considered when cargo is determining target settings. I noted that on the profile rustflags tracking issue.

Sounds like #11114 resolved the issue here, so closing as resolved by that.