`cargo config get` doesn't seem to pick up RUSTFLAGS or RUSTDOCFLAGS

[LukeMathWalker]

Problem

cargo config get build.rustdocflags and cargo config get build.rustflags do not seem to behave as I would have expected.
If the respective environment variables are set (i.e. RUSTDOCFLAGS and RUSTFLAGS) but no value is set in .cargo/config.toml, I'd expect the environment variable value to be returned.

Instead, the command fails with:

error: config value `build.rustdocflags` is not set

Steps

export RUSTDOCFLAGS="--cfg tokio_unstable"
cargo -Z unstable-options config get build.rustdocflags

and

export RUSTFLAGS="--cfg tokio_unstable"
cargo -Z unstable-options config get build.rustflags

Possible Solution(s)

No response

Notes

Is this intended behaviour?

Version

No response

[weihanglo]

I would say it is kinda intentional. The Cargo configuration system only cares about environment variables starting with CARGO_¹². Or more precisely, those that listed in the doc are officially supported; those not may be subject to change.

Try this instead:

CARGO_BUILD_RUSTFLAGS="--cfg tokio_unstable" cargo +nightly -Z unstable-options config get build.rustflags

However, we must admit that the story around rustflags management is a bit messy.

Footnotes

1. https://github.com/rust-lang/cargo/blob/2d693e20ea2dd3a55af94c56372c960503a6c673/src/cargo/util/config/key.rs#L31 ↩

2. https://doc.rust-lang.org/cargo/reference/config.html#environment-variables ↩

[LukeMathWalker]

I see, but the docs do mention RUSTFLAGS as well:

build.rustflags

    Type: string or array of strings
    Default: none
    Environment: CARGO_BUILD_RUSTFLAGS or CARGO_ENCODED_RUSTFLAGS or RUSTFLAGS

Extra command-line flags to pass to rustc. The value may be an array of strings or a space-separated string.

[LukeMathWalker]

The usecase here is being able to accurately predict what extra flags will be passed when invoking rustc or rustdoc without actually performing the invocation.
If cargo config get shows a value that differs from what cargo actually does, its value is greatly diminished.

[weihanglo]

Since RUSTFLAGS is essentially not a cargo configuration, I guess it won't be included in cargo config get in the near future. #11452 (comment) also pointed out this issue. This comment also listed ways to configure rustflags.

For now, I'll triage this issue as A-documenting-cargo-itself Area: Cargo's documentation . Like you just said in #12087 (comment), we can improve the doc of build.rustflags a bit. We can state that setting CARGO_ENCODED_RUSTFLAGS or RUSTFLAGS won't set the config value. Instead, we can say they affect build.rustflags.

[Bhardwaj-Himanshu]

After going through the discussion above, a with very little knowledge of what's going on, I think you guys are talking about changing this-

build.rustflags

    Type: string or array of strings
    Default: none
    Environment: CARGO_BUILD_RUSTFLAGS or CARGO_ENCODED_RUSTFLAGS or RUSTFLAGS

Extra command-line flags to pass to rustc. The value may be an array of strings or a space-separated string.

to-

build.rustflags

    Type: string or array of strings
    Default: none
   --> Environment: CARGO_BUILD_RUSTFLAGS or CARGO_ENCODED_RUSTFLAGS or RUSTFLAGS <-- N-stating this line    

Extra command-line flags to pass to rustc. The value may be an array of strings or a space-separated string.

But Idk what N-stating a line means, so if you could shower some more light on the issue,I would be happy to do the doc work for this issue!
Thanks

[LukeMathWalker]

I'd like to understand this better:

Since RUSTFLAGS is essentially not a cargo configuration

Are we saying that cargo ignores entirely RUSTFLAGS and it's rustc that picks it up on its own? If that's the case, I agree, this is a documentation issue.
But if that is not the case (i.e. cargo does look at RUSTFLAGS and performs some behaviour according to its value), I think the documentation is correct and this should be treated as a bug in cargo config.

My impression is that the latter is happening.