Cargo accepts conditional dependencies based on feature, but documented not to

[jsgf]

Problem
According to the Cargo documentation:

Unlike in your Rust source code, you cannot use [target.'cfg(feature = "my_crate")'.dependencies] to add dependencies based on optional crate features. Use the [features] section instead.

However, the tracing crates are apparently doing this successfully:

[target.'cfg(not(feature = "std"))'.dependencies]
spin = "0.5"
lazy_static = { version = "1", features = ["spin_no_std"] }

I would expect this to fail.

Possible Solution(s)
It's not clear to me whether the code or the documentation is correct.

I encountered this because I'm writing tools to process Cargo metadata which understands platform-specific dependencies, and it failed on encountering a dependency predicated on a feature. I'm not sure if I should implement support for that or not.

Notes

Output of cargo version:
cargo 1.37.0 (9edd089 2019-08-02)

[ehuss]

That expression is always true, so it is essentially always on, it looks to be a mistake.

Cargo could maybe display a warning about this, though? I'm not sure, because one can say RUSTFLAGS='--cfg feature="std"', which will enable it. But that seems somewhat unusual.

[jsgf]

@ehuss Ok, so in that context cfg(feature = "foo") will always evaluate to false? Does that apply to any unsupported predicate?

[ehuss]

I believe so. If it is not in the output of rustc --print=cfg $RUSTFLAGS, then it is false.

[ehuss]

Closing as warnings are now emitted via #7660.