papercut: cargo does not provide feedback when using a `cfg(feature = "...")` that is not declared in `Cargo.toml`

[lopopolo]

Problem

Rust code in a crate can use cfg(feature = "...") attributes even if those features are not defined in Cargo.toml.

I would expect this to be an error or a warning.

Steps

git clone https://github.com/artichoke/intaglio.git
cd intaglio/benchmarks
git checkout 28dc9784a59336aac5b879a9c90a56b85bee4216
cargo bench

Then observe that none of the bytes benches are ran.

This was fixed after stumbling upon this code during an ad hoc review:

• Benchmark bytes::SymbolTable artichoke/intaglio#114

Possible Solution(s)

Emit a warning when Rust sources use a cfg(feature = "...") attribute for a feature that cargo will never pass for the crate because it is not declared in Cargo.toml.

Notes

No response

Version

$ cargo --version --verbose
cargo 1.56.0 (4ed5d137b 2021-10-04)
release: 1.56.0
commit-hash: 4ed5d137baff5eccf1bae5a7b2ae4b57efad4a7d
commit-date: 2021-10-04

[ehuss]

Thanks for the suggestion! RFC 3013 has been accepted to implement this. You can follow the tracking issue at rust-lang/rust#82450 for updates.

[lopopolo]

ooo thank you. I looked in the cargo issues but didn't think to check the Rust repo. 😅