error: unexpected cfg condition name: fuzzing

[larseggert]

cargo 1.80.0-nightly (05364cb2f 2024-05-03) now throws the following clippy error:

error: unexpected `cfg` condition name: `fuzzing`
  --> neqo-transport/src/lib.rs:21:7
   |
21 | #[cfg(fuzzing)]
   |       ^^^^^^^
   |
   = help: expected names are: `clippy`, `debug_assertions`, `doc`, `docsrs`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, `windows`
   = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(fuzzing)");` to the top of the `build.rs`
   = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration
   = note: `-D unexpected-cfgs` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(unexpected_cfgs)]`

[nagisa]

This is when running clippy or is this reproducible with cargo check as well? Well, unfortunately, I don't think this is something we can fix on our side. Using and suggesting use of cfg(fuzzing) follows a well established example and it would be the tools that use the --check-cfg the way they do that are overzealous here (especially if they fail to provide mechanisms to modify the behaviour easily.)

I would report this issue against cargo (if the issue is reproducible with cargo check.)

[kpcyrd]

I assume this is related to this recent change: https://blog.rust-lang.org/2024/05/06/check-cfg.html

[larseggert]

@kpcyrd Yes, it is. Sorry, I should have said so in the report.

[Urgau]

With the release of rust-lang/cargo#13913 (in nightly-2024-05-19), Cargo has gain the ability to declare --check-cfg args directly in the [lints] table with [lints.rust.unexpected_cfgs.check-cfg]¹:

Cargo.toml:

[lints.rust]
unexpected_cfgs = { level = "warn", check-cfg = ['cfg(fuzzing)'] }

I recommand directly suggesting it in the README.md, although the lint diagnostic should already suggest that.

Footnotes

1. take effect on Rust 1.80 (current nightly), is ignored on Rust 1.79 (current beta), and produce an unused warning below ↩