Using #[cfg(kani)] results in a warning

[zhassan-aws]

As of the nightly-2024-05-05 toolchain, using #[cfg(kani)] in a crate results in a warning, e.g.

fn main() { }

#[cfg(kani)]
mod kani_checks {
    #[kani::proof]
    fn check() {
        assert_eq!(1, 1);
    }
}

using the following command line invocation:

cargo kani

with Kani version:

$ cargo kani
Kani Rust Verifier 0.51.0 (cargo plugin)
   Compiling kani_cfg v0.1.0 (/home/ubuntu/examples/kani_cfg)
warning: unexpected `cfg` condition name: `kani`
 --> src/main.rs:3:7
  |
3 | #[cfg(kani)]
  |       ^^^^
  |
  = help: expected names are: `clippy`, `debug_assertions`, `doc`, `docsrs`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `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(kani)");` 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: `#[warn(unexpected_cfgs)]` on by default

warning: 1 warning emitted

    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.06s
Checking harness kani_checks::check...

References:
https://blog.rust-lang.org/2024/05/06/check-cfg.html
rust-lang/rust#124800

[zhassan-aws]

This change silences the warning when running cargo kani:

diff --git a/kani-driver/src/call_single_file.rs b/kani-driver/src/call_single_file.rs
index 07899c8c489..90b3f3a58d2 100644
--- a/kani-driver/src/call_single_file.rs
+++ b/kani-driver/src/call_single_file.rs
@@ -133,6 +133,7 @@ pub fn kani_rustc_flags(&self) -> Vec<OsString> {
                 "panic_abort_tests=yes",
                 "-Z",
                 "mir-enable-passes=-RemoveStorageMarkers",
+                "--check-cfg=cfg(kani)"
             ]
             .map(OsString::from),
         );

The warning will still be emitted when running cargo build though (if the crate uses the yet-to-be-released rust 1.80 or a nightly beyond 2024-05-05).