Include --check-cfg=cfg(kani) in the rust flags to avoid a warning …

…about an unknown `cfg`. (rust-lang#3187)

Starting with the 2024-05-05 toolchain (and the upcoming Rust 1.80
release), the `unexpected_cfgs` lint has been turned on by default. As a
result, running `cargo kani` on a crate that has a `#[cfg(kani)]`
results in a warning (see rust-lang#3186). To avoid this warning, this PR adds
`--check-cfg=cfg(kani)` to `RUSTFLAGS` when Kani invokes `cargo`.

Call-outs: On such packages, doing a `cargo build` will also result in
this warning, unless:
```rust
println!("cargo::rustc-check-cfg=cfg(kani)");
```
is added to the package's `build.rs` file. However, this warning would
only occur with `cargo build` if the package uses the 2024-05-05
toolchain (or newer), or the Rust version used in the package is
upgraded to 1.80 (when it's released at the end of July 2024). Since
we're likely to release a new version of Kani sooner than the 1.80
release, this PR mitigates the issue that is more likely to impact users
(a warning from `cargo kani`).

Resolves rust-lang#3186 

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

kani-driver/src/call_single_file.rs

@@ -133,6 +133,7 @@ impl KaniSession {
                 "panic_abort_tests=yes",
                 "-Z",
                 "mir-enable-passes=-RemoveStorageMarkers",
+                "--check-cfg=cfg(kani)",
             ]
             .map(OsString::from),
         );

tests/cargo-kani/unexpected_cfgs/Cargo.toml

@@ -0,0 +1,9 @@
+# Copyright Kani Contributors
+# SPDX-License-Identifier: Apache-2.0 OR MIT
+
+[package]
+name = "unexpected_cfgs"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

tests/cargo-kani/unexpected_cfgs/expected

@@ -0,0 +1 @@
+VERIFICATION:- SUCCESSFUL

tests/cargo-kani/unexpected_cfgs/src/main.rs

@@ -0,0 +1,19 @@
+// Copyright Kani Contributors
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+// This test checks that the `unexpected_cfgs` lint (enabled by default as of
+// the 2024-05-05 toolchain) does not cause `cargo kani` to emit warnings when
+// the code has `#[cfg(kani)]`. Kani avoids the warning by adding
+// `--check-cfg=cfg(kani)` to the rust flags.
+
+#![deny(unexpected_cfgs)]
+
+fn main() {}
+
+#[cfg(kani)]
+mod kani_checks {
+    #[kani::proof]
+    fn check_unexpected_cfg() {
+        assert_eq!(1, 1);
+    }
+}