Concrete playback needs tweak with overridden rust-toolchain

[zhassan-aws]

The instructions for the concrete playback feature mention that once a concrete playback test is generated, one can run it with cargo test. For packages that pin a specific rust toolchain (e.g. via a rust-toolchain file), this could result in an error if the pinned rust toolchain is a stable release because compiling Kani requires using nightly due to the usage of unstable features.

Steps to reproduce:

1. cargo new --lib conc_playback
2. cd conc_playback
3. Add kani to the dev dependencies in Cargo.toml:

[dev-dependencies]
kani = { git = "https://github.com/model-checking/kani", features = ["concrete_playback"] }

4. Create a file to pin the rust toolchain:

echo "1.63.0" > rust-toolchain

5. Replace src/lib.rs with the following:

#[kani::proof]
fn main() {
    let x: u8 = kani::any();
    let y: u8 = kani::any();
    assert_eq!(x, y);
}

6. Run Kani:

cargo kani --harness main --enable-unstable --concrete-playback=inplace

7. Run cargo test kani_concrete_playback_main_241008145652399799 to run the concrete trace. This errors out with:

   Compiling conc_playback v0.1.0 (/home/ubuntu/examples/new-iss8/conc_playback)
   Compiling quote v1.0.21
   Compiling syn v1.0.99
   Compiling kani_macros v0.9.0 (https://github.com/model-checking/kani#6405617b)
   Compiling kani v0.9.0 (https://github.com/model-checking/kani#6405617b)
error[E0554]: `#![feature]` may not be used on the stable release channel
 --> /home/ubuntu/.cargo/git/checkouts/kani-0ce0dacf5e98886d/6405617/library/kani/src/lib.rs:5:1
  |
5 | #![feature(rustc_attrs)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0554`.
error: could not compile `kani` due to previous error

[zhassan-aws]

The workaround is to run cargo test with nightly, e.g.:

cargo +nightly-2022-08-16 test kani_concrete_playback_main_241008145652399799