Resolve unexpected_cfgs warning

    warning: unexpected `cfg` condition name: `async_trait_nightly_testing`
     --> tests/test.rs:2:5
      |
    2 |     async_trait_nightly_testing,
      |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^
      |
      = 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(async_trait_nightly_testing)");` 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: unexpected `cfg` condition name: `async_trait_nightly_testing`
       --> tests/test.rs:380:7
        |
    380 | #[cfg(async_trait_nightly_testing)]
        |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(async_trait_nightly_testing)");` 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

    warning: unexpected `cfg` condition name: `async_trait_nightly_testing`
        --> tests/test.rs:1298:7
         |
    1298 | #[cfg(async_trait_nightly_testing)]
         |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
         |
         = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(async_trait_nightly_testing)");` 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

    warning: unexpected `cfg` condition name: `async_trait_nightly_testing`
        --> tests/test.rs:1627:7
         |
    1627 | #[cfg(async_trait_nightly_testing)]
         |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
         |
         = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(async_trait_nightly_testing)");` 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

Cargo.toml

@@ -6,6 +6,7 @@ categories = ["asynchronous", "no-std"]
 description = "Type erasure for async trait methods"
 documentation = "https://docs.rs/async-trait"
 edition = "2021"
+exclude = ["build.rs"]
 keywords = ["async"]
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/dtolnay/async-trait"

build.rs

@@ -0,0 +1,5 @@
+fn main() {
+    // Warning: build.rs is not published to crates.io.
+
+    println!("cargo:rustc-check-cfg=cfg(async_trait_nightly_testing)");
+}