diff --git a/Cargo.toml b/Cargo.toml
index 1b3afaa..a0693c1 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -6,6 +6,7 @@ categories = ["development-tools", "no-std", "no-std::no-alloc"]
 description = "Compile-time checks that an enum, struct, or match is written in sorted order."
 documentation = "https://docs.rs/remain"
 edition = "2021"
+exclude = ["build.rs"]
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/dtolnay/remain"
 rust-version = "1.56"
diff --git a/build.rs b/build.rs
new file mode 100644
index 0000000..113f8a0
--- /dev/null
+++ b/build.rs
@@ -0,0 +1,7 @@
+fn main() {
+    // Warning: build.rs is not published to crates.io.
+
+    println!("cargo:rustc-cfg=check_cfg");
+    println!("cargo:rustc-check-cfg=cfg(check_cfg)");
+    println!("cargo:rustc-check-cfg=cfg(remain_stable_testing)");
+}
diff --git a/tests/unstable.rs b/tests/unstable.rs
index dd9fa80..b306de9 100644
--- a/tests/unstable.rs
+++ b/tests/unstable.rs
@@ -1,6 +1,7 @@
 #![allow(dead_code)]
 #![cfg(not(remain_stable_testing))]
 #![feature(proc_macro_hygiene, stmt_expr_attributes)]
+#![cfg_attr(not(check_cfg), allow(unexpected_cfgs))]
 #![allow(
     clippy::derive_partial_eq_without_eq,
     clippy::extra_unused_type_parameters,