diff --git a/Cargo.toml b/Cargo.toml
index aab4449..8ffa282 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -6,7 +6,7 @@ categories = ["value-formatting", "no-std", "no-std::no-alloc"]
 description = "Fast floating point to string conversion"
 documentation = "https://docs.rs/ryu"
 edition = "2018"
-exclude = ["performance.png", "chart/**"]
+exclude = ["build.rs", "performance.png", "chart/**"]
 keywords = ["float"]
 license = "Apache-2.0 OR BSL-1.0"
 repository = "https://github.com/dtolnay/ryu"
diff --git a/build.rs b/build.rs
new file mode 100644
index 0000000..ecd2acf
--- /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(exhaustive)");
+}
diff --git a/src/lib.rs b/src/lib.rs
index 0ec6038..fb1d071 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -82,6 +82,7 @@
 
 #![no_std]
 #![doc(html_root_url = "https://docs.rs/ryu/1.0.17")]
+#![cfg_attr(not(check_cfg), allow(unexpected_cfgs))]
 #![allow(
     clippy::cast_lossless,
     clippy::cast_possible_truncation,