diff --git a/build.rs b/build.rs
new file mode 100644
index 0000000..b2de2dc
--- /dev/null
+++ b/build.rs
@@ -0,0 +1,27 @@
+// Copyright 2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// It would be nice to use the rustversion crate here instead,
+// but that doesn't work with inner attributes.
+fn main() {
+    if let Some(ver) = rustc_version() {
+        if ver.contains("nightly") {
+            println!("cargo:rustc-cfg=nightly")
+        }
+    }
+}
+
+fn rustc_version() -> Option<String> {
+    let rustc = std::env::var_os("RUSTC")?;
+    let output = std::process::Command::new(rustc)
+        .arg("--version")
+        .output()
+        .ok()?;
+    let version = String::from_utf8(output.stdout).ok()?;
+    Some(version)
+}