diff --git a/library/core/build.rs b/library/core/build.rs
new file mode 100644
index 0000000000000..676f5979af625
--- /dev/null
+++ b/library/core/build.rs
@@ -0,0 +1,13 @@
+fn main() {
+    if !std::env::var("RUSTC_BOOTSTRAP").is_ok() {
+        eprintln!("error: you are attempting to build libcore without going through bootstrap");
+        eprintln!("help: use `x.py build --stage 0 library/std`, not `cargo build`");
+        eprintln!("help: use `x.py check`, not `cargo check`");
+        eprintln!(
+            "note: if you're sure you want to do this, use `RUSTC_BOOTSTRAP=0` or some other dummy value"
+        );
+        panic!();
+    }
+    println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo:rerun-if-env-changed=RUSTC_BOOTSTRAP");
+}