diff --git a/core/src/clone.rs b/core/src/clone.rs
index 939b2be6dfaf1..76a89eaaff86e 100644
--- a/core/src/clone.rs
+++ b/core/src/clone.rs
@@ -160,6 +160,9 @@ pub trait Clone: Sized {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[must_use = "cloning is often expensive and is not expected to have side effects"]
+    // Clone::clone is special because the compiler generates MIR to implement it for some types.
+    // See InstanceKind::CloneShim.
+    #[cfg_attr(not(bootstrap), lang = "clone_fn")]
     fn clone(&self) -> Self;
 
     /// Performs copy-assignment from `source`.