Single commit implementing the enzyme/autodiff frontend

Co-authored-by: Lorenz Schmidt <bytesnake@mailbox.org>

library/std/src/lib.rs

@@ -267,6 +267,7 @@
 #![allow(unused_features)]
 //
 // Features:
+#![cfg_attr(not(bootstrap), feature(autodiff))]
 #![cfg_attr(test, feature(internal_output_capture, print_internals, update_panic_count, rt))]
 #![cfg_attr(
     all(target_vendor = "fortanix", target_env = "sgx"),
@@ -627,7 +628,13 @@ pub mod simd {
     #[doc(inline)]
     pub use crate::std_float::StdFloat;
 }
-
+#[cfg(not(bootstrap))]
+#[unstable(feature = "autodiff", issue = "124509")]
+/// This module provides support for automatic differentiation.
+pub mod autodiff {
+    /// This macro handles automatic differentiation.
+    pub use core::autodiff::autodiff;
+}
 #[stable(feature = "futures_api", since = "1.36.0")]
 pub mod task {
     //! Types and Traits for working with asynchronous tasks.