Single commit implementing the enzyme/autodiff frontend

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

compiler/rustc_builtin_macros/src/lib.rs

@@ -5,6 +5,7 @@
 #![allow(internal_features)]
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
+#![cfg_attr(not(bootstrap), feature(autodiff))]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(assert_matches)]
@@ -29,6 +30,7 @@ use crate::deriving::*;
 
 mod alloc_error_handler;
 mod assert;
+mod autodiff;
 mod cfg;
 mod cfg_accessible;
 mod cfg_eval;
@@ -106,6 +108,7 @@ pub fn register_builtin_macros(resolver: &mut dyn ResolverExpand) {
 
     register_attr! {
         alloc_error_handler: alloc_error_handler::expand,
+        autodiff: autodiff::expand,
         bench: test::expand_bench,
         cfg_accessible: cfg_accessible::Expander,
         cfg_eval: cfg_eval::expand,