Rollup merge of rust-lang#142640 - Sa4dUs:ad-intrinsic, r=ZuseZ4

Implement autodiff using intrinsics

This PR aims to move autodiff logic to `autodiff` intrinsic. Allowing us to delete a great part of our frontend code and overall, simplify the compilation pipeline of autodiff functions.

Author: Zalathar

core/src/intrinsics/mod.rs

@@ -3157,6 +3157,44 @@ pub const unsafe fn copysignf64(x: f64, y: f64) -> f64;
 #[rustc_intrinsic]
 pub const unsafe fn copysignf128(x: f128, y: f128) -> f128;
 
+/// Generates the LLVM body for the automatic differentiation of `f` using Enzyme,
+/// with `df` as the derivative function and `args` as its arguments.
+///
+/// Used internally as the body of `df` when expanding the `#[autodiff_forward]`
+/// and `#[autodiff_reverse]` attribute macros.
+///
+/// Type Parameters:
+/// - `F`: The original function to differentiate. Must be a function item.
+/// - `G`: The derivative function. Must be a function item.
+/// - `T`: A tuple of arguments passed to `df`.
+/// - `R`: The return type of the derivative function.
+///
+/// This shows where the `autodiff` intrinsic is used during macro expansion:
+///
+/// ```rust,ignore (macro example)
+/// #[autodiff_forward(df1, Dual, Const, Dual)]
+/// pub fn f1(x: &[f64], y: f64) -> f64 {
+///     unimplemented!()
+/// }
+/// ```
+///
+/// expands to:
+///
+/// ```rust,ignore (macro example)
+/// #[rustc_autodiff]
+/// #[inline(never)]
+/// pub fn f1(x: &[f64], y: f64) -> f64 {
+///     ::core::panicking::panic("not implemented")
+/// }
+/// #[rustc_autodiff(Forward, 1, Dual, Const, Dual)]
+/// pub fn df1(x: &[f64], bx_0: &[f64], y: f64) -> (f64, f64) {
+///     ::core::intrinsics::autodiff(f1::<>, df1::<>, (x, bx_0, y))
+/// }
+/// ```
+#[rustc_nounwind]
+#[rustc_intrinsic]
+pub const fn autodiff<F, G, T: crate::marker::Tuple, R>(f: F, df: G, args: T) -> R;
+
 /// Inform Miri that a given pointer definitely has a certain alignment.
 #[cfg(miri)]
 #[rustc_allow_const_fn_unstable(const_eval_select)]

core/src/macros/mod.rs

@@ -1495,6 +1495,7 @@ pub(crate) mod builtin {
     ///   (or explicitly returns `-> ()`). Otherwise, it must be set to one of the allowed activities.
     #[unstable(feature = "autodiff", issue = "124509")]
     #[allow_internal_unstable(rustc_attrs)]
+    #[allow_internal_unstable(core_intrinsics)]
     #[rustc_builtin_macro]
     pub macro autodiff_forward($item:item) {
         /* compiler built-in */
@@ -1513,6 +1514,7 @@ pub(crate) mod builtin {
     ///   (or explicitly returns `-> ()`). Otherwise, it must be set to one of the allowed activities.
     #[unstable(feature = "autodiff", issue = "124509")]
     #[allow_internal_unstable(rustc_attrs)]
+    #[allow_internal_unstable(core_intrinsics)]
     #[rustc_builtin_macro]
     pub macro autodiff_reverse($item:item) {
         /* compiler built-in */