Allow applying autodiff macros to trait functions.

It will use enzyme to generate a default derivative implementation,
which can be overwritten by the user.

compiler/rustc_attr_parsing/src/attributes/autodiff.rs

@@ -24,6 +24,7 @@ impl<S: Stage> SingleAttributeParser<S> for RustcAutodiffParser {
         Allow(Target::Fn),
         Allow(Target::Method(MethodKind::Inherent)),
         Allow(Target::Method(MethodKind::Trait { body: true })),
+        Allow(Target::Method(MethodKind::Trait { body: false })),
         Allow(Target::Method(MethodKind::TraitImpl)),
     ]);
     const TEMPLATE: AttributeTemplate = template!(

compiler/rustc_builtin_macros/src/autodiff.rs

@@ -224,16 +224,18 @@ pub(crate) fn expand_with_mode(
                 }
                 _ => None,
             },
-            Annotatable::AssocItem(assoc_item, Impl { of_trait: _ }) => match &assoc_item.kind {
-                ast::AssocItemKind::Fn(box ast::Fn { sig, ident, generics, .. }) => Some((
-                    assoc_item.vis.clone(),
-                    sig.clone(),
-                    ident.clone(),
-                    generics.clone(),
-                    true,
-                )),
-                _ => None,
-            },
+            Annotatable::AssocItem(assoc_item, _ctxt @ (Impl { of_trait: _ } | Trait)) => {
+                match &assoc_item.kind {
+                    ast::AssocItemKind::Fn(box ast::Fn { sig, ident, generics, .. }) => Some((
+                        assoc_item.vis.clone(),
+                        sig.clone(),
+                        ident.clone(),
+                        generics.clone(),
+                        true,
+                    )),
+                    _ => None,
+                }
+            }
             _ => None,
         }) else {
             dcx.emit_err(errors::AutoDiffInvalidApplication { span: item.span() });
@@ -393,14 +395,14 @@ fn same_attribute(attr: &ast::AttrKind, item: &ast::AttrKind) -> bool {
                 }
                 Annotatable::Item(iitem.clone())
             }
-            Annotatable::AssocItem(ref mut assoc_item, i @ Impl { .. }) => {
+            Annotatable::AssocItem(ref mut assoc_item, ctxt @ (Impl { .. } | Trait)) => {
                 if !assoc_item.attrs.iter().any(|a| same_attribute(&a.kind, &attr.kind)) {
                     assoc_item.attrs.push(attr);
                 }
                 if assoc_item.attrs.iter().any(|a| same_attribute(&a.kind, &inline_never.kind)) {
                     has_inline_never = true;
                 }
-                Annotatable::AssocItem(assoc_item.clone(), i)
+                Annotatable::AssocItem(assoc_item.clone(), ctxt)
             }
             Annotatable::Stmt(ref mut stmt) => {
                 match stmt.kind {
@@ -441,7 +443,7 @@ fn same_attribute(attr: &ast::AttrKind, item: &ast::AttrKind) -> bool {
         }
 
         let d_annotatable = match &item {
-            Annotatable::AssocItem(_, _) => {
+            Annotatable::AssocItem(_, ctxt) => {
                 let assoc_item: AssocItemKind = ast::AssocItemKind::Fn(d_fn);
                 let d_fn = Box::new(ast::AssocItem {
                     attrs: d_attrs,
@@ -451,7 +453,7 @@ fn same_attribute(attr: &ast::AttrKind, item: &ast::AttrKind) -> bool {
                     kind: assoc_item,
                     tokens: None,
                 });
-                Annotatable::AssocItem(d_fn, Impl { of_trait: false })
+                Annotatable::AssocItem(d_fn, *ctxt)
             }
             Annotatable::Item(_) => {
                 let mut d_fn = ecx.item(span, d_attrs, ItemKind::Fn(d_fn));

compiler/rustc_expand/src/base.rs

@@ -149,14 +149,14 @@ pub fn expect_item(self) -> Box<ast::Item> {
     pub fn expect_trait_item(self) -> Box<ast::AssocItem> {
         match self {
             Annotatable::AssocItem(i, AssocCtxt::Trait) => i,
-            _ => panic!("expected Item"),
+            _ => panic!("expected trait item"),
         }
     }
 
     pub fn expect_impl_item(self) -> Box<ast::AssocItem> {
         match self {
             Annotatable::AssocItem(i, AssocCtxt::Impl { .. }) => i,
-            _ => panic!("expected Item"),
+            _ => panic!("expected impl item"),
         }
     }
 

tests/pretty/autodiff/trait.pp

@@ -0,0 +1,43 @@
+//@ compile-flags: -Zautodiff=Enable -Zautodiff=NoPostopt -C opt-level=3 -Clto=fat
+//@ no-prefer-dynamic
+//@ needs-enzyme
+
+// Just check it does not crash for now
+// CHECK: ;
+#![feature(autodiff)]
+#![feature(core_intrinsics)]
+#![feature(rustc_attrs)]
+
+use std::autodiff::autodiff_reverse;
+
+struct Foo {
+    a: f64,
+}
+
+trait MyTrait {
+    #[rustc_autodiff]
+    fn f(&self, x: f64) -> f64;
+    #[rustc_autodiff(Reverse, 1, Const, Active, Active)]
+    fn df(&self, x: f64, seed: f64) -> (f64, f64) {
+        std::hint::black_box(seed);
+        std::hint::black_box(x);
+        ::std::intrinsics::autodiff(
+            Self::f as for<'a> fn(&'a Self, _: f64) -> f64,
+            Self::df,
+            (self, x, seed),
+        )
+
+    }
+}
+
+impl MyTrait for Foo {
+    fn f(&self, x: f64) -> f64 {
+        x.sin()
+    }
+}
+
+fn main() {
+    let foo = Foo { a: 3.0f64 };
+    dbg!(foo.df(2.0, 1.0));
+    dbg!(2.0_f64.cos());
+}

tests/pretty/autodiff/trait.rs

@@ -0,0 +1,32 @@
+//@ compile-flags: -Zautodiff=Enable -Zautodiff=NoPostopt -C opt-level=3 -Clto=fat
+//@ no-prefer-dynamic
+//@ needs-enzyme
+
+// Just check it does not crash for now
+// CHECK: ;
+#![feature(autodiff)]
+#![feature(core_intrinsics)]
+#![feature(rustc_attrs)]
+
+use std::autodiff::autodiff_reverse;
+
+struct Foo {
+    a: f64,
+}
+
+trait MyTrait {
+    #[autodiff_reverse(df, Const, Active, Active)]
+    fn f(&self, x: f64) -> f64;
+}
+
+impl MyTrait for Foo {
+    fn f(&self, x: f64) -> f64 {
+        x.sin()
+    }
+}
+
+fn main() {
+    let foo = Foo { a: 3.0f64 };
+    dbg!(foo.df(2.0, 1.0));
+    dbg!(2.0_f64.cos());
+}