Single commit implementing the enzyme/autodiff frontend

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

compiler/rustc_passes/src/check_attr.rs

@@ -243,6 +243,9 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                     self.check_generic_attr(hir_id, attr, target, Target::Fn);
                     self.check_proc_macro(hir_id, target, ProcMacroKind::Derive)
                 }
+                [sym::autodiff, ..] => {
+                    self.check_autodiff(hir_id, attr, span, target)
+                }
                 [sym::coroutine, ..] => {
                     self.check_coroutine(attr, target);
                 }
@@ -2345,6 +2348,18 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
             self.dcx().emit_err(errors::RustcPubTransparent { span, attr_span });
         }
     }
+
+    /// Checks if `#[autodiff]` is applied to an item other than a function item.
+    fn check_autodiff(&self, _hir_id: HirId, _attr: &Attribute, span: Span, target: Target) {
+        debug!("check_autodiff");
+        match target {
+            Target::Fn => {}
+            _ => {
+                self.dcx().emit_err(errors::AutoDiffAttr { attr_span: span });
+                self.abort.set(true);
+            }
+        }
+    }
 }
 
 impl<'tcx> Visitor<'tcx> for CheckAttrVisitor<'tcx> {

compiler/rustc_passes/src/errors.rs

@@ -20,6 +20,14 @@ use crate::lang_items::Duplicate;
 #[diag(passes_incorrect_do_not_recommend_location)]
 pub(crate) struct IncorrectDoNotRecommendLocation;
 
+#[derive(Diagnostic)]
+#[diag(passes_autodiff_attr)]
+pub(crate) struct AutoDiffAttr {
+    #[primary_span]
+    #[label]
+    pub attr_span: Span,
+}
+
 #[derive(LintDiagnostic)]
 #[diag(passes_outer_crate_level_attr)]
 pub(crate) struct OuterCrateLevelAttr;