Auto merge of #136413 - EnzymeAD:fix-autodiff-comptime-regression, r=oli-obk

fix autodiff compile time regression

Tries to fix the regression from https://github.com/rust-lang/rust/pull/133429

Tracking:

- https://github.com/rust-lang/rust/issues/124509

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -403,7 +403,7 @@ fn generate_lto_work<B: ExtraBackendMethods>(
         assert!(needs_thin_lto.is_empty());
         let mut module =
             B::run_fat_lto(cgcx, needs_fat_lto, import_only_modules).unwrap_or_else(|e| e.raise());
-        if cgcx.lto == Lto::Fat {
+        if cgcx.lto == Lto::Fat && !autodiff.is_empty() {
             let config = cgcx.config(ModuleKind::Regular);
             module = unsafe { module.autodiff(cgcx, autodiff, config).unwrap() };
         }

compiler/rustc_monomorphize/src/partitioning.rs

@@ -1179,23 +1179,23 @@ fn collect_and_partition_mono_items(tcx: TyCtxt<'_>, (): ()) -> MonoItemPartitio
         }
     }
 
+    #[cfg(not(llvm_enzyme))]
+    let autodiff_mono_items: Vec<_> = vec![];
+    #[cfg(llvm_enzyme)]
+    let mut autodiff_mono_items: Vec<_> = vec![];
     let mono_items: DefIdSet = items
         .iter()
         .filter_map(|mono_item| match *mono_item {
-            MonoItem::Fn(ref instance) => Some(instance.def_id()),
+            MonoItem::Fn(ref instance) => {
+                #[cfg(llvm_enzyme)]
+                autodiff_mono_items.push((mono_item, instance));
+                Some(instance.def_id())
+            }
             MonoItem::Static(def_id) => Some(def_id),
             _ => None,
         })
         .collect();
 
-    let autodiff_mono_items: Vec<_> = items
-        .iter()
-        .filter_map(|item| match *item {
-            MonoItem::Fn(ref instance) => Some((item, instance)),
-            _ => None,
-        })
-        .collect();
-
     let autodiff_items =
         autodiff::find_autodiff_source_functions(tcx, &usage_map, autodiff_mono_items);
     let autodiff_items = tcx.arena.alloc_from_iter(autodiff_items);