From 909a480532d193e3d736d5e5925632adcf73456f Mon Sep 17 00:00:00 2001
From: Manuel Drehwald <git@manuel.drehwald.info>
Date: Mon, 17 Nov 2025 01:50:10 -0500
Subject: [PATCH] rlib handling

---
 compiler/rustc_codegen_llvm/src/intrinsic.rs    | 15 +++++++++++++--
 .../src/cross_crate_inline.rs                   |  4 ++++
 tests/ui/autodiff/autodiff_in_rlib.rs           | 17 +++++++++++++++++
 3 files changed, 34 insertions(+), 2 deletions(-)
 create mode 100644 tests/ui/autodiff/autodiff_in_rlib.rs

diff --git a/compiler/rustc_codegen_llvm/src/intrinsic.rs b/compiler/rustc_codegen_llvm/src/intrinsic.rs
index 84fc6ebbc3172..73d1aed564f49 100644
--- a/compiler/rustc_codegen_llvm/src/intrinsic.rs
+++ b/compiler/rustc_codegen_llvm/src/intrinsic.rs
@@ -21,6 +21,8 @@ use rustc_target::callconv::PassMode;
 use rustc_target::spec::Os;
 use tracing::debug;
 
+use rustc_session::config::CrateType;
+
 use crate::abi::FnAbiLlvmExt;
 use crate::builder::Builder;
 use crate::builder::autodiff::{adjust_activity_to_abi, generate_enzyme_call};
@@ -1136,8 +1138,17 @@ fn codegen_autodiff<'ll, 'tcx>(
     if !tcx.sess.opts.unstable_opts.autodiff.contains(&rustc_session::config::AutoDiff::Enable) {
         let _ = tcx.dcx().emit_almost_fatal(AutoDiffWithoutEnable);
     }
-    if tcx.sess.lto() != rustc_session::config::Lto::Fat {
-        let _ = tcx.dcx().emit_almost_fatal(AutoDiffWithoutLto);
+
+    let ct = tcx.crate_types();
+    let lto = tcx.sess.lto();
+    if ct.len() == 1 && ct.contains(&CrateType::Executable) {
+        if lto != rustc_session::config::Lto::Fat {
+            let _ = tcx.dcx().emit_almost_fatal(AutoDiffWithoutLto);
+        }
+    } else {
+        if lto != rustc_session::config::Lto::Fat && !tcx.sess.opts.cg.linker_plugin_lto.enabled(){
+            let _ = tcx.dcx().emit_almost_fatal(AutoDiffWithoutLto);
+        }
     }
 
     let fn_args = instance.args;
diff --git a/compiler/rustc_mir_transform/src/cross_crate_inline.rs b/compiler/rustc_mir_transform/src/cross_crate_inline.rs
index 7fc9fb9cca2d7..d15205dcf3cca 100644
--- a/compiler/rustc_mir_transform/src/cross_crate_inline.rs
+++ b/compiler/rustc_mir_transform/src/cross_crate_inline.rs
@@ -34,6 +34,10 @@ fn cross_crate_inlinable(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
         return true;
     }
 
+    if tcx.has_attr(def_id, sym::autodiff_forward) || tcx.has_attr(def_id, sym::autodiff_reverse) || tcx.has_attr(def_id, sym::rustc_autodiff) {
+        return true;
+    }
+
     if tcx.has_attr(def_id, sym::rustc_intrinsic) {
         // Intrinsic fallback bodies are always cross-crate inlineable.
         // To ensure that the MIR inliner doesn't cluelessly try to inline fallback
diff --git a/tests/ui/autodiff/autodiff_in_rlib.rs b/tests/ui/autodiff/autodiff_in_rlib.rs
new file mode 100644
index 0000000000000..1a96a6ff1a77c
--- /dev/null
+++ b/tests/ui/autodiff/autodiff_in_rlib.rs
@@ -0,0 +1,17 @@
+#![feature(autodiff)]
+#![crate_type = "rlib"]
+//@ needs-enzyme
+//@ compile-flags: -Zautodiff=Enable -C opt-level=3  -Clto=fat
+//@ build-fail
+
+// We test that we fail to compile if a user applied an autodiff_ macro in src/lib.rs,
+// since autodiff doesn't work in libraries yet. In the past we used to just return zeros in the
+// autodiffed functions, which is obviously confusing and wrong, so erroring is an improvement.
+
+use std::autodiff::autodiff_reverse;
+//~? ERROR: using the autodiff feature with library builds is not yet supported
+
+#[autodiff_reverse(d_square, Duplicated, Active)]
+pub fn square(x: &f64) -> f64 {
+    *x * *x
+}