From 63f4b36406016ad62cfe23580ed90ef1eabd4bfb Mon Sep 17 00:00:00 2001
From: Manuel Drehwald <git@manuel.drehwald.info>
Date: Tue, 11 Nov 2025 22:36:50 -0500
Subject: [PATCH] provide an error if an autodiff user does not set  in their
 Cargo.toml

---
 compiler/rustc_codegen_llvm/messages.ftl     | 1 +
 compiler/rustc_codegen_llvm/src/errors.rs    | 4 ++++
 compiler/rustc_codegen_llvm/src/intrinsic.rs | 5 ++++-
 compiler/rustc_session/src/session.rs        | 8 --------
 4 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/compiler/rustc_codegen_llvm/messages.ftl b/compiler/rustc_codegen_llvm/messages.ftl
index ce9a51b539d8..c9d28160d66f 100644
--- a/compiler/rustc_codegen_llvm/messages.ftl
+++ b/compiler/rustc_codegen_llvm/messages.ftl
@@ -1,4 +1,5 @@
 codegen_llvm_autodiff_without_enable = using the autodiff feature requires -Z autodiff=Enable
+codegen_llvm_autodiff_without_lto = using the autodiff feature requires setting `lto="fat"` in your Cargo.toml
 
 codegen_llvm_copy_bitcode = failed to copy bitcode to object file: {$err}
 
diff --git a/compiler/rustc_codegen_llvm/src/errors.rs b/compiler/rustc_codegen_llvm/src/errors.rs
index 627b0c9ff3b3..629afee8a667 100644
--- a/compiler/rustc_codegen_llvm/src/errors.rs
+++ b/compiler/rustc_codegen_llvm/src/errors.rs
@@ -32,6 +32,10 @@ impl<G: EmissionGuarantee> Diagnostic<'_, G> for ParseTargetMachineConfig<'_> {
     }
 }
 
+#[derive(Diagnostic)]
+#[diag(codegen_llvm_autodiff_without_lto)]
+pub(crate) struct AutoDiffWithoutLto;
+
 #[derive(Diagnostic)]
 #[diag(codegen_llvm_autodiff_without_enable)]
 pub(crate) struct AutoDiffWithoutEnable;
diff --git a/compiler/rustc_codegen_llvm/src/intrinsic.rs b/compiler/rustc_codegen_llvm/src/intrinsic.rs
index 0626cb3f2f16..bbe3b5b59f5b 100644
--- a/compiler/rustc_codegen_llvm/src/intrinsic.rs
+++ b/compiler/rustc_codegen_llvm/src/intrinsic.rs
@@ -24,7 +24,7 @@ use crate::abi::FnAbiLlvmExt;
 use crate::builder::Builder;
 use crate::builder::autodiff::{adjust_activity_to_abi, generate_enzyme_call};
 use crate::context::CodegenCx;
-use crate::errors::AutoDiffWithoutEnable;
+use crate::errors::{AutoDiffWithoutEnable, AutoDiffWithoutLto};
 use crate::llvm::{self, Metadata, Type, Value};
 use crate::type_of::LayoutLlvmExt;
 use crate::va_arg::emit_va_arg;
@@ -1145,6 +1145,9 @@ 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 fn_args = instance.args;
     let callee_ty = instance.ty(tcx, bx.typing_env());
diff --git a/compiler/rustc_session/src/session.rs b/compiler/rustc_session/src/session.rs
index 9fb3c35d5ef7..fe13bb821c9b 100644
--- a/compiler/rustc_session/src/session.rs
+++ b/compiler/rustc_session/src/session.rs
@@ -594,14 +594,6 @@ impl Session {
 
     /// Calculates the flavor of LTO to use for this compilation.
     pub fn lto(&self) -> config::Lto {
-        // Autodiff currently requires fat-lto to have access to the llvm-ir of all (indirectly) used functions and types.
-        // fat-lto is the easiest solution to this requirement, but quite expensive.
-        // FIXME(autodiff): Make autodiff also work with embed-bc instead of fat-lto.
-        // Don't apply fat-lto to proc-macro crates as they cannot use fat-lto without -Zdylib-lto
-        if self.opts.autodiff_enabled() && !self.opts.crate_types.contains(&CrateType::ProcMacro) {
-            return config::Lto::Fat;
-        }
-
         // If our target has codegen requirements ignore the command line
         if self.target.requires_lto {
             return config::Lto::Fat;