From 7389d18deb94cfcb7a019d008a0a21e6539c272a Mon Sep 17 00:00:00 2001
From: Haidong Zhang <zhanghaidong@uniontech.com>
Date: Wed, 3 Sep 2025 17:40:45 +0800
Subject: [PATCH] When compiling with the flag RUSTFLAGS="-Zautodiff=Enable",
 lto = "fat" is automatically set.

---
 compiler/rustc_session/src/config.rs  | 5 +++++
 compiler/rustc_session/src/session.rs | 5 +++++
 2 files changed, 10 insertions(+)

diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index 9793d8091e2ab..18f221dd9a238 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -1507,6 +1507,11 @@ impl Options {
     pub fn get_symbol_mangling_version(&self) -> SymbolManglingVersion {
         self.cg.symbol_mangling_version.unwrap_or(SymbolManglingVersion::Legacy)
     }
+
+    #[inline]
+    pub fn autodiff_enabled(&self) -> bool {
+        self.unstable_opts.autodiff.contains(&AutoDiff::Enable)
+    }
 }
 
 impl UnstableOptions {
diff --git a/compiler/rustc_session/src/session.rs b/compiler/rustc_session/src/session.rs
index 96767d054398c..790482ba2b063 100644
--- a/compiler/rustc_session/src/session.rs
+++ b/compiler/rustc_session/src/session.rs
@@ -658,6 +658,11 @@ impl Session {
 
     /// Calculates the flavor of LTO to use for this compilation.
     pub fn lto(&self) -> config::Lto {
+        // if autodiff is enabled, use fat lto
+        if self.opts.autodiff_enabled() {
+            return config::Lto::Fat;
+        }
+
         // If our target has codegen requirements ignore the command line
         if self.target.requires_lto {
             return config::Lto::Fat;