From 6e74905be29b861bcfd2780a8c81495deffec6c3 Mon Sep 17 00:00:00 2001
From: Haidong Zhang <zhanghaidong@uniontech.com>
Date: Wed, 3 Sep 2025 17:40:45 +0800
Subject: [PATCH] Set lto="fat" automatically  when compiling with
 RUSTFLAGS="-Zautodiff=Enable".

---
 compiler/rustc_codegen_ssa/messages.ftl  | 2 --
 compiler/rustc_codegen_ssa/src/errors.rs | 4 ----
 compiler/rustc_session/src/config.rs     | 5 +++++
 compiler/rustc_session/src/session.rs    | 7 +++++++
 4 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/compiler/rustc_codegen_ssa/messages.ftl b/compiler/rustc_codegen_ssa/messages.ftl
index 1dd65d38a2be7..91c3806df4c34 100644
--- a/compiler/rustc_codegen_ssa/messages.ftl
+++ b/compiler/rustc_codegen_ssa/messages.ftl
@@ -8,8 +8,6 @@ codegen_ssa_aix_strip_not_used = using host's `strip` binary to cross-compile to
 
 codegen_ssa_archive_build_failure = failed to build archive at `{$path}`: {$error}
 
-codegen_ssa_autodiff_without_lto = using the autodiff feature requires using fat-lto
-
 codegen_ssa_bare_instruction_set = `#[instruction_set]` requires an argument
 
 codegen_ssa_binary_output_to_tty = option `-o` or `--emit` is used to write binary output type `{$shorthand}` to stdout, but stdout is a tty
diff --git a/compiler/rustc_codegen_ssa/src/errors.rs b/compiler/rustc_codegen_ssa/src/errors.rs
index fb5a82051405d..d5c30c5c7a6b0 100644
--- a/compiler/rustc_codegen_ssa/src/errors.rs
+++ b/compiler/rustc_codegen_ssa/src/errors.rs
@@ -37,10 +37,6 @@ pub(crate) struct CguNotRecorded<'a> {
     pub cgu_name: &'a str,
 }
 
-#[derive(Diagnostic)]
-#[diag(codegen_ssa_autodiff_without_lto)]
-pub struct AutodiffWithoutLto;
-
 #[derive(Diagnostic)]
 #[diag(codegen_ssa_unknown_reuse_kind)]
 pub(crate) struct UnknownReuseKind {
diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index 297df7c2c9765..795cb2b2cfeba 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -1509,6 +1509,11 @@ 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 3525c7c1d1a99..d0dd2cdac0c48 100644
--- a/compiler/rustc_session/src/session.rs
+++ b/compiler/rustc_session/src/session.rs
@@ -600,6 +600,13 @@ 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.
+        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;