Rollup merge of #146229 - Hayden602:issue-142796-fix, r=ZuseZ4

Automatically switch to lto-fat when flag RUSTFLAGS="- Zautodiff=Enable" is set

…t" is automatically set.

closes: [#142796](https://github.com/rust-lang/rust/issues/142796)

4 files changed, 12 insertions, 6 deletions

diff --git a/compiler/rustc_codegen_ssa/messages.ftl b/compiler/rustc_codegen_ssa/messages.ftl
index 1dd65d38a2b..91c3806df4c 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 fb5a8205140..d5c30c5c7a6 100644
--- a/compiler/rustc_codegen_ssa/src/errors.rs
+++ b/compiler/rustc_codegen_ssa/src/errors.rs
@@ -38,10 +38,6 @@ pub(crate) struct CguNotRecorded<'a> {
 }
 
 #[derive(Diagnostic)]
-#[diag(codegen_ssa_autodiff_without_lto)]
-pub struct AutodiffWithoutLto;
-
-#[derive(Diagnostic)]
 #[diag(codegen_ssa_unknown_reuse_kind)]
 pub(crate) struct UnknownReuseKind {
     #[primary_span]
diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index 297df7c2c97..795cb2b2cfe 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 3525c7c1d1a..d0dd2cdac0c 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;