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

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

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 {

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 {

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;