Raise an error if people use autodiff without -Zautodiff=Enable

Author: ZuseZ4

compiler/rustc_codegen_llvm/messages.ftl

@@ -1,4 +1,5 @@
 codegen_llvm_autodiff_without_lto = using the autodiff feature requires using fat-lto
+codegen_llvm_autodiff_without_enable = using the autodiff feature requires -Z autodiff=Enable
 
 codegen_llvm_copy_bitcode = failed to copy bitcode to object file: {$err}
 

compiler/rustc_codegen_llvm/src/builder/autodiff.rs

@@ -10,7 +10,7 @@ use crate::back::write::llvm_err;
 use crate::builder::SBuilder;
 use crate::context::SimpleCx;
 use crate::declare::declare_simple_fn;
-use crate::errors::LlvmError;
+use crate::errors::{AutoDiffWithoutEnable, LlvmError};
 use crate::llvm::AttributePlace::Function;
 use crate::llvm::{Metadata, True};
 use crate::value::Value;
@@ -288,6 +288,14 @@ pub(crate) fn differentiate<'ll>(
     let diag_handler = cgcx.create_dcx();
     let cx = SimpleCx { llmod: module.module_llvm.llmod(), llcx: module.module_llvm.llcx };
 
+    // First of all, did the user try to use autodiff without using the -Zautodiff=Enable flag?
+    if !diff_items.is_empty()
+        && !cgcx.opts.unstable_opts.autodiff.contains(&rustc_session::config::AutoDiff::Enable)
+    {
+        let dcx = cgcx.create_dcx();
+        return Err(dcx.handle().emit_almost_fatal(AutoDiffWithoutEnable));
+    }
+
     // Before dumping the module, we want all the TypeTrees to become part of the module.
     for item in diff_items.iter() {
         let name = item.source.clone();

compiler/rustc_codegen_llvm/src/errors.rs

@@ -92,9 +92,12 @@ impl<G: EmissionGuarantee> Diagnostic<'_, G> for ParseTargetMachineConfig<'_> {
 
 #[derive(Diagnostic)]
 #[diag(codegen_llvm_autodiff_without_lto)]
-#[note]
 pub(crate) struct AutoDiffWithoutLTO;
 
+#[derive(Diagnostic)]
+#[diag(codegen_llvm_autodiff_without_enable)]
+pub(crate) struct AutoDiffWithoutEnable;
+
 #[derive(Diagnostic)]
 #[diag(codegen_llvm_lto_disallowed)]
 pub(crate) struct LtoDisallowed;

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -405,7 +405,7 @@ fn generate_lto_work<B: ExtraBackendMethods>(
             B::run_fat_lto(cgcx, needs_fat_lto, import_only_modules).unwrap_or_else(|e| e.raise());
         if cgcx.lto == Lto::Fat && !autodiff.is_empty() {
             let config = cgcx.config(ModuleKind::Regular);
-            module = unsafe { module.autodiff(cgcx, autodiff, config).unwrap() };
+            module = unsafe { module.autodiff(cgcx, autodiff, config).unwrap_or_else(|e| e.raise()) };
         }
         // We are adding a single work item, so the cost doesn't matter.
         vec![(WorkItem::LTO(module), 0)]