get tests to work without -Z/-C flags

compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

@@ -688,6 +688,8 @@ struct LLVMRustSanitizerOptions {
   bool SanitizeKernelAddressRecover;
 };
 
+extern "C" void registerEnzyme(llvm::PassBuilder &PB);
+
 extern "C" LLVMRustResult LLVMRustOptimize(
     LLVMModuleRef ModuleRef, LLVMTargetMachineRef TMRef,
     LLVMRustPassBuilderOptLevel OptLevelRust, LLVMRustOptStage OptStage,
@@ -1010,6 +1012,14 @@ extern "C" LLVMRustResult LLVMRustOptimize(
     MPM.addPass(NameAnonGlobalPass());
   }
 
+  // now load "-enzyme" pass:
+  registerEnzyme(PB);
+  if (auto Err = PB.parsePassPipeline(MPM, "enzyme")) {
+    std::string ErrMsg = toString(std::move(Err));
+    LLVMRustSetLastError(ErrMsg.c_str());
+    return LLVMRustResult::Failure;
+  }
+
   // Upgrade all calls to old intrinsics first.
   for (Module::iterator I = TheModule->begin(), E = TheModule->end(); I != E;)
     UpgradeCallsToIntrinsic(&*I++); // must be post-increment, as we remove

src/bootstrap/src/core/build_steps/compile.rs

@@ -1052,9 +1052,9 @@ pub fn rustc_cargo(
     // We temporarily disable linking here as part of some refactoring.
     // This way, people can manually use -Z llvm-plugins and -C passes=enzyme for now.
     // In a follow-up PR, we will re-enable linking here and load the pass for them.
-    //if builder.config.llvm_enzyme {
-    //    cargo.rustflag("-l").rustflag("Enzyme-19");
-    //}
+    if builder.config.llvm_enzyme {
+        cargo.rustflag("-l").rustflag("Enzyme-19");
+    }
 
     // Building with protected visibility reduces the number of dynamic relocations needed, giving
     // us a faster startup time. However GNU ld < 2.40 will error if we try to link a shared object