fix fwd test case

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -860,9 +860,10 @@ unsafe fn create_call<'a>(tgt: &'a Value, src: &'a Value, rev_mode: bool,
     LLVMRustEraseInstBefore(bb, last_inst);
 
     let f_return_type = LLVMGetReturnType(LLVMGlobalGetValueType(src));
+    let f_is_struct = llvm::LLVMRustIsStructType(f_return_type);
     let void_type = LLVMVoidTypeInContext(llcx);
     // Now unwrap the struct_ret if it's actually a struct
-    if f_return_type != void_type {
+    if f_is_struct {
         let num_elem_in_ret_struct = LLVMCountStructElementTypes(f_return_type);
         if num_elem_in_ret_struct == 1 {
             let inner_grad_name = "foo".to_string();

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1035,6 +1035,7 @@ extern "C" {
     pub fn LLVMRustEraseInstFromParent(V: &Value);
     pub fn LLVMRustGetTerminator<'a>(B: &BasicBlock) -> &'a Value;
     pub fn LLVMGetReturnType(T: &Type) -> &Type;
+    pub fn LLVMRustIsStructType(T: &Type) -> bool;
     pub fn LLVMDumpModule(M: &Module);
     pub fn LLVMCountStructElementTypes(T: &Type) -> c_uint;
     pub fn LLVMDeleteFunction(V: &Value);

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -300,6 +300,10 @@ extern "C" void LLVMRustAddFunctionAttributes(LLVMValueRef Fn, unsigned Index,
   AddAttributes(F, Index, Attrs, AttrsLen);
 }
 
+extern "C" bool LLVMRustIsStructType(LLVMTypeRef Ty) {
+  return unwrap(Ty)->isStructTy();
+}
+
 extern "C" void LLVMRustAddCallSiteAttributes(LLVMValueRef Instr,
                                               unsigned Index,
                                               LLVMAttributeRef *Attrs,