Avoid unused allocas for immediate return values

There's no need to allocate a return slot for anykind of immediate
return value, not just not for nils. Also, when the return value is
ignored, we only have to copy it to a temporary alloca if it's actually
required to call drop_ty on it.

Author: dotdash

src/librustc/middle/trans/callee.rs

@@ -672,15 +672,8 @@ pub fn trans_call_inner(in_cx: block,
             expr::Ignore => {
                 // drop the value if it is not being saved.
                 unsafe {
-                    if llvm::LLVMIsUndef(llretslot) != lib::llvm::True {
-                        if ty::type_is_nil(ret_ty) {
-                            // When implementing the for-loop sugar syntax, the
-                            // type of the for-loop is nil, but the function
-                            // it's invoking returns a bool. This is a special
-                            // case to ignore instead of invoking the Store
-                            // below into a scratch pointer of a mismatched
-                            // type.
-                        } else if ty::type_is_immediate(bcx.tcx(), ret_ty) {
+                    if ty::type_needs_drop(bcx.tcx(), ret_ty) {
+                        if ty::type_is_immediate(bcx.tcx(), ret_ty) {
                             let llscratchptr = alloc_ty(bcx, ret_ty);
                             Store(bcx, llresult, llscratchptr);
                             bcx = glue::drop_ty(bcx, llscratchptr, ret_ty);
@@ -734,7 +727,7 @@ pub fn trans_ret_slot(bcx: block, fn_ty: ty::t, dest: expr::Dest)
     match dest {
         expr::SaveIn(dst) => dst,
         expr::Ignore => {
-            if ty::type_is_nil(retty) {
+            if ty::type_is_immediate(bcx.tcx(), retty) {
                 unsafe {
                     llvm::LLVMGetUndef(Type::nil().ptr_to().to_ref())
                 }