Rollup merge of rust-lang#41529 - bitshifter:issue-41479, r=eddyb

Add missing OperandPair struct field index adjustments. Fixes rust-lang#41479.

This is a bug fix for a regression in rust-lang@6d841da.

src/librustc_trans/mir/operand.rs

@@ -85,8 +85,15 @@ impl<'a, 'tcx> OperandRef<'tcx> {
         assert!(common::type_is_zero_size(ccx, ty));
         let llty = type_of::type_of(ccx, ty);
         let val = if common::type_is_imm_pair(ccx, ty) {
+            let layout = ccx.layout_of(ty);
+            let (ix0, ix1) = if let Layout::Univariant { ref variant, .. } = *layout {
+                (adt::struct_llfields_index(variant, 0),
+                adt::struct_llfields_index(variant, 1))
+            } else {
+                (0, 1)
+            };
             let fields = llty.field_types();
-            OperandValue::Pair(C_null(fields[0]), C_null(fields[1]))
+            OperandValue::Pair(C_null(fields[ix0]), C_null(fields[ix1]))
         } else {
             OperandValue::Immediate(C_null(llty))
         };
@@ -156,8 +163,16 @@ impl<'a, 'tcx> OperandRef<'tcx> {
             if common::type_is_imm_pair(bcx.ccx, self.ty) {
                 debug!("Operand::unpack_if_pair: unpacking {:?}", self);
 
-                let mut a = bcx.extract_value(llval, 0);
-                let mut b = bcx.extract_value(llval, 1);
+                let layout = bcx.ccx.layout_of(self.ty);
+                let (ix0, ix1) = if let Layout::Univariant { ref variant, .. } = *layout {
+                    (adt::struct_llfields_index(variant, 0),
+                    adt::struct_llfields_index(variant, 1))
+                } else {
+                    (0, 1)
+                };
+
+                let mut a = bcx.extract_value(llval, ix0);
+                let mut b = bcx.extract_value(llval, ix1);
 
                 let pair_fields = common::type_pair_fields(bcx.ccx, self.ty);
                 if let Some([a_ty, b_ty]) = pair_fields {

src/test/run-pass/issue-41479.rs

@@ -0,0 +1,18 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn split<A, B>(pair: (A, B)) {
+    let _a = pair.0;
+    let _b = pair.1;
+}
+
+fn main() {
+    split(((), ((), ())));
+}