[MIR] Avoid some code generation for stores of ZST

Fixes #30831

src/librustc_trans/trans/mir/operand.rs

@@ -169,6 +169,11 @@ impl<'bcx, 'tcx> MirContext<'bcx, 'tcx> {
                          operand: OperandRef<'tcx>)
     {
         debug!("store_operand: operand={}", operand.repr(bcx));
+        // Avoid generating stores of zero-sized values, because the only way to have a zero-sized
+        // value is through `undef`, and store itself is useless.
+        if common::type_is_zero_size(bcx.ccx(), operand.ty) {
+            return;
+        }
         match operand.val {
             OperandValue::Ref(r) => base::memcpy_ty(bcx, lldest, r, operand.ty),
             OperandValue::Immediate(s) => base::store_ty(bcx, s, lldest, operand.ty),

src/librustc_trans/trans/mir/rvalue.rs

@@ -108,11 +108,15 @@ impl<'bcx, 'tcx> MirContext<'bcx, 'tcx> {
                     },
                     _ => {
                         for (i, operand) in operands.iter().enumerate() {
+                            let op = self.trans_operand(bcx, operand);
+                            // Do not generate stores and GEPis for zero-sized fields.
+                            if !common::type_is_zero_size(bcx.ccx(), op.ty) {
                                 // Note: perhaps this should be StructGep, but
                                 // note that in some cases the values here will
                                 // not be structs but arrays.
-                            let lldest_i = build::GEPi(bcx, dest.llval, &[0, i]);
-                            self.trans_operand_into(bcx, lldest_i, operand);
+                                let dest = build::GEPi(bcx, dest.llval, &[0, i]);
+                                self.store_operand(bcx, dest, op);
+                            }
                         }
                     }
                 }

src/test/codegen/mir_zst_stores.rs

@@ -0,0 +1,27 @@
+// Copyright 2016 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.
+
+// compile-flags: -C no-prepopulate-passes
+
+#![feature(rustc_attrs)]
+#![crate_type = "lib"]
+use std::marker::PhantomData;
+
+
+struct Zst { phantom: PhantomData<Zst> }
+
+// CHECK-LABEL: @mir
+#[no_mangle]
+#[rustc_mir]
+fn mir(){
+    // CHECK-NOT: getelementptr
+    // CHECK-NOT: store{{.*}}undef
+    let x = Zst { phantom: PhantomData };
+}