From 75c79bdb0204271e09220c4c59239bb85772ee93 Mon Sep 17 00:00:00 2001
From: Ryan Cumming <etaoins@gmail.com>
Date: Sat, 27 Jan 2018 12:14:12 +1100
Subject: [PATCH] Fix ICE on const eval of union field

MIR's `Const::get_field()` attempts to retrieve the value for a given
field in a constant. In the case of a union constant it was falling
through to a generic `const_get_elt` based on the field index. As union
fields don't have an index this caused an ICE in `llvm_field_index`.

Fix by simply returning the current value when accessing any field in a
union. This works because all union fields start at byte offset 0.

The added test uses `const_fn` it ensure the field is extracted using
MIR's const evaluation. The crash is reproducible without it, however.

Fixes #47788
---
 src/librustc_trans/mir/constant.rs            |  5 +++-
 .../run-pass/union/union-const-eval-field.rs  | 26 +++++++++++++++++++
 2 files changed, 30 insertions(+), 1 deletion(-)
 create mode 100644 src/test/run-pass/union/union-const-eval-field.rs

diff --git a/src/librustc_trans/mir/constant.rs b/src/librustc_trans/mir/constant.rs
index 71ce0aa3da96..f519e27dce98 100644
--- a/src/librustc_trans/mir/constant.rs
+++ b/src/librustc_trans/mir/constant.rs
@@ -140,7 +140,10 @@ impl<'a, 'tcx> Const<'tcx> {
                 }
             }
             _ => {
-                const_get_elt(self.llval, layout.llvm_field_index(i))
+                match layout.fields {
+                    layout::FieldPlacement::Union(_) => self.llval,
+                    _ => const_get_elt(self.llval, layout.llvm_field_index(i)),
+                }
             }
         }
     }
diff --git a/src/test/run-pass/union/union-const-eval-field.rs b/src/test/run-pass/union/union-const-eval-field.rs
new file mode 100644
index 000000000000..a58b8bc34cd0
--- /dev/null
+++ b/src/test/run-pass/union/union-const-eval-field.rs
@@ -0,0 +1,26 @@
+// Copyright 2018 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.
+
+#![feature(const_fn)]
+
+union DummyUnion {
+    field1: i32,
+    field2: i32,
+}
+
+const fn read_field() -> i32 {
+    const UNION: DummyUnion = DummyUnion { field1: 5 };
+    const FIELD: i32 = unsafe { UNION.field2 };
+    FIELD
+}
+
+fn main() {
+    assert_eq!(read_field(), 5);
+}