rustc_trans: support ZST indexing involving uninhabited types.

src/librustc_trans/mir/operand.rs

@@ -267,9 +267,22 @@ impl<'a, 'tcx> MirContext<'a, 'tcx> {
 
         // Moves out of scalar and scalar pair fields are trivial.
         if let &mir::Place::Projection(ref proj) = place {
-            if let mir::ProjectionElem::Field(ref f, _) = proj.elem {
-                if let Some(o) = self.maybe_trans_consume_direct(bcx, &proj.base) {
-                    return Some(o.extract_field(bcx, f.index()));
+            if let Some(o) = self.maybe_trans_consume_direct(bcx, &proj.base) {
+                match proj.elem {
+                    mir::ProjectionElem::Field(ref f, _) => {
+                        return Some(o.extract_field(bcx, f.index()));
+                    }
+                    mir::ProjectionElem::Index(_) |
+                    mir::ProjectionElem::ConstantIndex { .. } => {
+                        // ZSTs don't require any actual memory access.
+                        // FIXME(eddyb) deduplicate this with the identical
+                        // checks in `trans_consume` and `extract_field`.
+                        let elem = o.layout.field(bcx.ccx, 0);
+                        if elem.is_zst() {
+                            return Some(OperandRef::new_zst(bcx.ccx, elem));
+                        }
+                    }
+                    _ => {}
                 }
             }
         }

src/test/run-pass/issue-46855.rs

@@ -0,0 +1,34 @@
+// 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.
+
+// compile-flags: -Zmir-opt-level=1
+
+#![feature(slice_patterns)]
+
+use std::mem;
+
+#[derive(Copy, Clone)]
+enum Never {}
+
+union Foo {
+    a: u64,
+    b: Never
+}
+
+fn foo(xs: [(Never, u32); 1]) -> u32 { xs[0].1 }
+
+fn bar([(_, x)]: [(Never, u32); 1]) -> u32 { x }
+
+fn main() {
+    println!("{}", mem::size_of::<Foo>());
+
+    let f = [Foo { a: 42 }, Foo { a: 10 }];
+    println!("{:?}", unsafe { f[0].a });
+}