Auto merge of #47009 - eddyb:issue-46855, r=arielb1

rustc_trans: support ZST indexing involving uninhabited types.

Fixes #46855 in a minimal way. I decided against supporting non-memory `Rvalue::Len` in this PR (see https://github.com/rust-lang/rust/issues/46855#issuecomment-352965807), as `PlaceContext::Inspect` is also used for `Rvalue::Discriminant`.

r? @arielb1

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 });
+}