rustc_mir: also promote interior borrows, not just whole temps.

src/test/run-pass/issue-49955.rs

@@ -0,0 +1,30 @@
+// 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.
+
+// compile-flags: -Z borrowck=compare
+
+const ALL_THE_NUMS: [u32; 1] = [
+    1
+];
+
+#[inline(never)]
+fn array(i: usize) -> &'static u32 {
+    return &ALL_THE_NUMS[i];
+}
+
+#[inline(never)]
+fn tuple_field() -> &'static u32 {
+    &(42,).0
+}
+
+fn main() {
+    assert_eq!(tuple_field().to_string(), "42");
+    // assert_eq!(array(0).to_string(), "1");
+}