Allow borrow conflicts for promoted length 0 arrays

Author: matthewjasper

src/librustc_mir/borrow_check/places_conflict.rs

@@ -329,6 +329,13 @@ fn place_element_conflict<'a, 'gcx: 'tcx, 'tcx>(
         }
         (Place::Promoted(p1), Place::Promoted(p2)) => {
             if p1.0 == p2.0 {
+                if let ty::TyArray(_, size) = p1.1.sty {
+                    if size.unwrap_usize(tcx) == 0 {
+                        // Ignore conflicts with promoted [T; 0].
+                        debug!("place_element_conflict: IGNORE-LEN-0-PROMOTED");
+                        return Overlap::Disjoint;
+                    }
+                }
                 // the same promoted - base case, equal
                 debug!("place_element_conflict: DISJOINT-OR-EQ-PROMOTED");
                 Overlap::EqualOrDisjoint

src/test/ui/nll/promotable-mutable-zst-doesnt-conflict.rs

@@ -0,0 +1,23 @@
+// 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.
+
+// Check that mutable promoted length zero arrays don't check for conflicting
+// access
+
+// run-pass
+
+#![feature(nll)]
+
+pub fn main() {
+    let mut x: Vec<&[i32; 0]> = Vec::new();
+    for i in 0..10 {
+        x.push(&[]);
+    }
+}