Rollup merge of rust-lang#52834 - matthewjasper:allow-zst-conflicts, r=pnkfelix

[NLL] Allow conflicting borrows of promoted length zero arrays

This is currently overkill as there's no way to create two conflicting borrows of any promoted.
It is possible that the following code might not fail due to const eval in the future (@oli-obk?). In which case either the array marked needs to not be promoted, or to be checked for conflicts

```rust
static mut A: () = {
    let mut y = None;
    let z;
    let mut done_y = false;
    loop {
        let x = &mut [1];  // < this array
        if done_y {
            z = x;
            break;
        }
        y = Some(x);
        done_y = true;
    }
    some_const_fn(y, z); // some_const_fn expects that y to not alias z.
};
```

r? @pnkfelix  @nikomatsakis

closes rust-lang#52671
cc rust-lang#51823

Author: pietroalbini

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(&[]);
+    }
+}