Fix variable name in E0502 double borrow error

src/librustc_mir/borrow_check/error_reporting.rs

@@ -364,6 +364,9 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
         };
 
         if let Some((_, var_span)) = old_closure_span {
+            let place = &issued_borrow.borrowed_place;
+            let desc_place = self.describe_place(place).unwrap_or("_".to_owned());
+
             err.span_label(
                 var_span,
                 format!(

src/test/ui/nll/issue-51268.rs

@@ -0,0 +1,35 @@
+// 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.
+
+// ignore-tidy-linelength
+
+#![feature(nll)]
+
+struct Bar;
+
+impl Bar {
+    fn bar(&mut self, _: impl Fn()) {}
+}
+
+struct Foo {
+    thing: Bar,
+    number: usize,
+}
+
+impl Foo {
+    fn foo(&mut self) {
+        self.thing.bar(|| {
+        //~^ ERROR cannot borrow `self.thing` as mutable because it is also borrowed as immutable [E0502]
+            &self.number;
+        });
+    }
+}
+
+fn main() {}

src/test/ui/nll/issue-51268.stderr

@@ -0,0 +1,20 @@
+error[E0502]: cannot borrow `self.thing` as mutable because it is also borrowed as immutable
+  --> $DIR/issue-51268.rs:28:9
+   |
+LL |            self.thing.bar(|| {
+   |            ^              -- immutable borrow occurs here
+   |   _________|
+   |  |_________|
+   | ||
+LL | ||         //~^ ERROR cannot borrow `self.thing` as mutable because it is also borrowed as immutable [E0502]
+LL | ||             &self.number;
+   | ||              ---- previous borrow occurs due to use of `self` in closure
+LL | ||         });
+   | ||          ^
+   | ||__________|
+   | |___________mutable borrow occurs here
+   |             borrow later used here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0502`.