add a test for variables used twice

Author: nikomatsakis

src/test/ui/nll/relate_tys/var-appears-twice.rs

@@ -0,0 +1,38 @@
+// 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.
+
+// Test that the NLL `relate_tys` code correctly deduces that a
+// function returning always its first argument can be upcast to one
+// that returns either first or second argument.
+
+#![feature(nll)]
+#![allow(warnings)]
+
+use std::cell::Cell;
+
+type DoubleCell<A> = Cell<(A, A)>;
+type DoublePair<A> = (A, A);
+
+fn make_cell<'b>(x: &'b u32) -> Cell<(&'static u32, &'b u32)> {
+    panic!()
+}
+
+fn main() {
+    let a: &'static u32 = &22;
+    let b = 44;
+
+    // Here we get an error because `DoubleCell<_>` requires the same type
+    // on both parts of the `Cell`, and we can't have that.
+    let x: DoubleCell<_> = make_cell(&b); //~ ERROR
+
+    // Here we do not get an error because `DoublePair<_>` permits
+    // variance on the lifetimes involved.
+    let y: DoublePair<_> = make_cell(&b).get();
+}

src/test/ui/nll/relate_tys/var-appears-twice.stderr

@@ -0,0 +1,14 @@
+error[E0597]: `b` does not live long enough
+  --> $DIR/var-appears-twice.rs:33:38
+   |
+LL |     let x: DoubleCell<_> = make_cell(&b); //~ ERROR
+   |                                      ^^ borrowed value does not live long enough
+...
+LL | }
+   | - `b` dropped here while still borrowed
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0597`.