Add dropck test

Author: Zoxc

src/test/ui/generator/dropck.rs

@@ -0,0 +1,28 @@
+// 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.
+
+#![feature(generators, generator_trait, box_leak)]
+
+use std::cell::RefCell;
+use std::ops::Generator;
+
+fn main() {
+    let (cell, mut gen);
+    cell = Box::new(RefCell::new(0));
+    let ref_ = Box::leak(Box::new(Some(cell.borrow_mut())));
+    // the upvar is the non-dropck `&mut Option<Ref<'a, i32>>`.
+    gen = || {
+        // but the generator can use it to drop a `Ref<'a, i32>`.
+        let _d = ref_.take(); //~ ERROR `ref_` does not live long enough
+        yield;
+    };
+    gen.resume();
+    // drops the RefCell and then the Ref, leading to use-after-free
+}

src/test/ui/generator/dropck.stderr

@@ -0,0 +1,16 @@
+error[E0597]: `ref_` does not live long enough
+  --> $DIR/dropck.rs:23:18
+   |
+21 |     gen = || {
+   |           -- capture occurs here
+22 |         // but the generator can use it to drop a `Ref<'a, i32>`.
+23 |         let _d = ref_.take(); //~ ERROR `ref_` does not live long enough
+   |                  ^^^^ borrowed value does not live long enough
+...
+28 | }
+   | - borrowed value dropped before borrower
+   |
+   = note: values in a scope are dropped in the opposite order they are created
+
+error: aborting due to previous error
+