run-pass tests for RFC 1238.

Illustrates cases that worked before and must continue to work, and a
case that shows how to use the `unsafe_destructor_blind_to_params`
attribute (aka "the UGEH attribute") to work around
cannot-assume-parametricity.

src/test/run-pass/issue28498-must-work-ex1.rs

+// Copyright 2015 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.
+
+// Example taken from RFC 1238 text
+
+// https://github.com/rust-lang/rfcs/blob/master/text/1238-nonparametric-dropck.md#examples-of-code-that-must-continue-to-work
+
+use std::cell::Cell;
+
+struct Concrete<'a>(u32, Cell<Option<&'a Concrete<'a>>>);
+
+fn main() {
+    let mut data = Vec::new();
+    data.push(Concrete(0, Cell::new(None)));
+    data.push(Concrete(0, Cell::new(None)));
+
+    data[0].1.set(Some(&data[1]));
+    data[1].1.set(Some(&data[0]));
+}

src/test/run-pass/issue28498-must-work-ex2.rs

+// Copyright 2015 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.
+
+// Example taken from RFC 1238 text
+
+// https://github.com/rust-lang/rfcs/blob/master/text/1238-nonparametric-dropck.md#examples-of-code-that-must-continue-to-work
+
+use std::cell::Cell;
+
+struct Concrete<'a>(u32, Cell<Option<&'a Concrete<'a>>>);
+
+struct Foo<T> { data: Vec<T> }
+
+fn main() {
+    let mut foo = Foo {  data: Vec::new() };
+    foo.data.push(Concrete(0, Cell::new(None)));
+    foo.data.push(Concrete(0, Cell::new(None)));
+
+    foo.data[0].1.set(Some(&foo.data[1]));
+    foo.data[1].1.set(Some(&foo.data[0]));
+}
+

src/test/run-pass/issue28498-ugeh-ex1.rs

+// Copyright 2015 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.
+
+// Example taken from RFC 1238 text
+
+// https://github.com/rust-lang/rfcs/blob/master/text/1238-nonparametric-dropck.md#example-of-the-unguarded-escape-hatch
+
+#![feature(dropck_parametricity)]
+use std::cell::Cell;
+
+struct Concrete<'a>(u32, Cell<Option<&'a Concrete<'a>>>);
+
+struct Foo<T> { data: Vec<T> }
+
+impl<T> Drop for Foo<T> {
+    #[unsafe_destructor_blind_to_params] // This is the UGEH attribute
+    fn drop(&mut self) { }
+}
+
+fn main() {
+    let mut foo = Foo {  data: Vec::new() };
+    foo.data.push(Concrete(0, Cell::new(None)));
+    foo.data.push(Concrete(0, Cell::new(None)));
+
+    foo.data[0].1.set(Some(&foo.data[1]));
+    foo.data[1].1.set(Some(&foo.data[0]));
+}
+