compile-fail tests.

One just checks that we are feature-gating the UGEH attribute (as
usual for attributes associated with unstable features).

The other is adapted from the RFC 1238 text, except that it has been
extended somewhat to actually *illustrate* the scenario that we are
trying to prevent, namely observing the state of data, from safe code,
after the destructor for that data has been executed.

src/test/compile-fail/feature-gate-dropck-ugeh.rs

@@ -0,0 +1,39 @@
+// 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.
+
+// Ensure that attempts to use the unsafe attribute are feature-gated.
+
+// Example adapted from RFC 1238 text (just left out the feature gate).
+
+// 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
+    //~^ ERROR unsafe_destructor_blind_to_params has unstable semantics
+    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]));
+}
+

src/test/compile-fail/issue28498-reject-ex1.rs

@@ -0,0 +1,74 @@
+// 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-will-start-to-be-rejected
+
+// Compare against test/run-pass/issue28498-must-work-ex2.rs
+
+use std::cell::Cell;
+
+#[derive(Copy, Clone, Debug)]
+enum Validity { Valid, Invalid }
+use self::Validity::{Valid, Invalid};
+
+struct Abstract<T> {
+    id: u32,
+    nbor: Cell<Option<T>>,
+    valid: Validity,
+    observe: fn(&Cell<Option<T>>) -> (u32, Validity),
+}
+
+#[derive(Copy, Clone)]
+struct Neighbor<'a>(&'a Abstract<Neighbor<'a>>);
+
+fn observe(c: &Cell<Option<Neighbor>>) -> (u32, Validity) {
+    let r = c.get().unwrap().0;
+    (r.id, r.valid)
+}
+
+impl<'a> Abstract<Neighbor<'a>> {
+    fn new(id: u32) -> Self {
+        Abstract {
+            id: id,
+            nbor: Cell::new(None),
+            valid: Valid,
+            observe: observe
+        }
+    }
+}
+
+struct Foo<T> {
+    data: Vec<T>,
+}
+
+impl<T> Drop for Abstract<T> {
+    fn drop(&mut self) {
+        let (nbor_id, nbor_valid) = (self.observe)(&self.nbor);
+        println!("dropping element {} ({:?}), observed neighbor {} ({:?})",
+                 self.id,
+                 self.valid,
+                 nbor_id,
+                 nbor_valid);
+        self.valid = Invalid;
+    }
+}
+
+fn main() {
+    let mut foo: Foo<Abstract<Neighbor>> = Foo {  data: Vec::new() };
+    foo.data.push(Abstract::new(0));
+    foo.data.push(Abstract::new(1));
+
+    foo.data[0].nbor.set(Some(Neighbor(&foo.data[1])));
+    //~^ ERROR `foo.data` does not live long enough
+    foo.data[1].nbor.set(Some(Neighbor(&foo.data[0])));
+    //~^ ERROR `foo.data` does not live long enough
+}