Added tests illustrating when and when not to use the UGEH attribute …

…w.r.t. a trait bound.

src/test/compile-fail/issue28498-reject-trait-bound.rs

@@ -0,0 +1,50 @@
+// 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.
+
+// Demonstrate that having a trait bound causes dropck to reject code
+// that might indirectly access previously dropped value.
+//
+// Compare with run-pass/issue28498-ugeh-with-trait-bound.rs
+
+use std::fmt;
+
+#[derive(Debug)]
+struct ScribbleOnDrop(String);
+
+impl Drop for ScribbleOnDrop {
+    fn drop(&mut self) {
+        self.0 = format!("DROPPED");
+    }
+}
+
+struct Foo<T:fmt::Debug>(u32, T);
+
+impl<T:fmt::Debug> Drop for Foo<T> {
+    fn drop(&mut self) {
+        // Use of `unsafe_destructor_blind_to_params` is unsound,
+        // because we access `T` fmt method when we pass `self.1`
+        // below, and thus potentially read from borrowed data.
+        println!("Dropping Foo({}, {:?})", self.0, self.1);
+    }
+}
+
+fn main() {
+    let (last_dropped, foo0);
+    let (foo1, first_dropped);
+
+    last_dropped = ScribbleOnDrop(format!("last"));
+    first_dropped = ScribbleOnDrop(format!("first"));
+    foo0 = Foo(0, &last_dropped);
+    //~^ ERROR `last_dropped` does not live long enough
+    foo1 = Foo(1, &first_dropped);
+    //~^ ERROR `first_dropped` does not live long enough
+
+    println!("foo0.1: {:?} foo1.1: {:?}", foo0.1, foo1.1);
+}

src/test/run-pass/issue28498-ugeh-with-trait-bound.rs

@@ -0,0 +1,51 @@
+// 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.
+
+// Demonstrate the use of the unguarded escape hatch with a trait bound
+// to assert that destructor will not access any dead data.
+//
+// Compare with compile-fail/issue28498-reject-trait-bound.rs
+
+#![feature(dropck_parametricity)]
+
+use std::fmt;
+
+#[derive(Debug)]
+struct ScribbleOnDrop(String);
+
+impl Drop for ScribbleOnDrop {
+    fn drop(&mut self) {
+        self.0 = format!("DROPPED");
+    }
+}
+
+struct Foo<T:fmt::Debug>(u32, T);
+
+impl<T:fmt::Debug> Drop for Foo<T> {
+    #[unsafe_destructor_blind_to_params]
+    fn drop(&mut self) {
+        // Use of `unsafe_destructor_blind_to_params` is sound,
+        // because destructor never accesses the `Debug::fmt` method
+        // of `T`, despite having it available.
+        println!("Dropping Foo({}, _)", self.0);
+    }
+}
+
+fn main() {
+    let (last_dropped, foo0);
+    let (foo1, first_dropped);
+
+    last_dropped = ScribbleOnDrop(format!("last"));
+    first_dropped = ScribbleOnDrop(format!("first"));
+    foo0 = Foo(0, &last_dropped);
+    foo1 = Foo(1, &first_dropped);
+
+    println!("foo0.1: {:?} foo1.1: {:?}", foo0.1, foo1.1);
+}