Add a bunch of new tests per Alex's suggestion.

src/test/auxiliary/trait-safety-lib.rs

@@ -0,0 +1,19 @@
+// Copyright 2014 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.
+
+// Simple smoke test that unsafe traits can be compiled etc.
+
+pub unsafe trait Foo {
+    fn foo(&self) -> int;
+}
+
+unsafe impl Foo for int {
+    fn foo(&self) -> int { *self }
+}

src/test/compile-fail/trait-safety-trait-impl-cc.rs

@@ -0,0 +1,25 @@
+// Copyright 2014 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.
+
+// aux-build:trait-safety-lib.rs
+
+// Check that unsafe traits require unsafe impls and that inherent
+// impls cannot be unsafe.
+
+extern crate "trait-safety-lib" as lib;
+
+struct Bar;
+impl lib::Foo for Bar { //~ ERROR requires an `unsafe impl` declaration
+    fn foo(&self) -> int {
+        *self as int
+    }
+}
+
+fn main() { }

src/test/run-pass/trait-safety-ok-cc.rs

@@ -0,0 +1,32 @@
+// Copyright 2014 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.
+
+// aux-build:trait-safety-lib.rs
+
+// Simple smoke test that unsafe traits can be compiled across crates.
+
+extern crate "trait-safety-lib" as lib;
+
+use lib::Foo;
+
+struct Bar { x: int }
+unsafe impl Foo for Bar {
+    fn foo(&self) -> int { self.x }
+}
+
+fn take_foo<F:Foo>(f: &F) -> int { f.foo() }
+
+fn main() {
+    let x: int = 22;
+    assert_eq!(22, take_foo(&x));
+
+    let x: Bar = Bar { x: 23 };
+    assert_eq!(23, take_foo(&x));
+}

src/test/run-pass/trait-safety-ok.rs

@@ -0,0 +1,26 @@
+// Copyright 2014 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.
+
+// Simple smoke test that unsafe traits can be compiled etc.
+
+unsafe trait Foo {
+    fn foo(&self) -> int;
+}
+
+unsafe impl Foo for int {
+    fn foo(&self) -> int { *self }
+}
+
+fn take_foo<F:Foo>(f: &F) -> int { f.foo() }
+
+fn main() {
+    let x: int = 22;
+    assert_eq!(22, take_foo(&x));
+}