tests for lint that warns about mixing #[repr(C)] with Drop.

THis includes tests for struct and enum. (I suspect the closure case
is actually unreachable, but i see no harm in including it.)

Author: pnkfelix

src/test/compile-fail/lint-no-drop-on-repr-extern.rs

@@ -0,0 +1,59 @@
+// 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.
+
+// Check we reject structs that mix a `Drop` impl with `#[repr(C)]`.
+//
+// As a special case, also check that we do not warn on such structs
+// if they also are declared with `#[unsafe_no_drop_flag]`
+
+#![feature(unsafe_no_drop_flag)]
+#![deny(drop_with_repr_extern)]
+
+#[repr(C)] struct As { x: Box<i8> }
+#[repr(C)] enum Ae { Ae(Box<i8>), _None }
+
+struct Bs { x: Box<i8> }
+enum Be { Be(Box<i8>), _None }
+
+#[repr(C)] struct Cs { x: Box<i8> }
+//~^ NOTE the `#[repr(C)]` attribute is attached here
+
+impl Drop for Cs { fn drop(&mut self) { } }
+//~^ ERROR implementing Drop adds hidden state to types, possibly conflicting with `#[repr(C)]`
+
+#[repr(C)] enum Ce { Ce(Box<i8>), _None }
+//~^ NOTE the `#[repr(C)]` attribute is attached here
+
+impl Drop for Ce { fn drop(&mut self) { } }
+//~^ ERROR implementing Drop adds hidden state to types, possibly conflicting with `#[repr(C)]`
+
+#[unsafe_no_drop_flag]
+#[repr(C)] struct Ds { x: Box<i8> }
+
+impl Drop for Ds { fn drop(&mut self) { } }
+
+#[unsafe_no_drop_flag]
+#[repr(C)] enum De { De(Box<i8>), _None }
+
+impl Drop for De { fn drop(&mut self) { } }
+
+fn main() {
+    let a = As { x: Box::new(3) };
+    let b = Bs { x: Box::new(3) };
+    let c = Cs { x: Box::new(3) };
+    let d = Ds { x: Box::new(3) };
+
+    println!("{:?}", (*a.x, *b.x, *c.x, *d.x));
+
+    let _a = Ae::Ae(Box::new(3));
+    let _b = Be::Be(Box::new(3));
+    let _c = Ce::Ce(Box::new(3));
+    let _d = De::De(Box::new(3));
+}