Require that types cannot implement both Drop and Copy.

Opt-in built-in traits allowed one to explicitly implement both `Drop`
and `Copy` for a type. This can theoretically make some sense, but the
current implementation means it is codegened totally incorrectly which
can lead to memory unsafety, so this feature is disabled for now.

Fixes rust-lang#20126.

src/librustc/middle/ty.rs

@@ -6891,6 +6891,7 @@ pub enum CopyImplementationError {
     FieldDoesNotImplementCopy(ast::Name),
     VariantDoesNotImplementCopy(ast::Name),
     TypeIsStructural,
+    TypeHasDestructor,
 }
 
 pub fn can_type_implement_copy<'a,'tcx>(param_env: &ParameterEnvironment<'a, 'tcx>,
@@ -6900,14 +6901,15 @@ pub fn can_type_implement_copy<'a,'tcx>(param_env: &ParameterEnvironment<'a, 'tc
 {
     let tcx = param_env.tcx;
 
-    match self_type.sty {
+    let did = match self_type.sty {
         ty::ty_struct(struct_did, substs) => {
             let fields = ty::struct_fields(tcx, struct_did, substs);
             for field in fields.iter() {
                 if type_moves_by_default(param_env, span, field.mt.ty) {
                     return Err(FieldDoesNotImplementCopy(field.name))
                 }
             }
+            struct_did
         }
         ty::ty_enum(enum_did, substs) => {
             let enum_variants = ty::enum_variants(tcx, enum_did);
@@ -6920,8 +6922,13 @@ pub fn can_type_implement_copy<'a,'tcx>(param_env: &ParameterEnvironment<'a, 'tc
                     }
                 }
             }
+            enum_did
         }
         _ => return Err(TypeIsStructural),
+    };
+
+    if ty::has_dtor(tcx, did) {
+        return Err(TypeHasDestructor)
     }
 
     Ok(())

src/librustc_typeck/coherence/mod.rs

@@ -507,6 +507,11 @@ impl<'a, 'tcx> CoherenceChecker<'a, 'tcx> {
                                   for this type; type is not a structure or \
                                   enumeration")
                 }
+                Err(ty::TypeHasDestructor) => {
+                    span_err!(tcx.sess, span, E0184,
+                              "the trait `Copy` may not be implemented for this type; \
+                               the type has a destructor");
+                }
             }
         }
     }

src/librustc_typeck/diagnostics.rs

@@ -157,5 +157,6 @@ register_diagnostics! {
     E0180,
     E0181,
     E0182,
-    E0183
+    E0183,
+    E0184
 }

src/test/compile-fail/drop-on-non-struct.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-impl Drop for int {
+impl<'a> Drop for &'a mut int {
     //~^ ERROR the Drop trait may only be implemented on structures
     //~^^ ERROR cannot provide an extension implementation
     fn drop(&mut self) {

src/test/compile-fail/exclusive-drop-and-copy.rs

@@ -0,0 +1,30 @@
+// 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.
+
+#![feature(unsafe_destructor)]
+
+// issue #20126
+
+#[derive(Copy)] //~ ERROR the trait `Copy` may not be implemented
+struct Foo;
+
+impl Drop for Foo {
+    fn drop(&mut self) {}
+}
+
+#[derive(Copy)] //~ ERROR the trait `Copy` may not be implemented
+struct Bar<T>;
+
+#[unsafe_destructor]
+impl<T> Drop for Bar<T> {
+    fn drop(&mut self) {}
+}
+
+fn main() {}