Fix bug in collecting trait and impl items with derives.

Author: jseyfried

src/libsyntax/ext/expand.rs

@@ -736,12 +736,7 @@ impl<'a, 'b> InvocationCollector<'a, 'b> {
                     item: Annotatable,
                     kind: ExpansionKind)
                     -> Expansion {
-        if !traits.is_empty() &&
-           (kind == ExpansionKind::TraitItems || kind == ExpansionKind::ImplItems) {
-            self.cx.span_err(traits[0].span, "`derive` can be only be applied to items");
-            return kind.expect_from_annotatables(::std::iter::once(item));
-        }
-        self.collect(kind, InvocationKind::Attr { attr: attr, traits: traits, item: item })
+        self.collect(kind, InvocationKind::Attr { attr, traits, item })
     }
 
     // If `item` is an attr invocation, remove and return the macro attribute.

src/test/compile-fail/issue-43023.rs

@@ -0,0 +1,28 @@
+// Copyright 2017 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.
+
+struct S;
+
+impl S {
+    #[derive(Debug)] //~ ERROR `derive` may only be applied to structs, enums and unions
+    fn f() {
+        file!();
+    }
+}
+
+trait Tr1 {
+    #[derive(Debug)] //~ ERROR `derive` may only be applied to structs, enums and unions
+    fn f();
+}
+
+trait Tr2 {
+    #[derive(Debug)] //~ ERROR `derive` may only be applied to structs, enums and unions
+    type F;
+}