Fix bug in legacy #[derive] processing logic.

src/librustc_resolve/macros.rs

@@ -230,12 +230,12 @@ impl<'a> base::Resolver for Resolver<'a> {
                         attrs.remove(i);
                     } else {
                         let mut tokens = Vec::new();
-                        for (i, path) in traits.iter().enumerate() {
-                            if i > 0 {
+                        for (j, path) in traits.iter().enumerate() {
+                            if j > 0 {
                                 tokens.push(TokenTree::Token(attrs[i].span, Token::Comma).into());
                             }
-                            for (j, segment) in path.segments.iter().enumerate() {
-                                if j > 0 {
+                            for (k, segment) in path.segments.iter().enumerate() {
+                                if k > 0 {
                                     tokens.push(TokenTree::Token(path.span, Token::ModSep).into());
                                 }
                                 let tok = Token::Ident(segment.identifier);

src/test/run-pass-fulldeps/auxiliary/custom_derive_plugin.rs

@@ -34,8 +34,14 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_custom_derive(
         Symbol::intern("derive_TotalSum"),
         MultiDecorator(box expand));
+
+    reg.register_custom_derive(
+        Symbol::intern("derive_Nothing"),
+        MultiDecorator(box noop));
 }
 
+fn noop(_: &mut ExtCtxt, _: Span, _: &ast::MetaItem, _: &Annotatable, _: &mut FnMut(Annotatable)) {}
+
 fn expand(cx: &mut ExtCtxt,
           span: Span,
           mitem: &ast::MetaItem,

src/test/run-pass-fulldeps/issue-40663.rs

@@ -0,0 +1,19 @@
+// 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.
+
+// aux-build:custom_derive_plugin.rs
+
+#![feature(plugin, custom_derive)]
+#![plugin(custom_derive_plugin)]
+
+#[derive(Nothing, Nothing, Nothing)]
+struct S;
+
+fn main() {}