Re-add support for `impl Trait for ..` to the parser

src/librustc_passes/ast_validation.rs

@@ -215,8 +215,13 @@ fn visit_lifetime(&mut self, lifetime: &'a Lifetime) {
 
     fn visit_item(&mut self, item: &'a Item) {
         match item.node {
-            ItemKind::Impl(.., Some(..), _, ref impl_items) => {
+            ItemKind::Impl(.., Some(..), ref ty, ref impl_items) => {
                 self.invalid_visibility(&item.vis, item.span, None);
+                if ty.node == TyKind::Err {
+                    self.err_handler()
+                        .struct_span_err(item.span, "`impl Trait for .. {}` is an obsolete syntax")
+                        .help("use `auto trait Trait {}` instead").emit();
+                }
                 for impl_item in impl_items {
                     self.invalid_visibility(&impl_item.vis, impl_item.span, None);
                     if let ImplItemKind::Method(ref sig, _) = impl_item.node {

src/libsyntax/parse/parser.rs

@@ -5422,7 +5422,11 @@ fn parse_item_impl(&mut self,
         };
 
         if opt_trait.is_some() {
-            ty = self.parse_ty()?;
+            ty = if self.eat(&token::DotDot) {
+                P(Ty { node: TyKind::Err, span: self.prev_span, id: ast::DUMMY_NODE_ID })
+            } else {
+                self.parse_ty()?
+            }
         }
         generics.where_clause = self.parse_where_clause()?;
 

src/test/ui/obsolete-syntax-impl-for-dotdot.rs

+// 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.
+
+trait Trait1 {}
+trait Trait2 {}
+
+#[cfg(not_enabled)]
+impl Trait1 for .. {}
+
+impl Trait2 for .. {} //~ ERROR `impl Trait for .. {}` is an obsolete syntax
+
+fn main() {}

src/test/ui/obsolete-syntax-impl-for-dotdot.stderr

+error: `impl Trait for .. {}` is an obsolete syntax
+  --> $DIR/obsolete-syntax-impl-for-dotdot.rs:17:1
+   |
+17 | impl Trait2 for .. {} //~ ERROR `impl Trait for .. {}` is an obsolete syntax
+   | ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: use `auto trait Trait {}` instead
+
+error: aborting due to previous error
+