Auto merge of #38268 - withoutboats:parse_where_higher_rank_hack, r=eddyb

Prevent where < ident > from parsing.

In order to be forward compatible with `where<'a>` syntax for higher
rank parameters, prevent potential conflicts with UFCS from parsing
correctly for the near term.

src/test/parse-fail/where_with_bound.rs

@@ -0,0 +1,16 @@
+// Copyright 2016 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.
+
+// compile-flags: -Z parse-only
+
+fn foo<T>() where <T>::Item: ToString, T: Iterator { }
+               //~^ syntax `where<T>` is reserved for future use
+
+fn main() {}