RFC 558: Require parentheses for chained comparisons

Fixes #20724.

src/test/compile-fail/require-parens-for-chained-comparison.rs

@@ -0,0 +1,23 @@
+// 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.
+
+fn f<T>() {}
+
+fn main() {
+    false == false == false;
+    //~^ ERROR: Chained comparison operators require parentheses
+
+    false == 0 < 2;
+    //~^ ERROR: Chained comparison operators require parentheses
+
+    f<X>();
+    //~^ ERROR: Chained comparison operators require parentheses
+    //~^^ HELP: Use ::< instead of < if you meant to specify type arguments.
+}

src/test/compile-fail/unsized2.rs

@@ -13,5 +13,8 @@
 fn f<X>() {}
 
 pub fn main() {
-    f<type>(); //~ ERROR expected identifier, found keyword `type`
+    f<type>();
+    //~^ ERROR expected identifier, found keyword `type`
+    //~^^ ERROR: Chained comparison operators require parentheses
+    //~^^^ HELP: Use ::< instead of < if you meant to specify type arguments.
 }