hack: add a linker_flavor feature gate

to make tidy accept `-Z linker-flavor` documentation

src/doc/unstable-book/src/SUMMARY.md

@@ -106,7 +106,7 @@
 - [link_llvm_intrinsics](link-llvm-intrinsics.md)
 - [linkage](linkage.md)
 - [linked_list_extras](linked-list-extras.md)
-- [linker-flavor](linker-flavor.md)
+- [linker_flavor](linker-flavor.md)
 - [log_syntax](log-syntax.md)
 - [lookup_host](lookup-host.md)
 - [loop_break_value](loop-break-value.md)

src/libsyntax/feature_gate.rs

@@ -343,6 +343,9 @@ declare_features! (
 
     // Used to preserve symbols (see llvm.used)
     (active, used, "1.18.0", Some(40289)),
+
+    // Hack to document `-Z linker-flavor` in The Unstable Book
+    (active, linker_flavor, "1.18.0", Some(41142)),
 );
 
 declare_features! (

src/test/compile-fail/feature-gate-linker-flavor.rs

@@ -0,0 +1,13 @@
+// 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.
+
+#![linker_flavor] //~ ERROR the `#[linker_flavor]` attribute is
+
+fn main() {}