add feature gate doc_masked and tests

src/test/compile-fail/feature-gate-doc_masked.rs

@@ -0,0 +1,14 @@
+// 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.
+
+#[doc(masked)] //~ ERROR: #[doc(masked)] is experimental
+extern crate std as realstd;
+
+fn main() {}

src/test/rustdoc/doc-masked.rs

@@ -0,0 +1,29 @@
+// 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.
+
+#![feature(doc_masked)]
+
+#![doc(masked)]
+extern crate std as realstd;
+
+// @has doc_masked/struct.LocalStruct.html
+// @has - '//*[@class="impl"]//code' 'impl LocalTrait for LocalStruct'
+// @!has - '//*[@class="impl"]//code' 'impl Copy for LocalStruct'
+#[derive(Copy, Clone)]
+pub struct LocalStruct;
+
+// @has doc_masked/trait.LocalTrait.html
+// @has - '//*[@id="implementors-list"]//code' 'impl LocalTrait for LocalStruct'
+// @!has - '//*[@id="implementors-list"]//code' 'impl LocalTrait for usize'
+pub trait LocalTrait { }
+
+impl LocalTrait for LocalStruct { }
+
+impl LocalTrait for usize { }