add tests for new intra-doc-link behavior

src/test/rustdoc/auxiliary/intra-link-extern-crate.rs

+// Copyright 2018 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.
+
+#![crate_name="inner"]
+
+//! ooh, i'm a rebel just for [kicks]

src/test/rustdoc/intra-link-extern-crate.rs

+// Copyright 2018 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.
+
+// aux-build:intra-link-extern-crate.rs
+
+// When loading `extern crate` statements, we would pull in their docs at the same time, even
+// though they would never actually get displayed. This tripped intra-doc-link resolution failures,
+// for items that aren't under our control, and not actually getting documented!
+
+#![deny(intra_doc_link_resolution_failure)]
+
+extern crate inner;

src/test/rustdoc/intra-link-private.rs

+// Copyright 2018 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.
+
+// Rustdoc would previously report resolution failures on items that weren't in the public docs.
+// These failures were legitimate, but not truly relevant - the docs in question couldn't be
+// checked for accuracy anyway.
+
+#![deny(intra_doc_link_resolution_failure)]
+
+/// ooh, i'm a [rebel] just for kicks
+struct SomeStruct;