Use the extern item-path for documentation links

The local item-path includes the local crates path to the extern crate
declaration which breaks cross-crate rustdoc links if the extern crate
is not linked into the crate root or renamed via `extern foo as bar`.

src/librustc/middle/cstore.rs

@@ -142,6 +142,7 @@ fn closure_ty(&self, tcx: &ty::ctxt<'tcx>, def_id: DefId)
     fn item_type(&self, tcx: &ty::ctxt<'tcx>, def: DefId)
                  -> ty::TypeScheme<'tcx>;
     fn item_path(&self, def: DefId) -> Vec<hir_map::PathElem>;
+    fn extern_item_path(&self, def: DefId) -> Vec<hir_map::PathElem>;
     fn item_name(&self, def: DefId) -> ast::Name;
     fn item_predicates(&self, tcx: &ty::ctxt<'tcx>, def: DefId)
                        -> ty::GenericPredicates<'tcx>;
@@ -295,6 +296,7 @@ fn repr_attrs(&self, def: DefId) -> Vec<attr::ReprAttr> { unimplemented!() }
     fn item_type(&self, tcx: &ty::ctxt<'tcx>, def: DefId)
                  -> ty::TypeScheme<'tcx> { unimplemented!() }
     fn item_path(&self, def: DefId) -> Vec<hir_map::PathElem> { unimplemented!() }
+    fn extern_item_path(&self, def: DefId) -> Vec<hir_map::PathElem> { unimplemented!() }
     fn item_name(&self, def: DefId) -> ast::Name { unimplemented!() }
     fn item_predicates(&self, tcx: &ty::ctxt<'tcx>, def: DefId)
                        -> ty::GenericPredicates<'tcx> { unimplemented!() }

src/librustc_metadata/csearch.rs

@@ -21,7 +21,7 @@
 use middle::ty::{self, Ty};
 use middle::def_id::{DefId, DefIndex};
 
-use rustc::front::map as ast_map;
+use rustc::front::map as hir_map;
 use rustc::util::nodemap::{FnvHashMap, NodeMap, NodeSet};
 
 use std::cell::RefCell;
@@ -29,6 +29,7 @@
 use std::path::PathBuf;
 use syntax::ast;
 use syntax::attr;
+use syntax::parse::token;
 use rustc_back::svh::Svh;
 use rustc_back::target::Target;
 use rustc_front::hir;
@@ -115,7 +116,7 @@ fn method_arg_names(&self, did: DefId) -> Vec<String>
         decoder::get_method_arg_names(&cdata, did.index)
     }
 
-    fn item_path(&self, def: DefId) -> Vec<ast_map::PathElem> {
+    fn item_path(&self, def: DefId) -> Vec<hir_map::PathElem> {
         let cdata = self.get_crate_data(def.krate);
         let path = decoder::get_item_path(&*cdata, def.index);
 
@@ -127,6 +128,17 @@ fn item_path(&self, def: DefId) -> Vec<ast_map::PathElem> {
         })
     }
 
+    fn extern_item_path(&self, def: DefId) -> Vec<hir_map::PathElem> {
+        let cdata = self.get_crate_data(def.krate);
+        let path = decoder::get_item_path(&*cdata, def.index);
+
+        let mut r = Vec::with_capacity(path.len() + 1);
+        let crate_name = hir_map::PathMod(token::intern(&cdata.name));
+        r.push(crate_name);
+        r.push_all(&path);
+        r
+    }
+
     fn item_name(&self, def: DefId) -> ast::Name {
         let cdata = self.get_crate_data(def.krate);
         decoder::get_item_name(&self.intr, &cdata, def.index)
@@ -350,7 +362,7 @@ fn reachable_ids(&self, cnum: ast::CrateNum) -> Vec<DefId>
         decoder::get_reachable_ids(&*cdata)
     }
 
-    fn def_path(&self, def: DefId) -> ast_map::DefPath
+    fn def_path(&self, def: DefId) -> hir_map::DefPath
     {
         let cdata = self.get_crate_data(def.krate);
         let path = decoder::def_path(&*cdata, def.index);

src/librustdoc/clean/inline.rs

@@ -138,7 +138,7 @@ pub fn load_attrs(cx: &DocContext, tcx: &ty::ctxt,
 pub fn record_extern_fqn(cx: &DocContext, did: DefId, kind: clean::TypeKind) {
     match cx.tcx_opt() {
         Some(tcx) => {
-            let fqn = tcx.sess.cstore.item_path(did);
+            let fqn = tcx.sess.cstore.extern_item_path(did);
             let fqn = fqn.into_iter().map(|i| i.to_string()).collect();
             cx.external_paths.borrow_mut().as_mut().unwrap().insert(did, (fqn, kind));
         }

src/test/auxiliary/issue-30109-1.rs

@@ -0,0 +1,11 @@
+// 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.
+
+pub struct Bar;

src/test/rustdoc/issue-30109.rs

@@ -0,0 +1,24 @@
+// 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.
+
+// build-aux-docs
+// aux-build:issue-30109-1.rs
+// ignore-cross-compile
+
+pub mod quux {
+    extern crate issue_30109_1 as bar;
+    use self::bar::Bar;
+
+    pub trait Foo {}
+
+    // @has issue_30109/quux/trait.Foo.html \
+    //          '//a/@href' '../issue_30109_1/struct.Bar.html'
+    impl Foo for Bar {}
+}