rustdoc: Fix a couple of issues with src links to external crates

- src links/redirects to extern fn from another crate had an extra '/'.
- src links to `pub use` of a crate module had an extra '/'.
- src links to renamed reexports from another crate used the new name
  for the link but should use the original name.

src/librustdoc/clean/inline.rs

@@ -143,8 +143,14 @@ pub fn load_attrs<'a, 'tcx>(cx: &DocContext, tcx: TyCtxt<'a, 'tcx, 'tcx>,
 pub fn record_extern_fqn(cx: &DocContext, did: DefId, kind: clean::TypeKind) {
     if let Some(tcx) = cx.tcx_opt() {
         let crate_name = tcx.sess.cstore.crate_name(did.krate).to_string();
-        let relative = tcx.def_path(did).data.into_iter().map(|elem| {
-            elem.data.to_string()
+        let relative = tcx.def_path(did).data.into_iter().filter_map(|elem| {
+            // extern blocks have an empty name
+            let s = elem.data.to_string();
+            if !s.is_empty() {
+                Some(s)
+            } else {
+                None
+            }
         });
         let fqn = once(crate_name).chain(relative).collect();
         cx.renderinfo.borrow_mut().external_paths.insert(did, (fqn, kind));

src/librustdoc/html/render.rs

@@ -1519,20 +1519,23 @@ impl<'a> Item<'a> {
         // located, then we return `None`.
         } else {
             let cache = cache();
-            let path = match cache.external_paths.get(&self.item.def_id) {
+            let external_path = match cache.external_paths.get(&self.item.def_id) {
                 Some(path) => path,
                 None => return None,
             };
-            let root = match cache.extern_locations.get(&self.item.def_id.krate) {
+            let mut path = match cache.extern_locations.get(&self.item.def_id.krate) {
                 Some(&(_, Remote(ref s))) => s.to_string(),
                 Some(&(_, Local)) => self.cx.root_path.clone(),
                 Some(&(_, Unknown)) => return None,
                 None => return None,
             };
-            Some(format!("{root}{path}/{file}?gotosrc={goto}",
-                         root = root,
-                         path = path[..path.len() - 1].join("/"),
-                         file = item_path(shortty(self.item), self.item.name.as_ref().unwrap()),
+            for item in &external_path[..external_path.len() - 1] {
+                path.push_str(item);
+                path.push_str("/");
+            }
+            Some(format!("{path}{file}?gotosrc={goto}",
+                         path = path,
+                         file = item_path(shortty(self.item), external_path.last().unwrap()),
                          goto = self.item.def_id.index.as_usize()))
         }
     }

src/test/rustdoc/auxiliary/issue-34274.rs

@@ -0,0 +1,13 @@
+// Copyright 2016 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.
+
+extern {
+    pub fn extern_c_fn();
+}

src/test/rustdoc/auxiliary/src-links-external.rs

@@ -0,0 +1,11 @@
+// Copyright 2016 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 Foo;

src/test/rustdoc/issue-34274.rs

@@ -0,0 +1,20 @@
+// Copyright 2016 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:issue-34274.rs
+// build-aux-docs
+// ignore-cross-compile
+
+#![crate_name = "foo"]
+
+extern crate issue_34274;
+
+// @has foo/fn.extern_c_fn.html '//a/@href' '../issue_34274/fn.extern_c_fn.html?gotosrc='
+pub use issue_34274::extern_c_fn;

src/test/rustdoc/src-links-external.rs

@@ -0,0 +1,22 @@
+// Copyright 2016 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:src-links-external.rs
+// build-aux-docs
+// ignore-cross-compile
+
+#![crate_name = "foo"]
+
+extern crate src_links_external;
+
+// @has foo/bar/index.html '//a/@href' '../src_links_external/index.html?gotosrc='
+pub use src_links_external as bar;
+
+// @has foo/bar/struct.Foo.html '//a/@href' '../src_links_external/struct.Foo.html?gotosrc='