rustdoc: Fix ICE with cross-crate default impls

This adds a special code path for impls which are listed as default impls to
ensure that they're loaded correctly.

src/test/run-make/rustdoc-default-impl/Makefile

@@ -0,0 +1,5 @@
+-include ../tools.mk
+
+all: foo.rs bar.rs
+	$(RUSTC) foo.rs --crate-type lib
+	$(HOST_RPATH_ENV) $(RUSTDOC) -w html -o $(TMPDIR)/doc bar.rs -L $(TMPDIR)

src/test/run-make/rustdoc-default-impl/bar.rs

@@ -0,0 +1,17 @@
+// 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.
+
+extern crate foo;
+
+pub use foo::bar;
+
+pub fn wut<T: bar::Bar>() {
+}
+

src/test/run-make/rustdoc-default-impl/foo.rs

@@ -0,0 +1,33 @@
+// 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 mod bar {
+    use std::marker;
+
+    pub trait Bar: marker::MarkerTrait + 'static {}
+
+    impl Bar for .. {}
+
+    pub trait Foo {
+        fn foo(&self) {}
+    }
+
+    impl Foo {
+        pub fn test<T: Bar>(&self) {}
+    }
+
+    pub struct TypeId;
+
+    impl TypeId {
+        pub fn of<T: Bar + ?Sized>() -> TypeId {
+            panic!()
+        }
+    }
+}