Auto merge of #33002 - mitaa:rdoc-cross-impls, r=alexcrichton

rustdoc: refine cross-crate impl inlining

This changes the current rule that impls within `doc(hidden)` modules aren't inlined, to only inlining impls where the implemented trait and type are reachable in documentation.

fixes #14586
fixes #31948

.. and also applies the reachability checking to cross-crate links.

fixes #28480

r? @alexcrichton

src/librustdoc/html/format.rs

@@ -24,6 +24,7 @@ use syntax::abi::Abi;
 use rustc::hir;
 
 use clean;
+use core::DocAccessLevels;
 use html::item_type::ItemType;
 use html::render;
 use html::render::{cache, CURRENT_LOCATION_KEY};
@@ -298,6 +299,9 @@ pub fn href(did: DefId) -> Option<(String, ItemType, Vec<String>)> {
     let mut url = if did.is_local() || cache.inlined.contains(&did) {
         repeat("../").take(loc.len()).collect::<String>()
     } else {
+        if !cache.access_levels.is_doc_reachable(did) {
+            return None
+        }
         match cache.extern_locations[&did.krate] {
             (_, render::Remote(ref s)) => s.to_string(),
             (_, render::Local) => repeat("../").take(loc.len()).collect(),