Auto merge of #32439 - jseyfried:visible_suggestions, r=nrc

diagnostics: make paths to external items more visible

This PR changes the reported path for an external item so that it is visible from at least one local module (i.e. it does not use any inaccessible external modules) if possible. If the external item's crate was declared with an `extern crate`, the path is guarenteed to use the `extern crate`.

Fixes #23224, fixes #23355, fixes #26635, fixes #27165.

r? @nrc

Author: bors

src/librustc/middle/cstore.rs

@@ -32,7 +32,7 @@ use mir::repr::Mir;
 use mir::mir_map::MirMap;
 use session::Session;
 use session::search_paths::PathKind;
-use util::nodemap::{FnvHashMap, NodeMap, NodeSet};
+use util::nodemap::{FnvHashMap, NodeMap, NodeSet, DefIdMap};
 use std::any::Any;
 use std::cell::RefCell;
 use std::rc::Rc;
@@ -169,6 +169,7 @@ pub trait CrateStore<'tcx> : Any {
     fn item_type(&self, tcx: &TyCtxt<'tcx>, def: DefId)
                  -> ty::TypeScheme<'tcx>;
     fn relative_item_path(&self, def: DefId) -> Vec<hir_map::PathElem>;
+    fn visible_parent_map<'a>(&'a self) -> ::std::cell::RefMut<'a, DefIdMap<DefId>>;
     fn extern_item_path(&self, def: DefId) -> Vec<hir_map::PathElem>;
     fn item_name(&self, def: DefId) -> ast::Name;
     fn item_predicates(&self, tcx: &TyCtxt<'tcx>, def: DefId)
@@ -347,6 +348,9 @@ impl<'tcx> CrateStore<'tcx> for DummyCrateStore {
     fn item_type(&self, tcx: &TyCtxt<'tcx>, def: DefId)
                  -> ty::TypeScheme<'tcx> { unimplemented!() }
     fn relative_item_path(&self, def: DefId) -> Vec<hir_map::PathElem> { unimplemented!() }
+    fn visible_parent_map<'a>(&'a self) -> ::std::cell::RefMut<'a, DefIdMap<DefId>> {
+        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: &TyCtxt<'tcx>, def: DefId)

src/librustc/ty/item_path.rs

@@ -10,7 +10,7 @@
 
 use front::map::DefPathData;
 use middle::cstore::LOCAL_CRATE;
-use middle::def_id::DefId;
+use middle::def_id::{DefId, CRATE_DEF_INDEX};
 use ty::{self, Ty, TyCtxt};
 use syntax::ast;
 
@@ -75,9 +75,51 @@ impl<'tcx> TyCtxt<'tcx> {
         }
     }
 
+    /// If possible, this pushes a global path resolving to `external_def_id` that is visible
+    /// from at least one local module and returns true. If the crate defining `external_def_id` is
+    /// declared with an `extern crate`, the path is guarenteed to use the `extern crate`.
+    pub fn try_push_visible_item_path<T>(&self, buffer: &mut T, external_def_id: DefId) -> bool
+        where T: ItemPathBuffer
+    {
+        let visible_parent_map = self.sess.cstore.visible_parent_map();
+
+        let (mut cur_def, mut cur_path) = (external_def_id, Vec::<ast::Name>::new());
+        loop {
+            // If `cur_def` is a direct or injected extern crate, push the path to the crate
+            // followed by the path to the item within the crate and return.
+            if cur_def.index == CRATE_DEF_INDEX {
+                match self.sess.cstore.extern_crate(cur_def.krate) {
+                    Some(extern_crate) if extern_crate.direct => {
+                        self.push_item_path(buffer, extern_crate.def_id);
+                        cur_path.iter().rev().map(|segment| buffer.push(&segment.as_str())).count();
+                        return true;
+                    }
+                    None => {
+                        buffer.push(&self.crate_name(cur_def.krate));
+                        cur_path.iter().rev().map(|segment| buffer.push(&segment.as_str())).count();
+                        return true;
+                    }
+                    _ => {},
+                }
+            }
+
+            cur_path.push(self.sess.cstore.item_name(cur_def));
+            match visible_parent_map.get(&cur_def) {
+                Some(&def) => cur_def = def,
+                None => return false,
+            };
+        }
+    }
+
     pub fn push_item_path<T>(&self, buffer: &mut T, def_id: DefId)
         where T: ItemPathBuffer
     {
+        match *buffer.root_mode() {
+            RootMode::Local if !def_id.is_local() =>
+                if self.try_push_visible_item_path(buffer, def_id) { return },
+            _ => {}
+        }
+
         let key = self.def_key(def_id);
         match key.disambiguated_data.data {
             DefPathData::CrateRoot => {