Rollup merge of #41061 - arielb1:parent-lock, r=eddyb

cstore: return an immutable borrow from `visible_parent_map`

This prevents an ICE when `visible_parent_map` is called multiple times, for example when an item referenced in an impl signature is imported from an  `extern crate` statement occurs within an impl.

Fixes #41053.

r? @eddyb

diff --combined src/librustc/middle/cstore.rs
index ee0635ac9a179b7e02ea6fea51eb69703bc697b7,dd1941ba48af8f6140c39defa9d684280fac83f0..694321812836b2b943cebea312515ab54118ec5a
--- 1/src/librustc/middle/cstore.rs
--- 2/src/librustc/middle/cstore.rs
+++ b/src/librustc/middle/cstore.rs
@@@ -172,7 -172,7 +172,7 @@@ pub trait CrateStore 
      fn stability(&self, def: DefId) -> Option<attr::Stability>;
      fn deprecation(&self, def: DefId) -> Option<attr::Deprecation>;
      fn visibility(&self, def: DefId) -> ty::Visibility;
-     fn visible_parent_map<'a>(&'a self) -> ::std::cell::RefMut<'a, DefIdMap<DefId>>;
+     fn visible_parent_map<'a>(&'a self) -> ::std::cell::Ref<'a, DefIdMap<DefId>>;
      fn item_generics_cloned(&self, def: DefId) -> ty::Generics;
      fn item_attrs(&self, def_id: DefId) -> Vec<ast::Attribute>;
      fn fn_arg_names(&self, did: DefId) -> Vec<ast::Name>;
@@@ -230,7 -230,6 +230,7 @@@
                      -> Option<DefId>;
      fn def_key(&self, def: DefId) -> DefKey;
      fn def_path(&self, def: DefId) -> hir_map::DefPath;
 +    fn def_path_hash(&self, def: DefId) -> u64;
      fn struct_field_names(&self, def: DefId) -> Vec<ast::Name>;
      fn item_children(&self, did: DefId) -> Vec<def::Export>;
      fn load_macro(&self, did: DefId, sess: &Session) -> LoadedMacro;
@@@ -303,7 -302,7 +303,7 @@@ impl CrateStore for DummyCrateStore 
      fn stability(&self, def: DefId) -> Option<attr::Stability> { bug!("stability") }
      fn deprecation(&self, def: DefId) -> Option<attr::Deprecation> { bug!("deprecation") }
      fn visibility(&self, def: DefId) -> ty::Visibility { bug!("visibility") }
-     fn visible_parent_map<'a>(&'a self) -> ::std::cell::RefMut<'a, DefIdMap<DefId>> {
+     fn visible_parent_map<'a>(&'a self) -> ::std::cell::Ref<'a, DefIdMap<DefId>> {
          bug!("visible_parent_map")
      }
      fn item_generics_cloned(&self, def: DefId) -> ty::Generics
@@@ -378,9 -377,6 +378,9 @@@
      fn def_path(&self, def: DefId) -> hir_map::DefPath {
          bug!("relative_def_path")
      }
 +    fn def_path_hash(&self, def: DefId) -> u64 {
 +        bug!("wa")
 +    }
      fn struct_field_names(&self, def: DefId) -> Vec<ast::Name> { bug!("struct_field_names") }
      fn item_children(&self, did: DefId) -> Vec<def::Export> { bug!("item_children") }
      fn load_macro(&self, did: DefId, sess: &Session) -> LoadedMacro { bug!("load_macro") }

diff --combined src/librustc_metadata/cstore_impl.rs
index efcd2f007d66c5944466dcf6dabe541dc47be8eb,b286cb8050ea05d4f62dcab7442d27331eba1b02..37984e4c3718fd96231832ea22bae1f8f47e409d
--- 1/src/librustc_metadata/cstore_impl.rs
--- 2/src/librustc_metadata/cstore_impl.rs
+++ b/src/librustc_metadata/cstore_impl.rs
@@@ -73,7 -73,7 +73,7 @@@ provide! { <'tcx> tcx, def_id, cdat
      predicates => { cdata.get_predicates(def_id.index, tcx) }
      super_predicates => { cdata.get_super_predicates(def_id.index, tcx) }
      trait_def => {
 -        tcx.alloc_trait_def(cdata.get_trait_def(def_id.index, tcx))
 +        tcx.alloc_trait_def(cdata.get_trait_def(def_id.index))
      }
      adt_def => { cdata.get_adt_def(def_id.index, tcx) }
      adt_destructor => {
@@@ -370,10 -370,6 +370,10 @@@ impl CrateStore for cstore::CStore 
          self.get_crate_data(def.krate).def_path(def.index)
      }
  
 +    fn def_path_hash(&self, def: DefId) -> u64 {
 +        self.get_crate_data(def.krate).def_path_hash(def.index)
 +    }
 +
      fn struct_field_names(&self, def: DefId) -> Vec<ast::Name>
      {
          self.dep_graph.read(DepNode::MetaData(def));
@@@ -511,12 -507,19 +511,19 @@@
      /// Returns a map from a sufficiently visible external item (i.e. an external item that is
      /// visible from at least one local module) to a sufficiently visible parent (considering
      /// modules that re-export the external item to be parents).
-     fn visible_parent_map<'a>(&'a self) -> ::std::cell::RefMut<'a, DefIdMap<DefId>> {
-         let mut visible_parent_map = self.visible_parent_map.borrow_mut();
-         if !visible_parent_map.is_empty() { return visible_parent_map; }
+     fn visible_parent_map<'a>(&'a self) -> ::std::cell::Ref<'a, DefIdMap<DefId>> {
+         {
+             let visible_parent_map = self.visible_parent_map.borrow();
+             if !visible_parent_map.is_empty() {
+                 return visible_parent_map;
+             }
+         }
  
          use std::collections::vec_deque::VecDeque;
          use std::collections::hash_map::Entry;
+ 
+         let mut visible_parent_map = self.visible_parent_map.borrow_mut();
+ 
          for cnum in (1 .. self.next_crate_num().as_usize()).map(CrateNum::new) {
              let cdata = self.get_crate_data(cnum);
  
@@@ -560,6 -563,7 +567,7 @@@
              }
          }
  
-         visible_parent_map
+         drop(visible_parent_map);
+         self.visible_parent_map.borrow()
      }
  }