add a cache to impl_polarity

this is another one of these things that looks *much* worse on valgrind.

src/librustc/middle/cstore.rs

@@ -195,7 +195,6 @@ pub trait CrateStore {
     fn implementations_of_trait(&self, filter: Option<DefId>) -> Vec<DefId>;
 
     // impl info
-    fn impl_polarity(&self, def: DefId) -> hir::ImplPolarity;
     fn impl_parent(&self, impl_def_id: DefId) -> Option<DefId>;
 
     // trait/impl-item info
@@ -330,7 +329,6 @@ impl CrateStore for DummyCrateStore {
     fn implementations_of_trait(&self, filter: Option<DefId>) -> Vec<DefId> { vec![] }
 
     // impl info
-    fn impl_polarity(&self, def: DefId) -> hir::ImplPolarity { bug!("impl_polarity") }
     fn impl_parent(&self, def: DefId) -> Option<DefId> { bug!("impl_parent") }
 
     // trait/impl-item info

src/librustc/ty/maps.rs

@@ -10,6 +10,7 @@
 
 use dep_graph::{DepGraph, DepNode, DepTrackingMap, DepTrackingMapConfig};
 use hir::def_id::{CrateNum, DefId, LOCAL_CRATE};
+use hir;
 use middle::const_val;
 use middle::privacy::AccessLevels;
 use mir;
@@ -394,6 +395,7 @@ define_maps! { <'tcx>
     pub associated_item: AssociatedItems(DefId) -> ty::AssociatedItem,
 
     pub impl_trait_ref: ItemSignature(DefId) -> Option<ty::TraitRef<'tcx>>,
+    pub impl_polarity: ItemSignature(DefId) -> hir::ImplPolarity,
 
     /// Maps a DefId of a type to a list of its inherent impls.
     /// Contains implementations of methods that are inherent to a type.

src/librustc/ty/mod.rs

@@ -2149,14 +2149,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
     }
 
     pub fn trait_impl_polarity(self, id: DefId) -> hir::ImplPolarity {
-        if let Some(id) = self.hir.as_local_node_id(id) {
-            match self.hir.expect_item(id).node {
-                hir::ItemImpl(_, polarity, ..) => polarity,
-                ref item => bug!("trait_impl_polarity: {:?} not an impl", item)
-            }
-        } else {
-            self.sess.cstore.impl_polarity(id)
-        }
+        queries::impl_polarity::get(self, DUMMY_SP, id)
     }
 
     pub fn trait_relevant_for_never(self, did: DefId) -> bool {

src/librustc_metadata/cstore_impl.rs

@@ -89,6 +89,7 @@ provide! { <'tcx> tcx, def_id, cdata
     }
     associated_item => { cdata.get_associated_item(def_id.index) }
     impl_trait_ref => { cdata.get_impl_trait(def_id.index, tcx) }
+    impl_polarity => { cdata.get_impl_polarity(def_id.index) }
     coerce_unsized_info => {
         cdata.get_coerce_unsized_info(def_id.index).unwrap_or_else(|| {
             bug!("coerce_unsized_info: `{:?}` is missing its info", def_id);
@@ -177,12 +178,6 @@ impl CrateStore for cstore::CStore {
         result
     }
 
-    fn impl_polarity(&self, def: DefId) -> hir::ImplPolarity
-    {
-        self.dep_graph.read(DepNode::MetaData(def));
-        self.get_crate_data(def.krate).get_impl_polarity(def.index)
-    }
-
     fn impl_parent(&self, impl_def: DefId) -> Option<DefId> {
         self.dep_graph.read(DepNode::MetaData(impl_def));
         self.get_crate_data(impl_def.krate).get_parent_impl(impl_def.index)

src/librustc_typeck/collect.rs

@@ -99,6 +99,7 @@ pub fn provide(providers: &mut Providers) {
         trait_def,
         adt_def,
         impl_trait_ref,
+        impl_polarity,
         is_foreign_item,
         ..*providers
     };
@@ -1133,6 +1134,16 @@ fn impl_trait_ref<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     }
 }
 
+fn impl_polarity<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
+                           def_id: DefId)
+                           -> hir::ImplPolarity {
+    let node_id = tcx.hir.as_local_node_id(def_id).unwrap();
+    match tcx.hir.expect_item(node_id).node {
+        hir::ItemImpl(_, polarity, ..) => polarity,
+        ref item => bug!("trait_impl_polarity: {:?} not an impl", item)
+    }
+}
+
 // Is it marked with ?Sized
 fn is_unsized<'gcx: 'tcx, 'tcx>(astconv: &AstConv<'gcx, 'tcx>,
                                 ast_bounds: &[hir::TyParamBound],