cache def-path hashes across all items

This seems like approx a 2x win on syntex_syntax.

Author: nikomatsakis

src/librustc_incremental/calculate_svh/mod.rs

@@ -35,6 +35,7 @@ use rustc::hir;
 use rustc::hir::def_id::{CRATE_DEF_INDEX, DefId};
 use rustc::hir::intravisit as visit;
 use rustc::ty::TyCtxt;
+use rustc::util::nodemap::DefIdMap;
 use rustc_data_structures::fnv::FnvHashMap;
 
 use self::svh_visitor::StrictVersionHashVisitor;
@@ -47,7 +48,9 @@ pub fn compute_incremental_hashes_map<'a, 'tcx: 'a>(tcx: TyCtxt<'a, 'tcx, 'tcx>)
                                                     -> IncrementalHashesMap {
     let _ignore = tcx.dep_graph.in_ignore();
     let krate = tcx.map.krate();
-    let mut visitor = HashItemsVisitor { tcx: tcx, hashes: FnvHashMap() };
+    let mut visitor = HashItemsVisitor { tcx: tcx,
+                                         hashes: FnvHashMap(),
+                                         def_path_hashes: DefIdMap() };
     visitor.calculate_def_id(DefId::local(CRATE_DEF_INDEX), |v| visit::walk_crate(v, krate));
     krate.visit_all_items(&mut visitor);
     visitor.compute_crate_hash();
@@ -56,6 +59,7 @@ pub fn compute_incremental_hashes_map<'a, 'tcx: 'a>(tcx: TyCtxt<'a, 'tcx, 'tcx>)
 
 struct HashItemsVisitor<'a, 'tcx: 'a> {
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
+    def_path_hashes: DefIdMap<u64>,
     hashes: IncrementalHashesMap,
 }
 
@@ -75,7 +79,9 @@ impl<'a, 'tcx> HashItemsVisitor<'a, 'tcx> {
         // FIXME: this should use SHA1, not SipHash. SipHash is not
         // built to avoid collisions.
         let mut state = SipHasher::new();
-        walk_op(&mut StrictVersionHashVisitor::new(&mut state, self.tcx));
+        walk_op(&mut StrictVersionHashVisitor::new(&mut state,
+                                                   self.tcx,
+                                                   &mut self.def_path_hashes));
         let item_hash = state.finish();
         self.hashes.insert(DepNode::Hir(def_id), item_hash);
         debug!("calculate_item_hash: def_id={:?} hash={:?}", def_id, item_hash);

src/librustc_incremental/calculate_svh/svh_visitor.rs

@@ -35,23 +35,24 @@ pub struct StrictVersionHashVisitor<'a, 'tcx: 'a> {
     pub st: &'a mut SipHasher,
 
     // collect a deterministic hash of def-ids that we have seen
-    def_id_hashes: DefIdMap<u64>,
+    def_path_hashes: &'a mut DefIdMap<u64>,
 }
 
 impl<'a, 'tcx> StrictVersionHashVisitor<'a, 'tcx> {
     pub fn new(st: &'a mut SipHasher,
-               tcx: TyCtxt<'a, 'tcx, 'tcx>)
+               tcx: TyCtxt<'a, 'tcx, 'tcx>,
+               def_path_hashes: &'a mut DefIdMap<u64>)
                -> Self {
-        StrictVersionHashVisitor { st: st, tcx: tcx, def_id_hashes: DefIdMap() }
+        StrictVersionHashVisitor { st: st, tcx: tcx, def_path_hashes: def_path_hashes }
     }
 
     fn compute_def_id_hash(&mut self, def_id: DefId) -> u64 {
         let tcx = self.tcx;
-        *self.def_id_hashes.entry(def_id)
-                           .or_insert_with(|| {
-                               let def_path = tcx.def_path(def_id);
-                               def_path.deterministic_hash(tcx)
-                           })
+        *self.def_path_hashes.entry(def_id)
+                             .or_insert_with(|| {
+                                 let def_path = tcx.def_path(def_id);
+                                 def_path.deterministic_hash(tcx)
+                             })
     }
 }