add a deterministic_hash method to DefPath

Produces a deterministic hash, at least for a single platform /
compiler-version.

src/librustc/hir/map/definitions.rs

@@ -13,6 +13,7 @@
 use hir::map::def_collector::DefCollector;
 use rustc_data_structures::fnv::FnvHashMap;
 use std::fmt::Write;
+use std::hash::{Hash, Hasher, SipHasher};
 use syntax::{ast, visit};
 use syntax::parse::token::InternedString;
 use ty::TyCtxt;
@@ -133,6 +134,18 @@ pub fn to_string(&self, tcx: TyCtxt) -> String {
 
         s
     }
+
+    pub fn deterministic_hash(&self, tcx: TyCtxt) -> u64 {
+        let mut state = SipHasher::new();
+        self.deterministic_hash_to(tcx, &mut state);
+        state.finish()
+    }
+
+    pub fn deterministic_hash_to<H: Hasher>(&self, tcx: TyCtxt, state: &mut H) {
+        tcx.crate_name(self.krate).hash(state);
+        tcx.crate_disambiguator(self.krate).hash(state);
+        self.data.hash(state);
+    }
 }
 
 /// Root of an inlined item. We track the `DefPath` of the item within

src/librustc_incremental/calculate_svh.rs

@@ -144,7 +144,7 @@ pub fn new(st: &'a mut SipHasher,
         }
 
         fn hash_def_path(&mut self, path: &DefPath) {
-            path.to_string(self.tcx).hash(self.st);
+            path.deterministic_hash_to(self.tcx, self.st);
         }
     }