Avoid sorting the item_ids array the StableHash impl of hir::Mod.

src/librustc/hir/map/definitions.rs

@@ -23,6 +23,7 @@ use rustc_data_structures::indexed_vec::{IndexVec};
 use rustc_data_structures::stable_hasher::StableHasher;
 use serialize::{Encodable, Decodable, Encoder, Decoder};
 use session::CrateDisambiguator;
+use std::borrow::Borrow;
 use std::fmt::Write;
 use std::hash::Hash;
 use syntax::ast;
@@ -389,6 +390,13 @@ pub struct DefPathHash(pub Fingerprint);
 
 impl_stable_hash_for!(tuple_struct DefPathHash { fingerprint });
 
+impl Borrow<Fingerprint> for DefPathHash {
+    #[inline]
+    fn borrow(&self) -> &Fingerprint {
+        &self.0
+    }
+}
+
 impl Definitions {
     /// Create new empty definition map.
     pub fn new() -> Definitions {

src/librustc/ich/fingerprint.rs

@@ -45,6 +45,18 @@ impl Fingerprint {
         )
     }
 
+    // Combines two hashes in an order independent way. Make sure this is what
+    // you want.
+    #[inline]
+    pub fn combine_commutative(self, other: Fingerprint) -> Fingerprint {
+        let a = (self.1 as u128) << 64 | self.0 as u128;
+        let b = (other.1 as u128) << 64 | other.0 as u128;
+
+        let c = a.wrapping_add(b);
+
+        Fingerprint((c >> 64) as u64, c as u64)
+    }
+
     pub fn to_hex(&self) -> String {
         format!("{:x}{:x}", self.0, self.1)
     }

src/librustc/ich/impls_hir.rs

@@ -14,7 +14,7 @@
 use hir;
 use hir::map::DefPathHash;
 use hir::def_id::{DefId, LocalDefId, CrateNum, CRATE_DEF_INDEX};
-use ich::{StableHashingContext, NodeIdHashingMode};
+use ich::{StableHashingContext, NodeIdHashingMode, Fingerprint};
 use rustc_data_structures::stable_hasher::{HashStable, ToStableHashKey,
                                            StableHasher, StableHasherResult};
 use std::mem;
@@ -767,15 +767,21 @@ impl<'a> HashStable<StableHashingContext<'a>> for hir::Mod {
 
         inner_span.hash_stable(hcx, hasher);
 
-        let mut item_ids: Vec<DefPathHash> = item_ids.iter().map(|id| {
-            let (def_path_hash, local_id) = id.id.to_stable_hash_key(hcx);
-            debug_assert_eq!(local_id, hir::ItemLocalId(0));
-            def_path_hash
-        }).collect();
+        // Combining the DefPathHashes directly is faster than feeding them
+        // into the hasher. Because we use a commutative combine, we also don't
+        // have to sort the array.
+        let item_ids_hash = item_ids
+            .iter()
+            .map(|id| {
+                let (def_path_hash, local_id) = id.id.to_stable_hash_key(hcx);
+                debug_assert_eq!(local_id, hir::ItemLocalId(0));
+                def_path_hash.0
+            }).fold(Fingerprint::ZERO, |a, b| {
+                a.combine_commutative(b)
+            });
 
-        item_ids.sort_unstable();
-
-        item_ids.hash_stable(hcx, hasher);
+        item_ids.len().hash_stable(hcx, hasher);
+        item_ids_hash.hash_stable(hcx, hasher);
     }
 }