2 files changed, 24 insertions, 2 deletions
diff --git a/compiler/rustc_data_structures/src/hashes.rs b/compiler/rustc_data_structures/src/hashes.rs
index 291ee5bbe26..1564eeb4bae 100644
--- a/compiler/rustc_data_structures/src/hashes.rs
+++ b/compiler/rustc_data_structures/src/hashes.rs
@@ -75,11 +75,21 @@ impl fmt::LowerHex for Hash64 {
     }
 }
 
-#[derive(Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord, Default)]
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Default)]
 pub struct Hash128 {
     inner: u128,
 }
 
+// We expect Hash128 to be well mixed. So there's no point in hashing both parts.
+//
+// This also allows using Hash128-containing types in UnHash-based hashmaps, which would otherwise
+// debug_assert! that we're hashing more than a single u64.
+impl std::hash::Hash for Hash128 {
+    fn hash<H: std::hash::Hasher>(&self, h: &mut H) {
+        h.write_u64(self.truncate().as_u64());
+    }
+}
+
 impl Hash128 {
     #[inline]
     pub fn truncate(self) -> Hash64 {
diff --git a/compiler/rustc_data_structures/src/stable_hasher.rs b/compiler/rustc_data_structures/src/stable_hasher.rs
index afe26f80de8..52304c72a2f 100644
--- a/compiler/rustc_data_structures/src/stable_hasher.rs
+++ b/compiler/rustc_data_structures/src/stable_hasher.rs
@@ -314,7 +314,19 @@ impl_stable_traits_for_trivial_type!(char);
 impl_stable_traits_for_trivial_type!(());
 
 impl_stable_traits_for_trivial_type!(Hash64);
-impl_stable_traits_for_trivial_type!(Hash128);
+
+// We need a custom impl as the default hash function will only hash half the bits. For stable
+// hashing we want to hash the full 128-bit hash.
+impl<CTX> HashStable<CTX> for Hash128 {
+    #[inline]
+    fn hash_stable(&self, _: &mut CTX, hasher: &mut StableHasher) {
+        self.as_u128().hash(hasher);
+    }
+}
+
+unsafe impl StableOrd for Hash128 {
+    const CAN_USE_UNSTABLE_SORT: bool = true;
+}
 
 impl<CTX> HashStable<CTX> for ! {
     fn hash_stable(&self, _ctx: &mut CTX, _hasher: &mut StableHasher) {