1 files changed, 29 insertions, 0 deletions
diff --git a/compiler/rustc_data_structures/src/unhash.rs b/compiler/rustc_data_structures/src/unhash.rs
new file mode 100644
index 000000000..48e21a9da
--- /dev/null
+++ b/compiler/rustc_data_structures/src/unhash.rs
@@ -0,0 +1,29 @@
+use std::collections::{HashMap, HashSet};
+use std::hash::{BuildHasherDefault, Hasher};
+
+pub type UnhashMap<K, V> = HashMap<K, V, BuildHasherDefault<Unhasher>>;
+pub type UnhashSet<V> = HashSet<V, BuildHasherDefault<Unhasher>>;
+
+/// This no-op hasher expects only a single `write_u64` call. It's intended for
+/// map keys that already have hash-like quality, like `Fingerprint`.
+#[derive(Default)]
+pub struct Unhasher {
+    value: u64,
+}
+
+impl Hasher for Unhasher {
+    #[inline]
+    fn finish(&self) -> u64 {
+        self.value
+    }
+
+    fn write(&mut self, _bytes: &[u8]) {
+        unimplemented!("use write_u64");
+    }
+
+    #[inline]
+    fn write_u64(&mut self, value: u64) {
+        debug_assert_eq!(0, self.value, "Unhasher doesn't mix values!");
+        self.value = value;
+    }
+}