Avoid allocating a vector of elements when hashing frozenset (RustPython#5408)

Adds a `try_fold_keys` method to Dict which allows performing common
operations on all elements without needing to create a Vec first.

Author: crazymerlyn

vm/src/builtins/set.rs

@@ -436,12 +436,12 @@ impl PySetInner {
             ((h ^ 89869747) ^ (h.wrapping_shl(16))).wrapping_mul(3644798167)
         }
         // Factor in the number of active entries
-        let mut hash: u64 = (self.elements().len() as u64 + 1).wrapping_mul(1927868237);
+        let mut hash: u64 = (self.len() as u64 + 1).wrapping_mul(1927868237);
         // Xor-in shuffled bits from every entry's hash field because xor is
         // commutative and a frozenset hash should be independent of order.
-        for element in self.elements().iter() {
-            hash ^= _shuffle_bits(element.hash(vm)? as u64);
-        }
+        hash = self.content.try_fold_keys(hash, |h, element| {
+            Ok(h ^ _shuffle_bits(element.hash(vm)? as u64))
+        })?;
         // Disperse patterns arising in nested frozensets
         hash ^= (hash >> 11) ^ (hash >> 25);
         hash = hash.wrapping_mul(69069).wrapping_add(907133923);

vm/src/dictdatatype.rs

@@ -530,6 +530,17 @@ impl<T: Clone> Dict<T> {
             .collect()
     }
 
+    pub fn try_fold_keys<Acc, Fold>(&self, init: Acc, f: Fold) -> PyResult<Acc>
+    where
+        Fold: FnMut(Acc, &PyObject) -> PyResult<Acc>,
+    {
+        self.read()
+            .entries
+            .iter()
+            .filter_map(|v| v.as_ref().map(|v| v.key.as_object()))
+            .try_fold(init, f)
+    }
+
     /// Lookup the index for the given key.
     #[cfg_attr(feature = "flame-it", flame("Dict"))]
     fn lookup<K: DictKey + ?Sized>(