Reuse stored hashes in dict.fromkeys() (#8503)

* Reuse stored hashes in dict.fromkeys()

Closes #8490.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* Make fromkeys_known_hashes a PyDict associated fn

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Lib/test/test_set.py

@@ -330,7 +330,6 @@ class TestJointOps:
             name = repr(s).partition('(')[0]    # strip class name
             self.assertEqual(repr(s), '%s({%s(...)})' % (name, name))
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON
     def test_do_not_rehash_dict_keys(self):
         n = 10
         d = dict.fromkeys(map(HashCountingInt, range(n)))

crates/vm/src/builtins/dict.rs

@@ -1,6 +1,6 @@
 use super::{
     IterStatus, PositionIterInternal, PyBaseExceptionRef, PyGenericAlias, PyMappingProxy, PySet,
-    PyStr, PyStrRef, PyTupleRef, PyType, PyTypeRef, set::PySetInner,
+    PyStr, PyStrRef, PyTupleRef, PyType, PyTypeRef, set, set::PySetInner,
 };
 use crate::common::lock::LazyLock;
 use crate::object::{Traverse, TraverseFn};
@@ -9,7 +9,7 @@ use crate::{
     TryFromObject, atomic_func,
     builtins::{PyList, PyTuple, iter::builtins_iter, type_::PyAttributes},
     class::{PyClassDef, PyClassImpl},
-    common::ascii,
+    common::{ascii, hash::PyHash},
     dict_inner::{self, DictKey},
     function::{ArgIterable, FuncArgs, KwArgs, OptionalArg, PyArithmeticValue, PyComparisonValue},
     iter::PyExactSizeIterator,
@@ -354,6 +354,20 @@ impl PyDict {
     ) -> PyResult<Option<PyObjectRef>> {
         self.entries.get(vm, key)
     }
+
+    /// Keys of `obj` with their stored hashes, or `None` if it must be iterated
+    /// generically. Only exact dicts and sets qualify, as in CPython's
+    /// `_PyDict_FromKeys`: a subclass may override `__iter__`.
+    fn fromkeys_known_hashes(
+        obj: &PyObject,
+        vm: &VirtualMachine,
+    ) -> Option<Vec<(PyObjectRef, PyHash)>> {
+        if let Some(dict) = obj.downcast_ref_if_exact::<Self>(vm) {
+            Some(dict.entries.keys_with_hashes())
+        } else {
+            set::exact_set_keys_with_hashes(obj, vm)
+        }
+    }
 }
 
 // Python dict methods:
@@ -384,8 +398,16 @@ impl PyDict {
         let d = PyType::call(&class, ().into(), vm)?;
         match d.downcast_exact::<Self>(vm) {
             Ok(pydict) => {
-                for key in iterable.iter(vm)? {
-                    pydict.__setitem__(key?, value.clone(), vm)?;
+                if let Some(keys) = Self::fromkeys_known_hashes(iterable.as_object(), vm) {
+                    for (key, hash) in keys {
+                        pydict
+                            .entries
+                            .insert_known_hash(vm, &*key, hash, value.clone())?;
+                    }
+                } else {
+                    for key in iterable.iter(vm)? {
+                        pydict.__setitem__(key?, value.clone(), vm)?;
+                    }
                 }
                 Ok(pydict.into_pyref().into())
             }

crates/vm/src/builtins/set.rs

@@ -605,6 +605,23 @@ fn extract_set(obj: &PyObject) -> Option<&PySetInner> {
     })
 }
 
+/// Elements of `obj` with their stored hashes, or `None` unless `obj` is exactly
+/// a `set` or `frozenset` — `PyAnySet_CheckExact`, where [`extract_set`] is the
+/// subclass-inclusive `PyAnySet_Check`.
+pub(super) fn exact_set_keys_with_hashes(
+    obj: &PyObject,
+    vm: &VirtualMachine,
+) -> Option<Vec<(PyObjectRef, PyHash)>> {
+    let inner = obj
+        .downcast_ref_if_exact::<PySet>(vm)
+        .map(|set| &set.inner)
+        .or_else(|| {
+            obj.downcast_ref_if_exact::<PyFrozenSet>(vm)
+                .map(|frozen| &frozen.inner)
+        })?;
+    Some(inner.content.keys_with_hashes())
+}
+
 fn reduce_set(zelf: &PyObject, vm: &VirtualMachine) -> (PyTypeRef, PyTupleRef, Option<PyDictRef>) {
     (
         zelf.class().to_owned(),