fast-path hash() for exact PyStr

str.__hash__ can’t be overridden, so for exact PyStr instances we
skip the attribute lookup and MRO walk and call the string hash directly.
This reduces overhead on hot paths (e.g. dict/set key hashing) while keeping
subclass semantics intact (downcast_ref_if_exact).

Author: giammirove

vm/src/protocol/object.rs

@@ -642,6 +642,11 @@ impl PyObject {
     }
 
     pub fn hash(&self, vm: &VirtualMachine) -> PyResult<PyHash> {
+        // __hash__ function in str cannot be overwritten
+        if let Some(pystr) = self.downcast_ref_if_exact::<PyStr>(vm) {
+            return Ok(pystr.hash(vm));
+        }
+
         let hash = self.get_class_attr(identifier!(vm, __hash__)).unwrap();
         if vm.is_none(&hash) {
             return Err(vm.new_exception_msg(