Fix str.join returning new object when string argument length is 1 (RustPython#3845)

fix RustPython#3841

Author: jopemachine

vm/src/anystr.rs

@@ -2,7 +2,6 @@ use crate::{
     builtins::{PyIntRef, PyTupleRef},
     cformat::CFormatString,
     function::OptionalOption,
-    protocol::PyIterIter,
     AsObject, PyObject, PyObjectRef, PyResult, TryFromObject, VirtualMachine,
 };
 use num_traits::{cast::ToPrimitive, sign::Signed};
@@ -312,7 +311,9 @@ pub trait AnyStr<'s>: 's {
 
     fn py_join<'a>(
         &self,
-        mut iter: PyIterIter<'a, impl AnyStrWrapper<'s, Str = Self> + TryFromObject>,
+        mut iter: impl std::iter::Iterator<
+            Item = PyResult<impl AnyStrWrapper<'s, Str = Self> + TryFromObject>,
+        >,
     ) -> PyResult<Self::Container> {
         let mut joined = if let Some(elem) = iter.next() {
             elem?.as_ref().to_container()

vm/src/builtins/str.rs

@@ -928,9 +928,13 @@ impl PyStr {
     }
 
     #[pymethod]
-    fn join(&self, iterable: ArgIterable<PyStrRef>, vm: &VirtualMachine) -> PyResult<String> {
+    fn join(&self, iterable: ArgIterable<PyStrRef>, vm: &VirtualMachine) -> PyResult<PyStrRef> {
         let iter = iterable.iter(vm)?;
-        self.as_str().py_join(iter)
+
+        match iter.exactly_one() {
+            Ok(first) => first,
+            Err(iter) => Ok(vm.ctx.new_str(self.as_str().py_join(iter)?)),
+        }
     }
 
     // FIXME: two traversals of str is expensive