Returns new string when (str|bytes).replace get empty original string

Lib/test/string_tests.py

@@ -1202,6 +1202,9 @@ class MixinStrUnicodeUserStringTest:
         self.checkequal(False, 'asd', '__contains__', 'asdf')
         self.checkequal(False, '', '__contains__', 'asdf')
 
+
+    # TODO: RUSTPYTHON
+    @unittest.expectedFailure
     def test_subscript(self):
         self.checkequal('a', 'abc', '__getitem__', 0)
         self.checkequal('c', 'abc', '__getitem__', -1)

vm/src/builtins/str.rs

@@ -826,9 +826,11 @@ impl PyStr {
         let s = self.as_str();
         match count {
             OptionalArg::Present(max_count) if max_count >= 0 => {
-                if max_count == 0 || s.is_empty() {
+                if max_count == 0 || (s.is_empty() && !old.is_empty()) {
                     // nothing to do; return the original bytes
-                    s.into()
+                    s.to_owned()
+                } else if s.is_empty() && old.is_empty() {
+                    new.as_str().to_owned()
                 } else {
                     s.replacen(old.as_str(), new.as_str(), max_count as usize)
                 }

vm/src/bytesinner.rs

@@ -847,9 +847,11 @@ impl PyBytesInner {
         // stringlib_replace in CPython
         let maxcount = match maxcount {
             OptionalArg::Present(maxcount) if maxcount >= 0 => {
-                if maxcount == 0 || self.elements.is_empty() {
+                if maxcount == 0 || (self.elements.is_empty() && !from.is_empty()) {
                     // nothing to do; return the original bytes
                     return Ok(self.elements.clone());
+                } else if self.elements.is_empty() && from.is_empty() {
+                    return Ok(to.elements);
                 }
                 Some(maxcount as usize)
             }