gh-131798: JIT: Assign type to sliced string

Lib/test/test_capi/test_opt.py

@@ -1655,13 +1655,11 @@ def testfunc(n):
         self.assertIn("_CONTAINS_OP_DICT", uops)
         self.assertNotIn("_TO_BOOL_BOOL", uops)
 
-
     def test_remove_guard_for_known_type_str(self):
         def f(n):
             for i in range(n):
                 false = i == TIER2_THRESHOLD
                 empty = "X"[:false]
-                empty += ""  # Make JIT realize this is a string.
                 if empty:
                     return 1
             return 0

Python/optimizer_bytecodes.c

@@ -1096,6 +1096,18 @@ dummy_func(void) {
         sym_set_const(callable, len);
     }
 
+    op(_BINARY_SLICE, (container, start, stop -- res)) {
+        // Slicing a string always returns a string.
+        // TODO: We can apply this to lists and tuples as well.
+        //       We'll start with string to simplify the process.
+        PyTypeObject *type = sym_get_type(container);
+        if (type == &PyUnicode_Type) {
+            res = sym_new_type(ctx, type);
+        } else {
+            res = sym_new_not_null(ctx);
+        }
+    }
+
 // END BYTECODES //
 
 }