Minor numba Scan tweaks

pytensor/link/numba/dispatch/scan.py

@@ -222,14 +222,16 @@ def numba_funcify_Scan(op, node, **kwargs):
         # the storage array.
         # This is needed when the output storage array does not have a length
         # equal to the number of taps plus `n_steps`.
+        # If the storage size only allows one entry, there's nothing to rotate
         output_storage_post_proc_stmts.append(
             dedent(
                 f"""
-                if (i + {tap_size}) > {storage_size}:
+                if 1 < {storage_size} < (i + {tap_size}):
                     {outer_in_name}_shift = (i + {tap_size}) % ({storage_size})
-                    {outer_in_name}_left = {outer_in_name}[:{outer_in_name}_shift]
-                    {outer_in_name}_right = {outer_in_name}[{outer_in_name}_shift:]
-                    {outer_in_name} = np.concatenate(({outer_in_name}_right, {outer_in_name}_left))
+                    if {outer_in_name}_shift > 0:
+                        {outer_in_name}_left = {outer_in_name}[:{outer_in_name}_shift]
+                        {outer_in_name}_right = {outer_in_name}[{outer_in_name}_shift:]
+                        {outer_in_name} = np.concatenate(({outer_in_name}_right, {outer_in_name}_left))
                 """
             ).strip()
         )
@@ -417,4 +419,4 @@ def scan({", ".join(outer_in_names)}):
 
     scan_op_fn = compile_function_src(scan_op_src, "scan", {**globals(), **global_env})
 
-    return numba_basic.numba_njit(scan_op_fn)
+    return numba_basic.numba_njit(scan_op_fn, boundscheck=False)