py/sequence: Remove unused len argument from mp_seq_extract_slice.

Also put this function inside the `MICROPY_PY_BUILTINS_SLICE` guard,
because it's only usable when that option is enabled.

Signed-off-by: Damien George <damien@micropython.org>

py/obj.h

@@ -1258,7 +1258,7 @@ bool mp_seq_cmp_bytes(mp_uint_t op, const byte *data1, size_t len1, const byte *
 bool mp_seq_cmp_objs(mp_uint_t op, const mp_obj_t *items1, size_t len1, const mp_obj_t *items2, size_t len2);
 mp_obj_t mp_seq_index_obj(const mp_obj_t *items, size_t len, size_t n_args, const mp_obj_t *args);
 mp_obj_t mp_seq_count_obj(const mp_obj_t *items, size_t len, mp_obj_t value);
-mp_obj_t mp_seq_extract_slice(size_t len, const mp_obj_t *seq, mp_bound_slice_t *indexes);
+mp_obj_t mp_seq_extract_slice(const mp_obj_t *seq, mp_bound_slice_t *indexes);
 
 // Helper to clear stale pointers from allocated, but unused memory, to preclude GC problems
 #define mp_seq_clear(start, len, alloc_len, item_sz) memset((byte *)(start) + (len) * (item_sz), 0, ((alloc_len) - (len)) * (item_sz))

py/objlist.c

@@ -188,7 +188,7 @@ static mp_obj_t list_subscr(mp_obj_t self_in, mp_obj_t index, mp_obj_t value) {
         if (mp_obj_is_type(index, &mp_type_slice)) {
             mp_bound_slice_t slice;
             if (!mp_seq_get_fast_slice_indexes(self->len, index, &slice)) {
-                return mp_seq_extract_slice(self->len, self->items, &slice);
+                return mp_seq_extract_slice(self->items, &slice);
             }
             mp_obj_list_t *res = list_new(slice.stop - slice.start);
             mp_seq_copy(res->items, self->items + slice.start, res->len, mp_obj_t);

py/sequence.c

@@ -63,11 +63,7 @@ bool mp_seq_get_fast_slice_indexes(mp_uint_t len, mp_obj_t slice, mp_bound_slice
     return indexes->step == 1;
 }
 
-#endif
-
-mp_obj_t mp_seq_extract_slice(size_t len, const mp_obj_t *seq, mp_bound_slice_t *indexes) {
-    (void)len; // TODO can we remove len from the arg list?
-
+mp_obj_t mp_seq_extract_slice(const mp_obj_t *seq, mp_bound_slice_t *indexes) {
     mp_int_t start = indexes->start, stop = indexes->stop;
     mp_int_t step = indexes->step;
 
@@ -87,6 +83,8 @@ mp_obj_t mp_seq_extract_slice(size_t len, const mp_obj_t *seq, mp_bound_slice_t
     return res;
 }
 
+#endif
+
 // Special-case comparison function for sequences of bytes
 // Don't pass MP_BINARY_OP_NOT_EQUAL here
 bool mp_seq_cmp_bytes(mp_uint_t op, const byte *data1, size_t len1, const byte *data2, size_t len2) {