qstr: Use const consistently to avoid a cast.

Author: tyomitch

py/qstr.c

@@ -123,7 +123,7 @@ void qstr_init(void) {
 STATIC const char *find_qstr(qstr q, qstr_attr_t *attr) {
     // search pool for this qstr
     // total_prev_len==0 in the final pool, so the loop will always terminate
-    qstr_pool_t *pool = MP_STATE_VM(last_pool);
+    const qstr_pool_t *pool = MP_STATE_VM(last_pool);
     while (q < pool->total_prev_len) {
         pool = pool->prev;
     }
@@ -181,7 +181,7 @@ qstr qstr_find_strn(const char *str, size_t str_len) {
     mp_uint_t str_hash = qstr_compute_hash((const byte *)str, str_len);
 
     // search pools for the data
-    for (qstr_pool_t *pool = MP_STATE_VM(last_pool); pool != NULL; pool = pool->prev) {
+    for (const qstr_pool_t *pool = MP_STATE_VM(last_pool); pool != NULL; pool = pool->prev) {
         qstr_attr_t *attrs = pool->attrs;
         for (mp_uint_t at = 0, top = pool->len; at < top; at++) {
             if (attrs[at].hash == str_hash && attrs[at].len == str_len && memcmp(pool->qstrs[at], str, str_len) == 0) {
@@ -290,7 +290,7 @@ void qstr_pool_info(size_t *n_pool, size_t *n_qstr, size_t *n_str_data_bytes, si
     *n_qstr = 0;
     *n_str_data_bytes = 0;
     *n_total_bytes = 0;
-    for (qstr_pool_t *pool = MP_STATE_VM(last_pool); pool != NULL && pool != &CONST_POOL; pool = pool->prev) {
+    for (const qstr_pool_t *pool = MP_STATE_VM(last_pool); pool != NULL && pool != &CONST_POOL; pool = pool->prev) {
         *n_pool += 1;
         *n_qstr += pool->len;
         for (const qstr_attr_t *q = pool->attrs, *q_top = pool->attrs + pool->len; q < q_top; q++) {
@@ -310,8 +310,8 @@ void qstr_pool_info(size_t *n_pool, size_t *n_qstr, size_t *n_str_data_bytes, si
 #if MICROPY_PY_MICROPYTHON_MEM_INFO
 void qstr_dump_data(void) {
     QSTR_ENTER();
-    for (qstr_pool_t *pool = MP_STATE_VM(last_pool); pool != NULL && pool != &CONST_POOL; pool = pool->prev) {
-        for (const char **q = pool->qstrs, **q_top = pool->qstrs + pool->len; q < q_top; q++) {
+    for (const qstr_pool_t *pool = MP_STATE_VM(last_pool); pool != NULL && pool != &CONST_POOL; pool = pool->prev) {
+        for (const char *const *q = pool->qstrs, *const *q_top = pool->qstrs + pool->len; q < q_top; q++) {
             mp_printf(&mp_plat_print, "Q(%s)\n", *q);
         }
     }

py/qstr.h

@@ -65,7 +65,7 @@ typedef struct _qstr_attr_t {
 } qstr_attr_t;
 
 typedef struct _qstr_pool_t {
-    struct _qstr_pool_t *prev;
+    const struct _qstr_pool_t *prev;
     size_t total_prev_len;
     size_t alloc;
     size_t len;

tools/mpy-tool.py

@@ -878,7 +878,7 @@ def freeze_mpy(base_qstrs, raw_codes):
     print()
     print("extern const qstr_pool_t mp_qstr_const_pool;")
     print("const qstr_pool_t mp_qstr_frozen_const_pool = {")
-    print("    (qstr_pool_t*)&mp_qstr_const_pool, // previous pool")
+    print("    &mp_qstr_const_pool, // previous pool")
     print("    MP_QSTRnumber_of, // previous pool size")
     print("    %u, // allocated entries" % qstr_pool_alloc)
     print("    %u, // used entries" % len(new))