core: introduce utils.decode_bytes

This instructs the low-level code to use mp_obj_new_str_copy directly
and avoids the usage of qstr_find_strn lookup.

core/embed/extmod/modtrezorutils/modtrezorutils.c

@@ -17,6 +17,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include "py/objstr.h"
 #include "py/runtime.h"
 
 #include "version.h"
@@ -115,6 +116,18 @@ STATIC mp_obj_t mod_trezorutils_halt(size_t n_args, const mp_obj_t *args) {
 STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(mod_trezorutils_halt_obj, 0, 1,
                                            mod_trezorutils_halt);
 
+/// def decode_bytes(b: bytes) -> str:
+///     """
+///     Decodes string from bytes
+///     """
+STATIC mp_obj_t mod_trezorutils_decode_bytes(mp_obj_t b) {
+  mp_buffer_info_t bytes;
+  mp_get_buffer_raise(b, &bytes, MP_BUFFER_READ);
+  return mp_obj_new_str_copy(&mp_type_str, bytes.buf, bytes.len);
+}
+STATIC MP_DEFINE_CONST_FUN_OBJ_1(mod_trezorutils_decode_bytes_obj,
+                                 mod_trezorutils_decode_bytes);
+
 #define PASTER(s) MP_QSTR_##s
 #define MP_QSTR(s) PASTER(s)
 
@@ -131,6 +144,8 @@ STATIC const mp_rom_map_elem_t mp_module_trezorutils_globals_table[] = {
     {MP_ROM_QSTR(MP_QSTR_consteq), MP_ROM_PTR(&mod_trezorutils_consteq_obj)},
     {MP_ROM_QSTR(MP_QSTR_memcpy), MP_ROM_PTR(&mod_trezorutils_memcpy_obj)},
     {MP_ROM_QSTR(MP_QSTR_halt), MP_ROM_PTR(&mod_trezorutils_halt_obj)},
+    {MP_ROM_QSTR(MP_QSTR_decode_bytes),
+     MP_ROM_PTR(&mod_trezorutils_decode_bytes_obj)},
     // various built-in constants
     {MP_ROM_QSTR(MP_QSTR_GITREV), MP_ROM_QSTR(MP_QSTR(GITREV))},
     {MP_ROM_QSTR(MP_QSTR_VERSION_MAJOR), MP_ROM_INT(VERSION_MAJOR)},

core/mocks/generated/trezorutils.pyi

@@ -32,6 +32,13 @@ def halt(msg: str = None) -> None:
     """
     Halts execution.
     """
+
+
+# extmod/modtrezorutils/modtrezorutils.c
+def decode_bytes(b: bytes) -> str:
+    """
+    Decodes string from bytes
+    """
 GITREV: str
 VERSION_MAJOR: int
 VERSION_MINOR: int

core/src/trezor/utils.py

@@ -9,6 +9,7 @@ from trezorutils import (  # noqa: F401
     VERSION_MINOR,
     VERSION_PATCH,
     consteq,
+    decode_bytes,
     halt,
     memcpy,
 )