py/obj: Add static safety checks to mp_obj_is_type().

Commit d96cfd13e3a464862c introduced a regression by breaking existing
users of mp_obj_is_type(.., &mp_obj_bool).  This function (and associated
helpers like mp_obj_is_int()) have some specific nuances, and mistakes like
this one can happen again.

This commit adds mp_obj_is_exact_type() which behaves like the the old
mp_obj_is_type().  The new mp_obj_is_type() has the same prototype but it
attempts to statically assert that it's not called with types which should
be checked using mp_obj_is_type().  If called with any of these types: int,
str, bool, NoneType - it will cause a compilation error.  Additional
checked types (e.g function types) can be added in the future.

Existing users of mp_obj_is_type() with the now "invalid" types, were
translated to use mp_obj_is_exact_type().

The use of MP_STATIC_ASSERT() is not bulletproof - usually GCC (and other
compilers) can't statically check conditions that are only known during
link-time (like variables' addresses comparison).  However, in this case,
GCC is able to statically detect these conditions, probably because it's
the exact same object - `&mp_type_int == &mp_type_int` is detected.
Misuses of this function with runtime-chosen types (e.g:
`mp_obj_type_t *x = ...; mp_obj_is_type(..., x);` won't be detected.  MSC
is unable to detect this, so we use MP_STATIC_ASSERT_NOT_MSC().

Compiling with this commit and without the fix for d96cfd13e3a464862c shows
that it detects the problem.

Signed-off-by: Yonatan Goldschmidt <yon.goldschmidt@gmail.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/py/binary.c b/py/binary.c
index 05e658c95..f59e89ca4 100644
--- a/py/binary.c
+++ b/py/binary.c
@@ -334,7 +334,7 @@ void mp_binary_set_val(char struct_type, char val_type, mp_obj_t val_in, byte *p
         #endif
         default:
             #if MICROPY_LONGINT_IMPL != MICROPY_LONGINT_IMPL_NONE
-            if (mp_obj_is_type(val_in, &mp_type_int)) {
+            if (mp_obj_is_exact_type(val_in, &mp_type_int)) {
                 mp_obj_int_to_bytes_impl(val_in, struct_type == '>', size, p);
                 return;
             }
@@ -371,7 +371,7 @@ void mp_binary_set_val_array(char typecode, void *p, size_t index, mp_obj_t val_
             break;
         default:
             #if MICROPY_LONGINT_IMPL != MICROPY_LONGINT_IMPL_NONE
-            if (mp_obj_is_type(val_in, &mp_type_int)) {
+            if (mp_obj_is_exact_type(val_in, &mp_type_int)) {
                 size_t size = mp_binary_get_size('@', typecode, NULL);
                 mp_obj_int_to_bytes_impl(val_in, MP_ENDIANNESS_BIG,
                     size, (uint8_t *)p + index * size);