In C currently, pointers just are, and there are no qualifiers to tell programmers whether they can be null or not. Objective-C kind of solves this problem by adding _Nonnull and _Nullable. However these hints do not really do much besides raise compiler warnings in only a few of the many possible misuse cases.

I would like to add a stronger nonnull keyword to my C-style programming language (no nullable counterpart because nullable is just designated by the absence of nonnull). The behavior is simple: NULL is a trap representation for nonnull pointers. That is, if a pointer qualified with nonnull contains NULL and is read, immediate undefined behavior is invoked. Compilers would warn if a non nonnull pointer is dereferenced, because then NULL might be being dereferenced (also undefined behavior). This could prevent common mistakes such as not checking malloc()/fopen() for NULL, and also help avoid redundant checks on pointers that will already never be NULL.

What are the semantic options for users to convert a non nonnull to a nonnull? A basic option would be:

void *ptr = malloc(size);
if (ptr) {
    void *nonnull nnptr = ptr; // Conversion is valid because the code is unreachable if ptr is NULL
}

However, a dumb compiler may still warn at this, and this still creates redundant variables. There needs to be a syntactic way to 'convert' regular to nonnull in a way that is ergonomic and can be correctly and easily checked by the compiler. What are my options?