> a misguided optimization of strcmp could do "a == b || really_strcmp(a, b)",

In practice, that's less of a risk than:

    char* x = function_that_might_return_null_and_error();
    dangerous_function();
    if (x == NULL) {...}

Or code for which pointer identity is relevant (e.g. accessing slices/subparts of a data structure):

    char* x = returns_null_or_a_slice_pointer(data_structure);
    char* y = other_function_that_returns_null_or_a_slice_pointer(data_structure);
    if (x == y) { puts("you're fucked now!"); }

You can always write:

   x : Result<string, err> = function...
   dangerous_function();
   switch(x) { 
     Ok() ... 
     Error() ...
   }

There's no difference between result and null other than readability (there's a mechanical transformation between the two, where you use unwrap() in rust on an unchecked null dereference, and you read errno in C instead of looking inside the Err() result variant).

To be clear, I strongly prefer Result<> because of the readability, but any dumb thing you can write with NULL + errno can be written with Result (and vice versa).

Totally! I threw that first example in because it's the poster child for things (checked) exceptions solve, and I didn't want to unfairly favor result unions (which I also prefer).

> You have to handle NULL in C, or your program crashes

But it crashes at runtime, so such a bug can exist for years with nobody noticing.

The existence of such bugs is what motivated the musl developers to return "*" instead of NULL.