bug#31343: scm_c_primitive_load behavior/documentation bug

[Tom Balzer <niebieskitrociny@gmail.com>]

Hello -

In ./guile/libguile/load.c, the function scm_c_primitive_load converts a
c string to a SCM string via scm_from_locale_string. I was reading the
manual and in section 6.6.5.14, it says:

> C Function: SCM scm_from_locale_string (const char *str)
> C Function: SCM scm_from_locale_stringn (const char *str, size_t
>
> [...]
>
> Note that these functions should _not_ be used to convert C string
> constants, because there is no guarantee that the current locale
> will match that of the execution character set, used for string and
> character constants.  Most modern C compilers use UTF-8 by default,
> so to convert C string constants we recommend
> ‘scm_from_utf8_string’.

This implies to me that you should not use scm_c_primitive_load with any
constant, like this:

#include <libguile.h>
#include <stdlib.h>

#define FILE "/home/niebie/sc/sdl/states.scm"

void *some_func(void *arg){
  SCM scm_c_primitive_load(FILE);
  
  return NULL;
}

int main(int argc, char **argv){
  void *res = scm_with_guile(some_func, NULL);
  return EXIT_SUCCESS;
}

I saw this only by reading the source for this function, as from the
documentation it isn't obvious. I am sending this to bug-guile because I
think that either this is a documentation bug or an implementation
bug. In either case I am happy to send a patch that fixes whichever is
at fault.

A counter example is scm_c_public_variable in modules.c, which uses
scm_from_utf8_symbol on the inputs, which precludes the use of dynamic c
strings for the input. Again, not something documented. I would think
both of these functions would do things the same way.

Thanks,
Tom