Presence of RBIMPL_ATTR_NONNULL let C compilers to eliminate
must_not_null(). Because null pointers are not allowed to exist there
are no reason to call the function. In reality null pointers are still
passed to those functions in a number of ways. Runtime check for them
are definitely nice to have. fix [Feature#18280]
Many functions in string.c assume that capa + termlen to be readable
memory. Add comment in header to communicate this to extension authors.
See also: comment in str_fill_term()
https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html says:
> GCC never returns 1 when you call the inline function with a string
> constant or ...
Because rb_str_new and familiy take string constants, it was a bad idea
for them to contain RB_CONSTANT_P. We should instead do so in
corresponding macros. [ruby-core:98212]
