util.h: fix broken casts in Perl_instr() parameters

[mauke]

Why broken? Because haystack/needle are effectively macros (they expand to token sequences), so a call like

Perl_instr(a + b, x ? y : z)

expands to

strstr((char *)a + b, (char *)x ? y : z)

which is just wrong.

Normally I'd simply add the missing parentheses ((char *)(haystack), etc), but it's not clear to me why the casts were added in the first place (and fairly recently, too, in commit 4e52881).

Background:

instr() was originally implemented in pre-standard C (without const and without a prototype) in util.c, perl-1.0. The consts were added fairly early on in 08105a9 (1997): char * instr(register const char *big, register const char *little);

The hand-written C code was replaced by a call to strstr() in 5d1d68e. It cast away const from the arguments to strstr() for no reason I can see: strstr() takes pointers to const char (just like Perl_instr), so the pointers were immediately implicitly re-consted by the prototype of strstr().

Commit fea1d2d straight up turned instr() into an alias for strstr(), which was apparently fine without any casts for 4.5 years. The C code in Perl_instr() was moved from util.c to mathoms.c in commit 534dad4.

---

• This set of changes does not require a perldelta entry.

[khwilliamson]

The only reason I can think of for the casts is that some platform complained at the time. It could have just been carelessness