Remove ::gets for FreeBSD 13 and later

[dim]

Summary

In https://svnweb.freebsd.org/changeset/base/351659 @emaste removed gets() from
FreeBSD 13's libc, and our copies of libc++ and libstdc++. In that change, the
declarations were simply deleted, but I would like to propose this conditional
test instead.

[emaste]

LGTM from the FreeBSD perspective.

[mclow.lists]

I'm wondering if this has become complicated enough that we should define a _LIBCPP_C_HAS_NO_GETS config macro.

Then the code in cstdio becomes:

#if _LIBCPP_STD_VER <= 11 && !defined(_LIBCPP_C_HAS_NO_GETS)
using ::gets;
#endif

which I think is much clearer, and captures the intent
We do not import gets into namespace std when:

• It does not exist in the underlying C library, or
• We are compiling for C++14 or later.

[mclow.lists]

This comment was removed by mclow.lists.

[dim]

Add _LIBCPP_C_HAS_NO_GETS macro to <__config>, and use that in <cstdio>.

[mclow.lists]

LGTM with the one change I suggested.

include/__config

1147

Even better would be to have a way to define it at build time as well.

So wrap this in

#ifndef _LIBCPP_C_HAS_NO_GETS
...
#endif

and then commit it.

include/__config

 
 // CloudABI is intended for running networked services. Processes do not
 // have standard input and output channels.
 #ifdef __CloudABI__
 #define _LIBCPP_HAS_NO_STDIN
 #define _LIBCPP_HAS_NO_STDOUT
 #endif
 
+// Some systems do not provide gets() in their C library, for security reasons.
+#if defined(_LIBCPP_MSVCRT) || (defined(__FreeBSD__) && __FreeBSD__ >= 13)
+#define _LIBCPP_C_HAS_NO_GETS
+#endif
+
 #if defined(__BIONIC__) || defined(__CloudABI__) ||                            \
     defined(__Fuchsia__) || defined(__wasi__) || defined(_LIBCPP_HAS_MUSL_LIBC)
 #define _LIBCPP_PROVIDES_DEFAULT_RUNE_TABLE
 #endif
 
 // Thread-unsafe functions such as strtok() and localtime()
 // are not available.
 #ifdef __CloudABI__

include/cstdio

 using ::remove;
 using ::rename;
 using ::tmpfile;
 using ::tmpnam;
 #endif
 
 #ifndef _LIBCPP_HAS_NO_STDIN
 using ::getchar;
-#if _LIBCPP_STD_VER <= 11 && !defined(_LIBCPP_MSVCRT)
+#if _LIBCPP_STD_VER <= 11 && !defined(_LIBCPP_C_HAS_NO_GETS)
 using ::gets;
 #endif
 using ::scanf;
 using ::vscanf;
 #endif
 
 #ifndef _LIBCPP_HAS_NO_STDOUT
 using ::printf;
 using ::putchar;
 using ::puts;
 using ::vprintf;
 #endif
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif  // _LIBCPP_CSTDIO