Oh, I missed the last line... why did you need to copy this?

src/sys/contrib/zstd/lib/common/xxhash.h:1354

When __STDC_VERSION__ >= 201112L, we need assert.h. However, header files like stdio.h is not included from src/include but we mock userspace header files by creating an equivalent to src/sys/contrib/zstd/lib/freebsd. This has been done since zstd was first imported to kernel in d3692a4dee4d6ff0926365bcf95bbedb039ea70d.

assert() is a macro function so I though copying and paste it to src/sys/contrib/zstd/lib/freebsd would be better than integrating assert() in zstd_kfreebsd.h

If this is really needed, could this file just`#include "../../../../include/assert.h" instead of copying the file?

I guess the alternative would be to modify the xxhash.h file to use _Static_assert instead of including <assert.h>?

IMO we should separate kernel-specific codes from user space as sys/contrib is separate from contrib.
I'm trying not to modify files fetched from zstd release. In this way, it is much easier to import next releases of zstd.

Actually, what I'd like to do for the kernel is just go ahead and define static_assert() in <sys/systm.h> (really <sys/kassert.h>) instead of requiring the use of _Static_assert in the kernel. The intention of C11 from my reading is that code should really be using static_assert and that _Static_assert is just a shim so that <assert.h> can define the wrapper macro. Honestly I'd prefer it if for the kernel if we eventually too a similar route with defining things like alignof in the kernel always and not having to use the more obfuscated _Alignof type keywords. In the case of static_assert fixing that would probably mean we wouldn't need this hack at all for zstd to compile in the kernel.

_Static_assert() is a compiler extension in clang, so I guess there is no need to redefine it.

sys/cdefs.h

#if !__has_extension(c_static_assert)
#if (defined(__cplusplus) && __cplusplus >= 201103L) || \
    __has_extension(cxx_static_assert)
#define	_Static_assert(x, y)	static_assert(x, y)
#elif __GNUC_PREREQ__(4,6) && !defined(__cplusplus)
/* Nothing, gcc 4.6 and higher has _Static_assert built-in */
#elif defined(__COUNTER__)
...