In style(9) there is no space after the cast. Same comment below.

Which compiler is generating this warning? It seems pretty clear that this initialization isn't needed.

sys/param.h should be included first, per style(9).

Same here.

When including sys/param.h, do not also include sys/types.h per style(9).

It would be better to assert sz > 0.

In style(9) there is no space after the cast. Same comment below.

Ah, its one of those small annoying differences between illumos and FreeBSD styles:D will fix it.

Which compiler is generating this warning? It seems pretty clear that this initialization isn't needed.

gcc-14.

zfs/dsl.c: In function 'dsl_dir_alloc':
zfs/dsl.c:423:42: error: 'parent' may be used uninitialized [-Werror=maybe-uninitialized]
  423 |         dir->phys->dd_parent_obj = parent->dirid;
      |                                    ~~~~~~^~~~~~~
zfs/dsl.c:354:30: note: 'parent' was declared here
  354 |         zfs_dsl_dir_t *dir, *parent;
      |                              ^~~~~~
cc1: all warnings being treated as errors

Without discussing if thats possible scenario or not (static analyzers are only so good), we can hit it on first iteration if nextdir does not contain '/' - in which case strsep() will set nextdir to NULL, we break out of the loop and have parent with random content from the stack. I'd rather blow up with NULL pointer dereference if thats the case;)

maybe. I personally do not really like it because:

#ifdef NDEBUG
#define assert(e)       ((void)0)