unix: use a type equivalent to reflect.SliceHeader to manipulate slices

The regression test included in this change verifies that the type is,
in fact, equivalent, while allowing the actual header definitions to
avoid importing the (relatively heavy) "reflect" package itself.

This change is loosely based on Keyan Pishdadian's draft in CL 230557.

For golang/go#37805

Change-Id: I998c69cdeb852154cd66ab5fdaa542a6f19666a2
Reviewed-on: https://go-review.googlesource.com/c/sys/+/231177
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_darwin.1_13.go

@@ -6,7 +6,11 @@
 
 package unix
 
-import "unsafe"
+import (
+	"unsafe"
+
+	"golang.org/x/sys/internal/unsafeheader"
+)
 
 //sys	closedir(dir uintptr) (err error)
 //sys	readdir_r(dir uintptr, entry *Dirent, result **Dirent) (res Errno)
@@ -71,6 +75,7 @@ func Getdirentries(fd int, buf []byte, basep *uintptr) (n int, err error) {
 			cnt++
 			continue
 		}
+
 		reclen := int(entry.Reclen)
 		if reclen > len(buf) {
 			// Not enough room. Return for now.
@@ -79,13 +84,15 @@ func Getdirentries(fd int, buf []byte, basep *uintptr) (n int, err error) {
 			// restarting is O(n^2) in the length of the directory. Oh well.
 			break
 		}
+
 		// Copy entry into return buffer.
-		s := struct {
-			ptr unsafe.Pointer
-			siz int
-			cap int
-		}{ptr: unsafe.Pointer(&entry), siz: reclen, cap: reclen}
-		copy(buf, *(*[]byte)(unsafe.Pointer(&s)))
+		var s []byte
+		hdr := (*unsafeheader.Slice)(unsafe.Pointer(&s))
+		hdr.Data = unsafe.Pointer(&entry)
+		hdr.Cap = reclen
+		hdr.Len = reclen
+		copy(buf, s)
+
 		buf = buf[reclen:]
 		n += reclen
 		cnt++