Function nix::unistd::unlinkat  
pub fn unlinkat<P: ?Sized + NixPath>(dirfd: Option<RawFd>, path: &P, flag: UnlinkatFlags) -> Result<()>The portability is definied by: 
#[cfg(not(target_os = "redox"))]Documentation
Remove a directory entry
In the case of a relative path, the directory entry to be removed is determined relative to
the directory associated with the file descriptor dirfd or the current working directory
if dirfd is None. In the case of an absolute path dirfd is ignored. If flag is
UnlinkatFlags::RemoveDir then removal of the directory entry specified by dirfd and path
is performed.
References
See also unlinkat(2)