Here's how you return a hashmap from a function:

  fn buildMap(allocator: std.mem.Allocator) !std.AutoHashMap(u64, u64) {
      var result = std.AutoHashMap(u64, u64).init(allocator);
      errdefer result.deinit();
      try result.put(10, 100);
      try result.put(20, 200);
      return result;
  }

Your #1 option should be possible once Zig has comptime allocators -- it's on the roadmap, but not possible yet iirc.