We did implement it out of tree at first. [0] Because Rust requires annotations on macros to distinguish them from built in syntax, they are less ergonomic to use. In general, we believe in adopting a path of beginning as an out-of-tree macro, and then deciding to elevate that to first class syntax if its clear that they will be extremely commonly used.

[0]: https://github.com/alexcrichton/futures-await