The port now uses an unofficial bootstrap of Rust 1.17.0 and Cargo 0.18.0 to build on FreeBSD/aarch64.
Here are my notes on creating the bootstraps:
https://gist.github.com/dumbbell/b587da50ef014078da9e732a4331ebad
The port's Makefile was changed to:
* Allow to override the versions and directories of the bootstrap archives
* Patch the bootstrap script and manifest with those versions/directories
Beside changes to support aarch64, the port received the following fixes:
* Pass `python` path to the configure script.
* Fix and simplify the extraction of rust-std bootstrap by re-using existing variables.
* Make sure tar(1) doesn't produce sparse file entries when it recreates the rust-std bootstrap because Python 2's tarfile module doesn't support them.