Rust port: CARGO_CRATES in a file

Hello,

I'm looking for the correct way of using CARGO_CRATES list in a file
instead of Makefile.

I've found an example in www/so port:
---
USES=           cargo ssl
...
.include "${.CURDIR}/bsd.so.mk"
---
where bsd.so.mk is the CARGO_CRATES file list.

What you think about it? IMO a CARGO_CRATES file is good when we have a big
list.
CARGO_CRATES_FILE will be very nice option.

Thanks,

Nuno Teixeira