You can write GPL rust code, you can also write non-gpl rust code. Gnu coreutils are gpl, if I choose to write functionality compatible with coreutils from scratch in rust I can relicense that as I please (ie not GPL) OP is postulating the driver behind rust rewrites is not for the language features but to allow coreutils functionality to be relicensed as closed source software.
You can write GPL rust code, you can also write non-gpl rust code. Gnu coreutils are gpl, if I choose to write functionality compatible with coreutils from scratch in rust I can relicense that as I please (ie not GPL) OP is postulating the driver behind rust rewrites is not for the language features but to allow coreutils functionality to be relicensed as closed source software.