RUST_TYPE suggestion

[Greg Troxel <gdt%lexort.com@localhost>]

I am in a situation where I want to use RUST_TYPE=bin generally (to
avoid the cost of building it), but src on a few machines because they
need super-recent rust (due to the usual issues :-().

I find it awkward to have to manage RUST_TYPE in mk.conf.  I know rust
and rust-bin have different package names (now).  It would be nice if
dependencies on rust could be satisfied by either rust or rust-bin if
installed and high enough, and that way I could just install wip/rustNEW
on the machines where I need it.  I suspect many others have the same
sorts of problems.

Does this may sense?   Is anyone rust-clueful in the mood to make this
improvement?