The following patch, which I've tested only on my private branch of
Guix, dramatically reduces the amount of memory and time required to
build Rust.

With this patch applied, I'm able to build Rust on a Thinkpad X200 with
only 4 GB of RAM using both cores.  Without it, I'm unable to build Rust
at all, even with parallel builds disabled, running in a bare text-only
terminal, and with every nonessential daemon stopped.  When I tried, a
single compiler process used more than 9 GB of virtual memory with heavy
thrashing before the OOM killer finally put it out of its misery.

Moreover, with this patch the builds are dramatically faster than I've
previously observed.  In my earlier bootstraps of Rust (up to
rust-1.51), it took about 5 hours to build each compiler on my machine.
Now it takes only about 2 hours per compiler, except for the initial
bootstrap (1.54) which takes about 4 hours.

The following patch is meant to be applied to the 'staging' branch, but
I've only tested it on my own private branch, which is based on very old
'master' (April 2021) and is thus quite different by now.

I offer it here in case it might be of interest, although it hasn't been
tested on contemporary Guix.  I'm not subscribed to this mailing list,
so please make sure to CC me on any replies you'd like me to see.

     Regards,
       Mark