apt – held broken packages, on 64b 20.04, how to fix?

I have run 20.04 64b since it was released. Awesome.
I’d never seen this “held broken packages” error until just this week.
I installed the Rust language via their online install script ( and later as one try to fix, via the downloaded tar), and the simple “hello” test program compile/link threw this error.
yet dpkg and synaptic-pkexec show none, still an apt install of glibc6-dev or build-essential gets the error.
I’d exhausted the Rust team’s suggestions, they suggested I try here.
I’d tried all solutions I could find; I get “no broken packages” in synaptic-pkexec then try the rustc hello.rs and get it again; apt, aptitude and dpkg in addition to synaptic-pkexec have not yielded the secret yet, but im not as seasoned in linux.

Q: Is there a location /log of the specific lib/ version conflicting and thus causing this install dependency error?