He and I both agreed UB could show up in Rust although it defaults on quite a bit of safety. Far as compiler maturity, he could use that argument but counters himself with machine code verification due to no trust in compilers. As in, what was point of being up Rust compiler quality if he doesnt trust C's either.

Only valid point he has is that a system language needs tools to produce and/or test machine code output for source equivalence. Claims Rust doesnt have that but that's outside my knowledge. I know Ada, SPARK, CompCert C, and a Java subset have methods available.