“Safety” as defined in the context of the Rust language - the absence of Undefined Behavior - is important 100% of the time. Without it, you are not writing programs in the language you think you are using.

That’s a convoluted way to say that UB is much, much worse than you think. A C program with UB is not a C program, but something else.

curious, where is the definition of "undefined behavior" for rust specified?

Here: https://doc.rust-lang.org/reference/behavior-considered-unde...

mmm, no, that's an explicitly incomplete list of informally-described observable behaviors that qualify as undefined -- which is fine and good! but quite far from a specification!