Good post! Worth noting that undefined and unsafe are not synonymous -- there's a subtle difference. UB is a property of the entire program; a line of code can trigger UB in the context of the whole program.
Whereas unsafe is a more local concept; things can be unsafe even if they don't trigger UB in the context of the whole program, unsafe is about whether or not they *could* trigger UB by only mutating safe code in the program.
The simplest example is that having a safe-marked function foo() that lets you dereference arbitrary pointers can still not be UB if you "use it correctly" (or don't use it at all!) but it is unsafe.
UB is a property common to c++ and rust (though in rust, barring some bugs, youre not supposed to be able to trigger it in "safe code"). Unsafe is a stronger property that Rust has that leads to compartmentalization of safety
UB is "as I currently use this, is this a problem?" whereas unsafe is "is there _any way_ to use this to cause problems?"
Incidentally the lack of this distinction in C++ (everything is technically unsafe in c++ since there's no compartmentalization mechanism to help you define "unsafe") often makes it hard to compare concepts of safety.
