I can't tell if you're trolling but `unsafe { crash() }` is safe from the compiler's perspective. Otherwise you wouldn't be able to achieve anything in 'safe' rust, even print to stdout.
I think its a good question, just because the whole UB thing is such an ideological shibboleth.
Maybe its better to think about this in the reverse, where C and C++ has 'defined behavior', but unsafe rust intentionally does not, its just whatever the complier and platform lets you get away with. Ultimately its still just a computer which stores values in memory and jumps to subroutines.
Every language has defined behavior. It's what you expect to happen through a program's execution. Sometimes there will be multiple possibilities, but you can still define them regardless. Laying this out explicitly is the purpose of a standard.
Undefined behavior is everything else. C and C++ are relatively unique in that their standards explicitly say "combining these constructs in this way is undefined", and we call those cases explicit UB. There's also a larger universe of implicit UB that standards omit. Most (all?) languages have implicit UB, even if they lack the explicit stuff. What happens when you get ENOMEM is a common one.
Rust does something similar to C/C++ and lists a bunch of UB that's only possible with incorrect code in unsafe blocks. Correct code placed in an unsafe block remains defined, as does code without unsafe (up to compiler/language bugs).
Yeah, if I understand correctly, the Rust project has no intention to formally 'define' what unsafe actually does, so its very implicit. Could be anything... so it's the Does It Work? standard.
Maybe its better to think about this in the reverse, where C and C++ has 'defined behavior', but unsafe rust intentionally does not, its just whatever the complier and platform lets you get away with. Ultimately its still just a computer which stores values in memory and jumps to subroutines.