In C and C++, Undefined Behavior basically says it's safe for the compiler to assume this hasn't happened, because as a programmer it's your responsibility to ensure it's impossible. This might not have been so bad if completely normal things (such as signed integer overflow) weren't UB. It's not safe to assume the programmer did it correctly, and it never will be. So, these languages have tons of footguns.
UB also exists in Rust, however it's only supposed to exist in unsafe code, and even within unsafe code, you still benefit from Rust's great RAII, move semantics, deterministic destructors, and so on. It's still UB to index past the bounds of a memory region (well... uhh, insert Stacked Borrows or Tree Borrows here, this gets much more complicated, but you get the idea) but you can only do this unchecked from unsafe code, otherwise it'll always be checked and will panic if you attempt an out of bounds access.
When unsafe code is a special delineated section, you're less likely to forget to be very careful.
UB also exists in Rust, however it's only supposed to exist in unsafe code, and even within unsafe code, you still benefit from Rust's great RAII, move semantics, deterministic destructors, and so on. It's still UB to index past the bounds of a memory region (well... uhh, insert Stacked Borrows or Tree Borrows here, this gets much more complicated, but you get the idea) but you can only do this unchecked from unsafe code, otherwise it'll always be checked and will panic if you attempt an out of bounds access.
When unsafe code is a special delineated section, you're less likely to forget to be very careful.