* Dereferencing null or dangling pointers
* Reading uninitialized memory
* Breaking the pointer aliasing rules
* Producing invalid primitive values:
- dangling/null references
- a bool that isn't 0 or 1
- an undefined enum discriminant
- a char outside the ranges [0x0, 0xD7FF] and [0xE000, 0x10FFFF]
- A non-utf8 str
* Unwinding into another language
* Causing a data race
These are all guarantees unsafe code must uphold or there are no guarantees anymore.
IIRC LLVM IR also has UBs.
UB isn't something "caused" by optimizations, it's something that exists in the code before optimizations, optimizations can just trigger nasal demons. So you shouldn't be able to write UB in safe Rust assuming no compiler bugs.
(And assuming that any unsafe libraries being leveraged are bug free)
That Friendly C proposal is terrific, I'd dearly love to use that. Basically, use sane and conservative optimizations for most code, with the option of using aggressive optimizations for hotspots. Kind of similar to Rust -- mostly safe by default, more dangerous stuff available when you need it.