I’d like to add that there’s a difference between unsafe and unspecified behavior. Sometimes I’d like the compiler to produce my unsafe code that has specified behavior. In this case, I want the compiler to produce exactly that unsafe behavior that was specified according to the language semantics.

Especially when developing a kernel or in an embedded system, an example would be code that references a pointer from a hardcoded constant address. Perhaps this code then performs pointer arithmetic to access other addresses. It’s clear what the code should literally do, but it’s quite an unsafe thing to do unless you as the developer have some special knowledge that you know the address is accessible and contains data that makes sense to be processed in such a manner. This can be the case when interacting directly with registers representing some physical device or peripheral, but of course, there’s nothing in the language that would suggest doing this is safe. It’s making dangerous assumptions that are not enforced as part of the program. Those assumptions are only true in the program is running on the hardware that makes this a valid thing to do, where that magical address and offsets to that address do represent something I can read in memory.

Of course, pointer arithmetic can be quite dangerous, but I think the point still stands that behavior can be specified and unsafe in a sense.