Ownership Inventory #1 Q2 is wrong about UB

[fee1-dead]

• I have searched open and closed issues and pull requests for duplicates, using these search terms:
  • Ownership Inventory
• I have checked the latest main branch to see if this has already been fixed, in this file:
  • https://github.com/cognitive-engineering-lab/rust-book/blob/main/quizzes/ch06-04-inventory.toml#L66-L84

URL to the section(s) of the book with this problem:

https://rust-book.cs.brown.edu/ch06-04-inventory.html

Description of the problem:

Per https://doc.rust-lang.org/nomicon/what-unsafe-does.html, Undefined Behavior arises from producing an invalid value, which means the let s = make_separator(""); choice would also cause UB. We should set the record straight and correctly tell people that producing invalid values causes UBs even though the code might be harmless when you run it. A reference can't ever be dangling, null, or point to an invalid value. That's the contract of the type. If you ever violate the contract (even without reading the value from the reference) bad things are allowed to happen.

Suggested fix:

Make it a correct option, or remove that as a distractor.

[Nilstrieb]

producing a value

Or to be more precise, it arises from doing a typed copy of an invalid value. The assignment to s is a typed copy, and references have a validity invariant that they point to a valid allocation.

It's up to debate whether the referent needs to be valid, so a reference to uninitialized memory might or might not be UB.

[willcrichton]

This is the same issue reported in #33. I've gone back and forth on this issue (see the linked thread), but given my principle of being consistent with Miri, I've decided to remove the distractor.