Not knowing anything about rust, the problem in these type of case is making it rejecting that bad code without also rejecting valid programs. But maybe this is indeed easy to fix, I don't know.
A valid program would be exploiting undefined behavior in rust, but rust has no undefined behavior. Thus such a program must not exist.
Now a program may appear to work before the fix, but it was never a valid Rust program.
By Rice's theorem you cannot be sound and complete, i.e. accept all valid programs and reject all invalid ones. Most languages choose to be sound but not complete, i.e. reject all invalid programs but also some valid ones (C and C++ are the notable exception, although most languages have escape hatches). GP was saying that the fix might reject valid programs that compile today. This is true for some existing soundness bugs, but not necessarily for this one specifically.
That said, I was sarcastic.
Rice's theorem implies that it's impossible to precisely distinguish valid and invalid programs just by looking at their source code. Thus Rust rejects some valid programs in order to be sound (e.g. "if false { None.unwrap_unchecked() }").
In the case of the bug report in OP, I don't think that there are many valid programs in practice that exploit this soundness issue, so fixing it just means rejecting more invalid programs.
You’re ignoring the possibility that Rust semantics are sound, and the implementation doesn’t fully implement them. In that case, we do not have to find sound semantics.
That, I think, by far is the most likely option.
Reason: I’ve not seen a single comment claiming the example should compile, and zillions (implicitly) acknowledging this is not what Rust should do.
> but not necessarily for this one specifically
> Maybe, maybe not.
But "the Rust semantics" are not defined, formally. The closest we have is what Rust implements currently, and that was shown here to not be sound. Other than that, we maybe have vague ideas in the brains of some T-types members, which are not detailed enough to determine soundness (source: those type system people want to formalize the semantics so they will be).
And for the record, we know of other soundness issues where the likely resolution path will involve rejecting valid programs we accept now, and the only question is how many.