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> only using the reference after it has been invalidated can cause language-level UB.
When a reference is 'used' is much broader than you might expect; https://github.com/rust-lang/rust/issues/52898 is a SIGILL from merely creating a null reference, never 'using' it.
https://play.rust-lang.org/?version=stable&mode=debug&editio...
You can run through Miri and confirm there's no UB even though we're modifying `ptr`, whose lifetime has been extended beyond the length of the function.
However, Rust does have extra guarantees here which make this irrelevant to the pessimization problem in the linked article - you cannot ever legally convert a `&T` into a `&mut T` - this is always UB. This means that Rust guarantees that `example` does not modify `x` (unless e.g. it contains an `UnsafeCell`, like a `Mutex`'s contents), and so it does not need to defensively reload its value.