I've looked for this optimisation, and while it makes sense to me (Infallible is unhabitable ==> s: Option<Infallible> can only exist if s = None ==> all values in vector must be of the same value None that is known ahead of time ==> store a counter of how many Nones are in the vector instead of each None as an entry into a traditional vec), I cannot find any trace of such optimisation, whether by reading into the bytes backing the vector (with rustc -O / -C opt-level=3 to ensure this opt is triggered), or by calling `mem::size_of::<Vec<Option<std::convert::Infallible>>>()`.