Progress on #13405, which asks for an explicit characterisation of the
equivalence relation like the one given here.
Also progress on #11895, because we're using the term "build trace
entry" instead of "realisation".
Mention #9259, a future work item.
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
Makes the behavoral change of #13263 without the underlying refactor.
Hopefully this clearly safe from a perf and GC perspective, and will
make it easier to benchmark #13263.
