Many of the non-optimized ones (eg .nextUp, .binade, .ulp) seem to already be marked with:
@_inlineable // FIXME(sil-serialize-all)
While the optimized examples (eg .magnitude, .squareRoot()) seem to be marked also with @_transparent:
@_inlineable // FIXME(sil-serialize-all)
@_transparent
(I'm just looking here and here and am not particularly sure how to interpret things.)