Could you borrow a bit?

bascule:

or does the layout of &bool prevent that

This you can work out yourself.

There are usize::MAX possible places a bool could be, because align_of::() == 1.

&bool is non-null, so it has exactly usize::MAX possible value.

Thus all possible values are accounted for and there's no space to add bit information.

(Plus we wouldn't do that anyway, for other reasons.)