There are some proposed builder guidelines which indicate that &mut self should be preferred unless there’s a reason to do so otherwise, so I would personally prefer to see these as &mut self and perhaps add future convenience methods to handle the fold case better. For example we could add f.debug_map().entries(self) where the entries method took I: IntoIterator, ....
I’m kinda ambivalent about debug_tuple and debug_tuple_struct as one would also debug tuples themselves with debug_tuple (and an empty name), and I also don’t have a super strong opinion about field vs entry. I agree that the current terms match the colloquial usage, but it would perhaps be nice to select a name which is both consistent an colloquial (not sure if this exists though).