rust (original) (raw)

I have found this code confusing for years. I've always roughly
understood it, but never exactly. I just made my fourth(?) attempt and
finally cracked it.

This commit improves the comments. In particular, it explicitly
describes how you can't do a custom fold/visit of any type; there are
actually a handful of "types of interest" (e.g. Ty, Predicate,
Region, Const) that can be custom folded/visted, and all other types
just get a generic traversal. I think this was the part that eluded me
on all my prior attempts at understanding.

The commit also updates comments to account for some newer changes such
as the fallible/infallible folding distinction, does some minor
reorderings, and moves one impl to a better place.

r? @BoxyUwU