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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.
For more information about how rustc works, see the rustc dev guide.