@cramertj provides a good approach in his comment. To be specific, you might want to start by:

Add a new field to the AssociatedTypeNormalizer struct for storing already-seen types (values of ty parameter to fold_ty) that you're folding with. (I think @cramertj's suggestion of using the def-ids for the types would work in most cases, but may fail in some complex edge cases. Maybe he can chip in here.) A HashSet (FxHashSet?) may be the best type for this.
At the top of the fold_ty function, check for the newly-encountered type in the set of already-seen types. If you find it, raise the error; if not, add the type to the set and continue.
Model the error reporting on one of the examples @estebank gave in his above comment. Probably the second one is a better prototype. Here is the code for reporting it; I suggest creating a new method that's pretty similar to that one.
Create a test for this issue under src/tests/ui/issues/issue-47659 with an appropriate minimal reproducible case. Make sure you "bless" the test once everything is running.

The rustc development guide is pretty handy if you're curious about how the compiler fits together, or what purpose a particular type or function serves, in many cases. Still, if you have any difficulties with the above, post back here, or hop onto the #compiler channel on Discord. Good luck!

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Improve error message for infinitely recursive impl Trait type #47659

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