(the person who posted the article here isn't the author (me))

The bugs in part 1 all around using higher ranked trait bounds. I'd disagree with the characterization that they're "feature requests": five years ago, yes, I would agree, but this entire area of the compiler needs to be bug-free for an upcoming feature (GATs) anyway, and indeed, the issues I found were often fixed by people working on fixing related GAT bugs. Ultimately, my use of higher ranked trait bounds is an attempt to emulate some of what GATs get you in stable Rust, so it's not surprising that the bugs are in the same area of the code.