> It seems anything that affects trait solving is very complicated and is more likely to have bugs or combine non-trivially with other trait-solving features.

Yep and this is why many features die or linger on forever. Getting the trait solving working correctly across types and soundly across lifetimes is complicated enough to have killed several features previously (like specialization/min_specialization). It was the reason async trait took so long and why GAT were so important.