Better error message for late/early lifetime param mismatch
Rework the way we report early-/late-bound lifetime param mismatches to equate the trait and impl signatures using region variables, so that we can detect when a late-bound param is present in the signature in place of an early-bound param, or vice versa.
The diagnostic is a bit more technical, but it's more obviously clear to see what the problem is, even if it's not great at explaining how to fix it. I think this could be improved further, but I still think it's much better than what exists today.
Note to reviewer(s): I'd appreciate if we didn't bikeshed *too* much about this verbiage, b/c I hope it's clear that the old message sucked a lot. I'm happy to file bugs for interested new contributors to improve the messaging further.
Edit(fmease): Fixes https://github.com/rust-lang/rust/issues/33624.
a810f8ac89