Rollup merge of rust-lang#26963 - Manishearth:improve-diag, r=steveklabnik

I'll be adding more commits to this PR as the weekend progresses. Was hoping to make this a mega-PR, but getting some eyes on this early would be nice too.

r? @steveklabnik

r? @eddyb on the object safety bits

cc @michaelsproul

Part of rust-lang#24407

Author: Manishearth