Rollup merge of #154778 - BennoLossin:frt-invariant, r=oli-obk

make field representing types invariant over the base type

We probably don't want to have any subtype relationship between any FRTs.

Reported-by: @dvdhrm "David Rheinsberg" <david@readahead.eu>
Link: https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/Multiple.20LockedBy.20with.20Single.20Lock/near/579275908

r? @oli-obk

Author: JonathanBrouwer