Auto merge of #139000 - compiler-errors:rigid-missing-item, r=lcnr

Rigidly project missing item due to guaranteed impossible sized predicate

This is a somewhat involved change, but it amounts to treating missing impl items due to guaranteed impossible where clauses (dyn/str/slice sized, cc #135480) as *rigid projections* rather than projecting to an error term, since that was preventing either reporting a proper error (in an empty param env) *or* successfully type checking the code (in the presence of trivially false where clauses).

Fixes rust-lang/rust#138970

r? `@lcnr` `@oli-obk`

Author: bors