Closure return type outlives guarantees

[jackh726]

See summary and rationale in rust-lang/rust#84385 and https://rust-lang.zulipchat.com/#narrow/stream/326866-t-types.2Fnominated/topic/.2384385.3A.20Enforce.20that.20.60closure.3A.20'a.60.20requires.20that.20.60closure_r.E2.80.A6

[nikomatsakis]

Scheduled for Nov 18