Hack fix for ICE as seen in CI

[t-nelson]

Problem

#23233 added BuiltinFeatureTransition
to Bank which triggers rust-lang/rust#92987 while
attempting to resolve Sync on BankRc in AccountsBackgroundService::new ala,

query stack during panic:
#0 [evaluate_obligation] evaluating trait selection obligation `bank::BankRc: core::marker::Sync`
#1 [typeck] type-checking `accounts_background_service::<impl at runtime/src/accounts_background_service.rs:358:1: 520:2>::new`
#2 [typeck_item_bodies] type-checking all item bodies
#3 [analysis] running analysis passes on this crate
end of query stack

Summary of Changes

Yoloing a Sync onto it avoids the auto trait evaluation and thus the ICE.

We should remove this when upgrading to Rust 1.60.0, where the bug has been
fixed by rust-lang/rust#93064

[t-nelson]

Which is safe because the compiler was cool with the same types in tuple form before #23233, right?

[jstarry]

True, but how about adding some extra checks to ensure that our assumptions aren't broken in the future. Here are my suggested changes: t-nelson#32

[t-nelson]

I'm not sure I understand the benefit. Commented over there

[jackcmay]

@jstarry what assumptions are you referring to that may be broken in the future?

[jstarry]

@jackcmay the unsafe impl can introduce undefined behavior because the compiler won't complain if they are added for types which are not thread-safe.