Function rustc_middle::ty::context::tls::with_opt

pub fn with_opt<F, R>(f: F) -> R where
    F: for<'tcx> FnOnce(Option<TyCtxt<'tcx>>) -> R, 

Allows access to the TyCtxt in the current ImplicitCtxt. The closure is passed None if there is no ImplicitCtxt available.