Function rustc_middle::ty::context::tls::with_context_opt

pub fn with_context_opt<F, R>(f: F) -> R where
    F: for<'a, 'tcx> FnOnce(Option<&ImplicitCtxt<'a, 'tcx>>) -> R, 

Allows access to the current ImplicitCtxt in a closure if one is available.