Function try_fn_sig 

fn try_fn_sig<'tcx>(
    tcx: TyCtxt<'tcx>,
    did: LocalDefId,
    attr_span: Span,
) -> Option<EarlyBinder<'tcx, PolyFnSig<'tcx>>>

In some cases, attributes are only valid on functions, but it’s the check_attr pass that checks that they aren’t used anywhere else, rather than this module. In these cases, we bail from performing further checks that are only meaningful for functions (such as calling fn_sig, which ICEs if given a non-function). We also report a delayed bug, just in case check_attr isn’t doing its job.