Function is_niche_optimization_candidate 

fn is_niche_optimization_candidate<'tcx>(
    tcx: TyCtxt<'tcx>,
    typing_env: TypingEnv<'tcx>,
    ty: Ty<'tcx>,
) -> bool

A type is niche-optimization candidate iff:

• Is a zero-sized type with alignment 1 (a “1-ZST”).
• Is either a struct/tuple with no fields, or an enum with no variants.
• Does not have the #[non_exhaustive] attribute.