Function type_allowed_to_implement_const_param_ty 

pub fn type_allowed_to_implement_const_param_ty<'tcx>(
    tcx: TyCtxt<'tcx>,
    param_env: ParamEnv<'tcx>,
    self_type: Ty<'tcx>,
    parent_cause: ObligationCause<'tcx>,
) -> Result<(), ConstParamTyImplementationError<'tcx>>

Checks that the fields of the type (an ADT) all implement (Unsized?)ConstParamTy.

If fields don’t implement (Unsized?)ConstParamTy, return an error containing a list of those violating fields.

If it’s not an ADT, int ty, bool or char, returns Err(NotAnAdtOrBuiltinAllowed).