Function check_field 

pub(crate) fn check_field<'db>(
    db: &'db dyn Db,
    field: SymField<'db>,
) -> Errors<Binder<'db, Binder<'db, SymTy<'db>>>>

Check the type of a field. The returned type has two binders, the outer binder is the class, the inner binder is the self place.