Function try_par_for_each_in 

pub fn try_par_for_each_in<T: IntoIterator, E: DynSend>(
    t: T,
    for_each: impl Fn(&<T as IntoIterator>::Item) -> Result<(), E> + DynSync + DynSend,
) -> Result<(), E>
where
    <T as IntoIterator>::Item: DynSend,

This runs for_each in parallel for each iterator item. If one or more of the for_each calls returns Err, the function will also return Err. The error returned will be non-deterministic, but this is expected to be used with ErrorGuaranteed which are all equivalent.