Make iana-time-zone dep and local_tz function optional

[lopopolo]

If strawlab/iana-time-zone#104 is merged, I'd like to depend on iana-time-zone directly so I can control which features are activated. I believe I'd be able to implement local_tz myself by calling:

let tz = tzdb::raw_tz_by_name(iana_time_zone::get_timezone()?);

Additionally, un-nesting iana-time-zone from this crate's dependency tree should improve compile times for apps depending on tzdb by allowing this crate (which takes 6 seconds to compile on my machine) to move earlier in the crate DAG.

[Kijewski]

I think the biggest "offender" is my macro

macro_rules! unwrap {
    ($($tt:tt)*) => {
        match $($tt)* {
            Ok(value) => value,
            Err(_) => {
                #[allow(unconditional_panic)]
                let err = [][0];
                err
            }
        }
    }
}

If I replace it with ↓, then the compile time is about 4 times faster in the simple test I did.

pub(crate) const fn time_zone_ref_new(
    transitions: &'static [tz::timezone::Transition],
    local_time_types: &'static [tz::LocalTimeType],
    leap_seconds: &'static [tz::timezone::LeapSecond],
    extra_rule: &'static Option<tz::timezone::TransitionRule>,
) -> TimeZoneRef<'static> {
    match TimeZoneRef::new(transitions, local_time_types, leap_seconds, extra_rule) {
        Ok(value) => value,
        Err(_) => {
            #[allow(unconditional_panic)]
            let err = [][0];
            err
        },
    }
}

I'll test it properly in the next few days.

Changing the default features would be a breaking change. But I could use breaking change this to increase the msrv from 1.56 to 1.57, so I could use panic!() in time_zone_ref_new(). I'll think about it. :)

[Kijewski]

Please have a look if the path I took in tzdb = "0.6.0-pre.1" would work for you:

• features = ["local"] enables function that use iana_time_zone.
• If included, iana_time_zone is always used with feature = ["fallback"]

[lopopolo]

This works great! thanks @Kijewski 🙇

I've gone ahead and upgraded to tzdb v0.6.0-pre.1.

[Kijewski]

Thank you for testing the pre-release! :) I just pushed the stable release 0.6.0.