TimeZone should be able to be used as a trait object

[mleonhard]

Hi chrono maintainers,
Is there any way to store a TimeZone in an Arc?

Best,
Michael

// src/main.rs
struct TimezoneHolder {
    tz: std::sync::Arc<dyn chrono::TimeZone>,
}

pub fn main() -> () {
    let _holder = TimezoneHolder { tz: std::sync::Arc::new(chrono::Utc) };
}

# Cargo.toml
[package]
name = "example"
version = "0.1.0"
edition = "2018"

[dependencies]
chrono = "0.4.11"

$ cargo run
   Compiling example v0.1.0 (/Users/user/rust-in-production/example)
error[E0191]: the value of the associated type `Offset` (from trait `chrono::offset::TimeZone`) must be specified
 --> src/main.rs:3:28
  |
3 |     tz: std::sync::Arc<dyn chrono::TimeZone>,
  |                            ^^^^^^^^^^^^^^^^ help: specify the associated type: `chrono::TimeZone<Offset = Type>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0191`.
error: could not compile `example`.

To learn more, run the command again with --verbose.

[mleonhard]

The solution is to use chrono_tz::Tz enum values instead of the chrono::TimeZone trait.

# Cargo.toml
[package]
name = "example"
version = "0.1.0"
edition = "2018"

[dependencies]
chrono = "0.4.11"
chrono-tz = "0.5.2"

// src/main.rs
use std::sync::Arc;

use chrono_tz::Tz;

#[derive(Debug)]
struct TimezoneHolder {
    value: Arc<Tz>,
}

pub fn main() -> () {
    let holder = TimezoneHolder { value: Arc::new(Tz::America__Los_Angeles) };
    println!("holder is {:?}", holder);
}

$ cargo run
holder is TimezoneHolder { value: America/Los_Angeles }

[quodlibetor]

Yeah unfortunately I believe that the inner Offset inside of TimeZone means it cannot be used as a trait object. I'd like to get rid of the Offset associated type, but I haven't figured out how to make it work yet.

[WhyNotHugo]

See also: #822