Define variable of type TimeZone - rust

Question

The following compiles and runs fine:
use chrono_tz::Tz;
use chrono::{TimeZone, NaiveDate};
use arrow2::temporal_conversions::parse_offset;
fn my_func(tz: &str) -> (){
let ndt = NaiveDate::from_ymd_opt(2018, 9, 28).unwrap().and_hms_opt(2, 30, 0).unwrap();
match parse_offset(&tz) {
Ok(time_zone) => {
println!("converted: {:?}", time_zone.from_utc_datetime(&ndt));
},
Err(_) => match tz.parse::<Tz>() {
Ok(time_zone) => {
println!("converted: {:?}", time_zone.from_utc_datetime(&ndt));
}
Err(error) => panic!("Problem opening the file: {:?}", error)
},
};
}
fn main() {
let time_zone = "Asia/Seoul";
my_func(&time_zone);
}
Noticed though that I'm doing
println!("converted: {:?}", time_zone.from_utc_datetime(&ndt));
twice.
I've tried refactoring to
use chrono_tz::Tz;
use chrono::{TimeZone, NaiveDate};
use arrow2::temporal_conversions::parse_offset;
fn my_func(tz: &str) -> (){
let ndt = NaiveDate::from_ymd_opt(2018, 9, 28).unwrap().and_hms_opt(2, 30, 0).unwrap();
let parsed_time_zone: TimeZone = match parse_offset(&tz) {
Ok(time_zone) => {
time_zone
},
Err(_) => match tz.parse::<Tz>() {
Ok(time_zone) => {
time_zone
}
Err(error) => panic!("Problem opening the file: {:?}", error)
},
};
println!("converted: {:?}", parsed_time_zone.from_utc_datetime(&ndt));
}
fn main() {
let time_zone = "Asia/Seoul";
my_func(&time_zone);
}
and get a long error:
error[E0782]: trait objects must include the `dyn` keyword
--> src/main.rs:7:27
|
7 | let parsed_time_zone: TimeZone = match parse_offset(&tz) {
| ^^^^^^^^
|
help: add `dyn` keyword before this trait
|
7 | let parsed_time_zone: dyn TimeZone = match parse_offset(&tz) {
| +++
error[E0191]: the value of the associated type `Offset` (from trait `TimeZone`) must be specified
--> src/main.rs:7:27
|
7 | let parsed_time_zone: TimeZone = match parse_offset(&tz) {
| ^^^^^^^^ help: specify the associated type: `TimeZone<Offset = Type>`
error[E0038]: the trait `TimeZone` cannot be made into an object
--> src/main.rs:7:27
|
7 | let parsed_time_zone: TimeZone = match parse_offset(&tz) {
| ^^^^^^^^ `TimeZone` cannot be made into an object
|
= note: the trait cannot be made into an object because it requires `Self: Sized`
= note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
error[E0308]: mismatched types
--> src/main.rs:9:13
|
9 | time_zone
| ^^^^^^^^^ expected trait object `dyn TimeZone`, found struct `FixedOffset`
|
= note: expected trait object `dyn TimeZone`
found struct `FixedOffset`
error[E0308]: mismatched types
--> src/main.rs:13:17
|
13 | time_zone
| ^^^^^^^^^ expected trait object `dyn TimeZone`, found enum `Tz`
|
= note: expected trait object `dyn TimeZone`
found enum `Tz`
error: the `from_utc_datetime` method cannot be invoked on a trait object
--> src/main.rs:18:50
|
18 | println!("converted: {:?}", parsed_time_zone.from_utc_datetime(&ndt));
| ^^^^^^^^^^^^^^^^^
|
::: /home/marcogorelli/.cargo/registry/src/github.com-1ecc6299db9ec823/chrono-0.4.23/src/offset/mod.rs:205:21
|
205 | pub trait TimeZone: Sized + Clone {
| ----- this has a `Sized` requirement
Some errors have detailed explanations: E0038, E0191, E0308, E0782.
For more information about an error, try `rustc --explain E0038`.
error: could not compile `tmp` due to 6 previous errors
I've tried the suggestion to do
let parsed_time_zone: dyn TimeZone = match parse_offset(&tz) {
but then it still errors with
error[E0191]: the value of the associated type `Offset` (from trait `TimeZone`) must be specified
--> src/main.rs:7:31
|
7 | let parsed_time_zone: dyn TimeZone = match parse_offset(&tz) {
| ^^^^^^^^ help: specify the associated type: `TimeZone<Offset = Type>`
How can I just parse tz as TimeZone, without specified whether it's Tz or FixedOffset, and then use it in the rest of the function?

Answer 1

You can't refactor this easily. Rust is strongly typed, and your two time_zone variables aren't the same type. One is chrono::FixedOffset, the other is chrono_tz::Tz.
If you do want to store both of them in the same variable, you have two possibilities:
trait object references (&dyn TimeZone)
Boxed trait objects (Box<dyn TimeZone).
dyn Timezone itself cannot directly be the type of a variable. It doesn't have a known size, because it could be any actual type of any size. Therefore you need one of the indirections above.
Both are non-ideal for your case though:
&dyn TimeZone won't work because someone has to own the actual object. You can't return a reference to a variable of an inner scope, as it gets dropped earlier than the reference does.
Box<dyn TimeZone> would work, no problem, but it introduces overhead: It will cause a heap allocation where the actual object will be put. The reason, as described earlier, is that a dyn object doesn't have a known size. Therefore, with Box, the actual object gets moved to the heap, and on the stack is the Box object, which has a known size.
So what I would do is: don't worry. The code duplication you have here is really minimal. I don't think it's worth the trouble.
Further detail I noticed after I wrote this answer:
TimeZone is : Sized, which means it isn't object safe and dyn TimeZone is disallowed entirely. So not even Box<dyn TimeZone> would work.