How to convert timestamp from UTC to arbitrary timezone with generics in Rust

Question

My Rust application stores timestamps in UTC, but needs to output in the local timezone. The following code shows is trivial to do this:

use chrono::{DateTime, Local, Utc};

fn main() {
    let utc = Utc::now();
    utc_to_local(utc);
}

fn utc_to_local(utc: DateTime<Utc>) {
    let local = DateTime::<Local>::from(utc);
    println!("{}", utc.to_string());
    println!("{}", local.to_string());
}

The output is as follows:

2021-08-26 14:55:31.590057252 UTC
2021-08-26 15:55:31.590057252 +01:00

However, I am not comfortable relying on environment settings like the local timezone in unit tests, and would like to test the formatting code (omitted here for simplicity) using a specific timezone or UTC. I am therefore trying to convert utc_to_local() to a generic function:

use chrono::{DateTime, Local, TimeZone, Utc};

fn main() {
    let utc = Utc::now();
    utc_to_local::<Local>(utc);
}

fn utc_to_local<Tz: TimeZone>(utc: DateTime<Utc>) {
    let local = DateTime::<Tz>::from(utc);
    println!("{}", utc.to_string());
    println!("{}", local.to_string());
}

This fails to compile:

error[E0308]: mismatched types
 --> src/main.rs:9:38
  |
8 | fn utc_to_local<Tz: TimeZone>(utc: DateTime<Utc>) {
  |                 -- this type parameter
9 |     let local = DateTime::<Tz>::from(utc);
  |                                      ^^^ expected type parameter `Tz`, found struct `Utc`
  |
  = note: expected struct `DateTime<Tz>`
             found struct `DateTime<Utc>`

Can anyone tell me where I'm going wrong please?

UPDATE

Following advice in the post linked by Svetlin, this works:

use chrono::{DateTime, Local, TimeZone, Utc, Datelike, Timelike};

fn main() {
    let utc = Utc::now();
    utc_to_local(utc, Local);
}

fn utc_to_local<Tz: TimeZone>(utc: DateTime<Utc>, zone: Tz) {
    let local = utc.with_timezone(&zone);
    println!("{:04}-{:02}-{:02} {:02}:{:02}:{:02}", utc.year(), utc.month(), utc.day(), utc.hour(), utc.minute(), utc.second());
    println!("{:04}-{:02}-{:02} {:02}:{:02}:{:02}", local.year(), local.month(), local.day(), local.hour(), local.minute(), local.second());
}

I had to do my own formatting instead of calling to_string(), because I got a compiler error:

error[E0599]: the method `to_string` exists for struct `DateTime<Tz>`, but its trait bounds were not satisfied
  --> src/main.rs:11:26
   |
11 |     println!("{}", local.to_string());
   |                          ^^^^^^^^^ method cannot be called on `DateTime<Tz>` due to unsatisfied trait bounds
   | 
  ::: /home/hwalters/.cargo/registry/src/github.com-1ecc6299db9ec823/chrono-0.4.19/src/datetime.rs:70:1
   |
70 | pub struct DateTime<Tz: TimeZone> {
   | ---------------------------------
   | |
   | doesn't satisfy `DateTime<Tz>: ToString`
   | doesn't satisfy `DateTime<Tz>: std::fmt::Display`
   |
   = note: the following trait bounds were not satisfied:
           `DateTime<Tz>: std::fmt::Display`
           which is required by `DateTime<Tz>: ToString`

But that's ok, I'm doing my own formatting anyway (that's the point of the unit test).