Set year/month/day all at once

[zyxw59]

With the Datelike trait, there are methods to return a new date with a different year/month/day. However, these do not provide the ability to easily set more than one of those at once, since the intermediate value might not be valid.

e.g.

let a = NaiveDate::from_ymd(2019, 1, 29);
let b = NaiveDate::from_ymd(2020, 2, 29);

fn with_year_month_1<D: Datelike>(date: &D, year: i32, month: u32) -> Option<D> {
  date.with_year(year)?.with_month(month)?
}

fn with_year_month_2<D: Datelike><date: &D, year: i32, month: u32) -> Option<D> {
  date.with_month(month)?.with_year(year)?
}

with_year_month_1(a, 2020, 2); // -> Some(2020-02-29)
with_year_month_2(a, 2020, 2); // -> None (2019-02-29 is invalid)
with_year_month_1(b, 2019, 1); // -> None (2019-02-29 is invalid)
with_year_month_2(b, 2019, 1); // -> Some(2020-01-29)

It would be useful to have a method which applied multiple of these methods at once, without checking for validity of intermediate dates.

[pitdicker]

If you want to replace two of the three parts of a date, is it not just as easy to create a new date?
Example:

use chrono::{NaiveDate, Datelike};

let a = NaiveDate::from_ymd(2019, 1, 29);
let b = NaiveDate::from_ymd(2020, 2, 29);

NaiveDate::from_ymd_opt(2020, 2, a.day()); // Some(2020-02-29)
NaiveDate::from_ymd_opt(2019, 1, b.day()); // Some(2019-01-29)

This is less tricky to get right.

[jtmoon79]

At first glance, I'm hesitant to add more API "surface area" for something that is fairly trivial for the user to do.

However, the OP makes a good point that there is this not-immediately-obvious "gotcha" that will probably trip-up others.
While @pitdicker provide some good alternate examples of how to achieve the desired end, the "gotcha" still exists and it could easily be protected against with fairly trivial new functions.

I recommend re-opening this Issue. (My two cents)

[pitdicker]

Maybe we should add some documentation to Datelike::with_month or even to the Datelike trait itself on how to change multiple values at once.

[jtmoon79]

Maybe we should add some documentation to Datelike::with_month or even to the Datelike trait itself on how to change multiple values at once.

My preference is the OPs suggestion but documentation is better than nothing.

[pitdicker]

Added some documentation in #1199.