Add #[must_use] to some methods

[aceArt-GmbH]

Problem:

some_date.checked_add_days(Days::new(1)).unwrap();

Expected:

some_date to change by 1 day, or a warning to be thrown.
Currently neither happens.

After:

warning: unused return value of `NaiveDate::checked_add_days` that must be used
   --> src/main.rs:10:9
    |
10  |         some_date.checked_add_days(Days::new(1));
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(unused_must_use)]` on by default

Adding #[must_use] indicates that the method does not change the original object and the return value has to be used.

As I am not using chrono too much, I only added must_use to some methods and not all that should have it

[jtmoon79]

This is a good idea.
I reviewed other code. It seems adding #[must_use] in only one file might be inconsistent. Here is one of many examples.

@djc my own recommendation is to allow this PR and then create a follow-up Issue like add must_use attribute that addresses the remaining code base. So this partially-complete work is not forgotten.

Or maybe @aceArt-GmbH would like to make an attempt to apply #[must_use] to the entire code-base for this PR? IMO, #[must_use] is only needed for pub functions that take &self (and of course that 1. return something new 2. don't modify self).

[aceArt-GmbH]

thanks @jtmoon79 for your feedback.
I added #[must_use] to all pub fn. Didn't see any that changed self

[jtmoon79]

I visually reviewed the changes. It all looks correct to me.

This is a great usability improvement. Thanks so much @aceArt-GmbH !

[djc]

Thanks for working on this! Can this be put on the 0.4.x branch? I think that would probably make sense.

[aceArt-GmbH]

I switched the branch

[djc]

Thanks!