feat: Implement dt.days_in_month function

[mrkn]

Implements dt.days_in_month() for Date and Datetime columns.

Uses the existing DAYS_PER_MONTH array and is_leap_year() function for performance instead of chrono's num_days_in_month().

>>> df = pl.DataFrame({"date": [date(2001, 1, 1), date(2000, 2, 1)]})
>>> df.with_columns(pl.col("date").dt.days_in_month())
┌────────────┬───────────────┐
│ date       ┆ days_in_month │
│ ---        ┆ ---           │
│ date       ┆ i8            │
╞════════════╪═══════════════╡
│ 2001-01-01 ┆ 31            │
│ 2000-02-01 ┆ 29            │
└────────────┴───────────────┘

This is my first contribution to a Rust project. As a Rust beginner, I'd appreciate thorough code review and feedback to help me improve.

Fixes #14193

[codecov]

Codecov Report

❌ Patch coverage is 93.65079% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.46%. Comparing base (ee338f7) to head (a608607).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...ates/polars-plan/src/dsl/function_expr/datetime.rs	0.00%	1 Missing ⚠️
...ars-plan/src/plans/aexpr/function_expr/datetime.rs	83.33%	1 Missing ⚠️
.../polars-python/src/lazyframe/visitor/expr_nodes.rs	0.00%	1 Missing ⚠️
crates/polars-time/src/series/mod.rs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23119      +/-   ##
==========================================
+ Coverage   81.44%   81.46%   +0.01%     
==========================================
  Files        1655     1663       +8     
  Lines      224589   224634      +45     
  Branches     2881     2881              
==========================================
+ Hits       182923   182994      +71     
+ Misses      40949    40923      -26     
  Partials      717      717              

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[MarcoGorelli]

thanks for your pr

this looks like it's off to a good start, but quite a few lines are reported as uncovered - could you make sure your tests cover them all (or at least, nearly all?) please?

[mrkn]

@MarcoGorelli Thanks for the feedback! I’ll work on improving the test coverage.

[mrkn]

@MarcoGorelli Is the coverage in polars-plan/src/dsl/function_expr/datetime.rs something I should look into? I believe this can be covered by checking the result of repr(pl.col("x").dt.days_in_month()).

[MarcoGorelli]

thanks for updating!

could you fetch and rebase onto the polars main branch please? there's some unrelated changes in https://github.com/pola-rs/polars/pull/23119/files which make it harder to review

[mrkn]

@MarcoGorelli I've finished rebasing onto the main branch.

[MarcoGorelli]

thanks! will try to get to this in the coming week

[MarcoGorelli]

thanks @mrkn ! all looks good

my only suggestion is - rather than adding an extra pattern to to_temporal_unit, to make a separate macro. this is partially to keep to_temporal_unit simple, but also because I'm not sure that "days in month" qualifies as a temporal unit

apart from that, this looks good to me, thanks for sticking with it 💪

[mrkn]

@MarcoGorelli Thanks for the review! Understood—I’ll try separating the macro as you suggested. I was also unsure whether days_in_month truly counts as a temporal unit, so I’ll think about a more appropriate name as well.

[mrkn]

@MarcoGorelli, I've updated the branch. I extracted the second variant of the to_temporal_unit macro into a separate macro named to_calendar_value. What do you think?

[MarcoGorelli]

thanks @mrkn , looks good to me!