refactor(rust): More refactor for PlSmallStr

[nameexhaustion]

• Organize the impls
• Some remaining cleanup of .as_ref().into() -> .clone()
• Removes From<&&str>: *breaks a lot of things - will do separately
  • Existing code calling with constant arrays are changed from e.g. Series::new(_, &["a", ...]) to Series::new(_, ["a", ...])
  • &[&str] can no longer be passed to most API functions accepting impl IntoIterator<Item = impl Into<PlSmallStr>>
• Add AsRef<Path>, AsRef<[u8]>, AsRef<OsStr>

[nameexhaustion]

Ensure we use the Display impl from &str

[ritchie46]

When do we have double indirection in &&str?, I think we should just deref.

[orlp]

Yes, please don't add this.

[nameexhaustion]

We have some functions that take impl IntoIterator<Item = impl Into<PlSmallStr>> -

polars/crates/polars-ops/src/frame/join/mod.rs

Lines 84 to 93 in b3172aa

	fn join<I, S>(
	&self,
	other: &DataFrame,
	left_on: I,
	right_on: I,
	args: JoinArgs,
	) -> PolarsResult<DataFrame>
	where
	I: IntoIterator<Item = S>,
	S: Into<PlSmallStr>,

and they get passed &[&str] -

polars/crates/polars/tests/it/core/joins.rs

Lines 508 to 515 in b3172aa

	let df_inner_join = df_left
	.join(
	&df_right,
	&["col1", "join_col2"],
	&["join_col1", "col2"],
	JoinType::Inner.into(),
	)
	.unwrap();

Whose IntoIterator then becomes &&str, so we needed the From<&&str>.

[ritchie46]

I think we can just pass ["col1", "join_col2"] instead of &["col1", "join_col2"].

[nameexhaustion]

That still gives &&str for the IntoIter *nvm: this has changed since a while ago in Rust

[nameexhaustion]

@ritchie46 removing this causes a huge amount of breakage, I will do it in a later PR

[codecov]

Codecov Report

Attention: Patch coverage is 78.50467% with 23 lines in your changes missing coverage. Please review.

Project coverage is 79.83%. Comparing base (915f164) to head (96ac672).
Report is 32 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-utils/src/pl_str.rs	57.14%	12 Missing ⚠️
crates/polars-sql/src/context.rs	50.00%	4 Missing ⚠️
crates/polars-expr/src/expressions/column.rs	0.00%	1 Missing ⚠️
crates/polars-io/src/ipc/ipc_reader_async.rs	0.00%	1 Missing ⚠️
crates/polars-io/src/partition.rs	50.00%	1 Missing ⚠️
crates/polars-plan/src/plans/ir/tree_format.rs	50.00%	1 Missing ⚠️
...rates/polars-python/src/lazyframe/visitor/nodes.rs	0.00%	1 Missing ⚠️
crates/polars-sql/src/functions.rs	0.00%	1 Missing ⚠️
crates/polars-sql/src/sql_expr.rs	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18456      +/-   ##
==========================================
- Coverage   79.84%   79.83%   -0.01%     
==========================================
  Files        1497     1498       +1     
  Lines      201828   201803      -25     
  Branches     2867     2867              
==========================================
- Hits       161141   161115      -26     
- Misses      40141    40142       +1     
  Partials      546      546              

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[nameexhaustion]

pre-work for removing From<&&str> - as then calling join(left_on=["a"], right_on=["a", "b"]) becomes 2 distinct generics <[&str; 1] as IntoIterator>, <[&str; 2] as IntoIterator>

*although also in general, allows for 2 distinct types