fix: unwrap identity Date cast in comparison unwrapping

[adriangb]

Which issue does this PR close?

No existing issue; this PR both reports and fixes the bug. Happy to file a tracking issue if preferred.

Rationale for this change

unwrap_cast_in_comparison fails to fold an identity CAST(col AS DATE) on a Date32 column. Instead of rewriting the predicate to compare against the bare column, it leaves a residual Cast(col AS Date32) in place, which defeats downstream optimizations (pruning / filter pushdown) that expect a bare-column comparison.

Reproduction (logical plan for SELECT * FROM t WHERE cast(d AS date) = DATE '2024-01-01', where d is Date32): the cast(d AS Date32) survives simplification rather than collapsing to d. Note arrow_cast(d, 'Date32') already folds, because the arrow_cast UDF's simplify() short-circuits an identity cast; the SQL CAST ... AS date planner plants a real Expr::Cast with no such elision, so it reaches try_cast_literal_to_type and is wrongly rejected.

The root cause is in is_lossy_temporal_cast (datafusion/expr-common/src/casts.rs):

(is_date_type(from_type) && to_type.is_temporal())
    || (is_date_type(to_type) && from_type.is_temporal())

For an identity Date32 -> Date32 cast this evaluates to true && true, because DataType::is_temporal() is true for both Date32 and Date64. The identity cast is therefore misclassified as a lossy temporal cast, try_cast_literal_to_type returns None, and unwrap_cast_in_comparison leaves the cast in the plan.

What changes are included in this PR?

Short-circuit an identity cast as non-lossy at the top of is_lossy_temporal_cast:

if from_type == to_type {
    return false; // an identity cast never changes comparison semantics
}

This is deliberately limited to identical types, not "any date-to-date". Date32 counts days while Date64 counts milliseconds, but try_cast_numeric_literal uses mul = 1 for both, so allowing a Date32 <-> Date64 unwrap would convert units incorrectly. The from_type == to_type identity guard is the exact correct scope, and Date32 <-> Date64 remains blocked.

Are these changes tested?

Yes. The PR is structured as a test-driven, stacked sequence so the effect of the fix is visible in the diff:

1. test: characterize identity Date cast in comparison unwrapping — adds an SLT test to simplify_expr.slt (explain select d from dates where cast(d as date) = DATE '2024-01-01') whose assertion records the current, buggy output: the logical plan keeps the residual Filter: CAST(dates.d AS Date32) = Date32(...). Passes on main.

2. fix: unwrap identity Date cast in comparison unwrapping — the production change plus two expr-common unit tests:

   • test_try_cast_identity_date_allowed — identity Date32 -> Date32 / Date64 -> Date64 now fold (try_cast_literal_to_type returns Some), and is_lossy_temporal_cast reports them non-lossy.
   • test_try_cast_date32_date64_still_blocked — Date32 <-> Date64 stays lossy/blocked (guards the units caveat above, which SLT can't easily express).

   At this commit the SLT test is intentionally red, proving it exercises the bug.

3. test: update identity Date cast assertion to folded plan — updates the SLT assertion to the corrected Filter: dates.d = Date32(...). The one-line diff is the observable effect of the fix.

cargo fmt --check and cargo clippy -D warnings are clean on the changed crate; datafusion-expr-common and the simplify_expr sqllogictest pass.

Are there any user-facing changes?

No API changes. Queries with CAST(date_col AS DATE) predicates on Date32 columns simplify to bare-column comparisons, which can enable additional pruning/pushdown. No behavioral change to query results.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.70%. Comparing base (5b65e70) to head (0ec7b72).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #23727   +/-   ##
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  Coverage   80.70%   80.70%           
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  Files        1089     1089           
  Lines      368137   368170   +33     
  Branches   368137   368170   +33     
=======================================
+ Hits       297121   297149   +28     
- Misses      53311    53313    +2     
- Partials    17705    17708    +3     

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[adriangb]

@kosiew another hopefully small one to look at