[CuteChuanChuan]

Which issue does this PR close?

• Closes Deprecate AggregateUDFImpl::is_nullable in favour of return_field #18882.

Rationale for this change

An aggregate's nullability is described twice: by AggregateUDFImpl::is_nullable and by the field returned from AggregateUDFImpl::return_field. AggregateFunctionExpr stored both, so is_nullable() and field().is_nullable() could disagree. This makes return_field the single source of truth, following #17074 for scalar UDFs.

What changes are included in this PR?

• Deprecate AggregateUDFImpl::is_nullable and AggregateUDF::is_nullable (since 56.0.0).
• AggregateExprBuilder::build now derives is_nullable from the return field.
• Migrate the built-in overrides (count, approx_distinct, regr_*) and the Count test stub to return_field.
• FFI: keep the FFI_AggregateUDF::is_nullable field for ABI stability and silence the deprecation warning; it can be removed together with the method.

This addresses the review feedback on #18934.

What is the testing strategy for this PR?

Covered by existing tests.

Are there any user-facing changes?

Yes. Implementations overriding is_nullable should set nullability in return_field instead.

[github-actions]

Thank you for opening this pull request!

Reviewer note: cargo-semver-checks reported the current version number is not SemVer-compatible with the changes in this pull request (compared against the base branch).

Details

     Cloning 36f393559620235c250f68727e3243cc7f429db8
    Building datafusion-expr v55.1.0 (current)
       Built [  35.529s] (current)
     Parsing datafusion-expr v55.1.0 (current)
      Parsed [   0.108s] (current)
    Building datafusion-expr v55.1.0 (baseline)
       Built [   7.839s] (baseline)
     Parsing datafusion-expr v55.1.0 (baseline)
      Parsed [   0.111s] (baseline)
    Checking datafusion-expr v55.1.0 -> v55.1.0 (no change; assume patch)
     Checked [   1.608s] 229 checks: 227 pass, 2 fail, 0 warn, 31 skip

--- failure trait_method_marked_deprecated: trait method #[deprecated] added ---

Description:
A trait method is now #[deprecated]. Downstream crates will get a compiler warning when using this method.
        ref: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-deprecated-attribute
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/trait_method_marked_deprecated.ron

Failed in:
  method is_nullable in trait datafusion_expr::AggregateUDFImpl in /home/runner/work/datafusion/datafusion/datafusion/expr/src/udaf.rs:461

--- failure type_method_marked_deprecated: type method #[deprecated] added ---

Description:
A type method is now #[deprecated]. Downstream crates will get a compiler warning when using this method.
        ref: https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-deprecated-attribute
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.51.0/src/lints/type_method_marked_deprecated.ron

Failed in:
  method datafusion_expr::AggregateUDF::is_nullable in /home/runner/work/datafusion/datafusion/datafusion/expr/src/udaf.rs:212

     Summary semver requires new minor version: 0 major and 2 minor checks failed
    Finished [  46.585s] datafusion-expr
    Building datafusion-ffi v55.1.0 (current)
       Built [  50.865s] (current)
     Parsing datafusion-ffi v55.1.0 (current)
      Parsed [   0.073s] (current)
    Building datafusion-ffi v55.1.0 (baseline)
       Built [  22.027s] (baseline)
     Parsing datafusion-ffi v55.1.0 (baseline)
      Parsed [   0.072s] (baseline)
    Checking datafusion-ffi v55.1.0 -> v55.1.0 (no change; assume patch)
     Checked [   0.276s] 229 checks: 229 pass, 31 skip
     Summary no semver update required
    Finished [  74.870s] datafusion-ffi
    Building datafusion-functions-aggregate v55.1.0 (current)
       Built [  11.675s] (current)
     Parsing datafusion-functions-aggregate v55.1.0 (current)
      Parsed [   0.057s] (current)
    Building datafusion-functions-aggregate v55.1.0 (baseline)
       Built [   8.118s] (baseline)
     Parsing datafusion-functions-aggregate v55.1.0 (baseline)
      Parsed [   0.059s] (baseline)
    Checking datafusion-functions-aggregate v55.1.0 -> v55.1.0 (no change; assume patch)
     Checked [   0.189s] 229 checks: 229 pass, 31 skip
     Summary no semver update required
    Finished [  21.168s] datafusion-functions-aggregate
    Building datafusion-physical-expr v55.1.0 (current)
       Built [   3.791s] (current)
     Parsing datafusion-physical-expr v55.1.0 (current)
      Parsed [   0.058s] (current)
    Building datafusion-physical-expr v55.1.0 (baseline)
       Built [   3.906s] (baseline)
     Parsing datafusion-physical-expr v55.1.0 (baseline)
      Parsed [   0.060s] (baseline)
    Checking datafusion-physical-expr v55.1.0 -> v55.1.0 (no change; assume patch)
     Checked [   0.343s] 229 checks: 229 pass, 31 skip
     Summary no semver update required
    Finished [   9.516s] datafusion-physical-expr

[CuteChuanChuan]

Hi @Jefffrey,

could you PTAL when you have a chance? Thanks 🙏!