Refactor casting functionality for internal use

Change `cast_with_target_field_and_options` to crate-internal
by modifying its visibility. Remove public re-export of the
planner-only helper while retaining planner access via the
internal module path. Make the `cast` module available
within the crate for internal usage.

Author: kosiew

datafusion/physical-expr/src/expressions/cast.rs

@@ -330,7 +330,7 @@ pub fn cast_with_options(
 
 /// Return a [`PhysicalExpr`] representing `expr` casted to `target_field`,
 /// preserving the field metadata in the resulting expression.
-pub fn cast_with_target_field_and_options(
+pub(crate) fn cast_with_target_field_and_options(
     expr: Arc<dyn PhysicalExpr>,
     input_schema: &Schema,
     target_field: FieldRef,

datafusion/physical-expr/src/expressions/mod.rs

@@ -20,7 +20,7 @@
 #[macro_use]
 mod binary;
 mod case;
-mod cast;
+pub(crate) mod cast;
 mod cast_column;
 mod column;
 mod dynamic_filters;
@@ -41,7 +41,7 @@ pub use crate::aggregate::stats::StatsType;
 
 pub use binary::{BinaryExpr, binary, similar_to};
 pub use case::{CaseExpr, case};
-pub use cast::{CastExpr, cast, cast_with_target_field_and_options};
+pub use cast::{CastExpr, cast};
 pub use cast_column::CastColumnExpr;
 pub use column::{Column, col, with_new_schema};
 pub use datafusion_expr::utils::format_state_name;

datafusion/physical-expr/src/planner.rs

@@ -289,7 +289,7 @@ pub fn create_physical_expr(
             Ok(expressions::case(expr, when_then_expr, else_expr)?)
         }
         Expr::Cast(Cast { expr, field }) => {
-            expressions::cast_with_target_field_and_options(
+            expressions::cast::cast_with_target_field_and_options(
                 create_physical_expr(expr, input_dfschema, execution_props)?,
                 input_schema,
                 Arc::clone(field),