Improve error message on invalid casts for structs

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

@@ -50,8 +50,8 @@ const DEFAULT_SAFE_CAST_OPTIONS: CastOptions<'static> = CastOptions {
 /// planning-time validation matches runtime validation, enabling fail-fast behavior
 /// instead of deferring errors to execution. Handles structs at any nesting level
 /// (e.g., `List<Struct>`, `Dictionary<_, Struct>`).
-fn can_cast_named_struct_types(source: &DataType, target: &DataType) -> bool {
-    validate_data_type_compatibility("", source, target).is_ok()
+fn can_cast_named_struct_types(source: &DataType, target: &DataType) -> Result<bool> {
+    validate_data_type_compatibility("", source, target).map(|_| true)
 }
 
 /// CAST expression casts an expression to a specific data type and returns a runtime error on invalid cast
@@ -399,7 +399,7 @@ pub fn cast_with_target_field(
         // applied at planning time (now) to fail fast, rather than deferring errors
         // to execution time. The name-based casting logic will be executed at runtime
         // via ColumnarValue::cast_to.
-        can_cast_named_struct_types(&expr_type, cast_type)
+        can_cast_named_struct_types(&expr_type, cast_type)?
     } else {
         can_cast_types(&expr_type, cast_type)
     };
@@ -1046,7 +1046,7 @@ mod tests {
         let err = cast_with_options(col("a", &schema)?, &schema, invalid_target, None)
             .expect_err("missing required struct field should fail");
 
-        assert!(err.to_string().contains("Unsupported CAST"));
+        assert!(err.to_string().contains("Cannot cast struct"));
 
         Ok(())
     }

datafusion/sqllogictest/test_files/struct.slt

@@ -887,9 +887,13 @@ SELECT CAST({a: 1, b: 2, extra: 3} AS STRUCT(a INT, b INT));
 {a: 1, b: 2}
 
 # Test no overlap with mismatched field count - should fail because no field names match
-statement error DataFusion error: (Plan error|Error during planning|This feature is not implemented): (Cannot cast struct: at least one field name must match between source and target|Cannot cast struct with 3 fields to 2 fields without name overlap|Unsupported CAST from Struct)
+statement error DataFusion error: Error during planning: Cannot cast struct with 3 fields to 2
 SELECT CAST(struct(1, 'x', 'y') AS STRUCT(a INT, b VARCHAR));
 
+# Cannot cast to nullable struct field to non nullable struct field
+statement error Optimizer rule 'simplify_expressions' failed
+select arrow_cast(struct(10), 'Struct("c0": non-null Int64)')
+
 # Test nested struct with field reordering
 query ?
 SELECT CAST(