Added identity option to TO_DATE_RANGE, plus more tests

Author: craigtaverner

x-pack/plugin/esql/qa/testFixtures/src/main/resources/date.csv-spec

@@ -2048,3 +2048,27 @@ date_range:date_range  | decade:integer | description:keyword
 2010-01-01..2020-01-01 | 2010           | Renaissance Decade
 2020-01-01..2030-01-01 | 2020           | Empowerment Era
 ;
+
+dateRangeIdentity
+required_capability: date_range_field_type
+
+FROM date_ranges
+| WHERE decade == 1900
+| EVAL range = TO_DATE_RANGE(date_range)
+;
+
+date_range:date_range  | decade:integer | description:keyword | range:date_range
+1900-01-01..1910-01-01 | 1900           | Edwardian Era       | 1900-01-01..1910-01-01
+;
+
+dateRangeToString
+required_capability: date_range_field_type
+
+FROM date_ranges
+| WHERE decade == 1900
+| EVAL range = TO_STRING(date_range)
+;
+
+date_range:date_range  | decade:integer | description:keyword | range:keyword
+1900-01-01..1910-01-01 | 1900           | Edwardian Era       | 1900-01-01T00:00:00.000Z..1909-12-31T23:59:59.999Z
+;

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/expression/function/scalar/convert/ToDateRange.java

@@ -13,30 +13,37 @@
 import org.elasticsearch.xpack.esql.core.tree.NodeInfo;
 import org.elasticsearch.xpack.esql.core.tree.Source;
 import org.elasticsearch.xpack.esql.core.type.DataType;
+import org.elasticsearch.xpack.esql.expression.function.FunctionAppliesTo;
+import org.elasticsearch.xpack.esql.expression.function.FunctionAppliesToLifecycle;
 import org.elasticsearch.xpack.esql.expression.function.FunctionInfo;
 import org.elasticsearch.xpack.esql.expression.function.Param;
 
 import java.io.IOException;
 import java.util.List;
 import java.util.Map;
 
+import static org.elasticsearch.xpack.esql.core.type.DataType.DATE_RANGE;
+
 public class ToDateRange extends AbstractConvertFunction {
     public static final NamedWriteableRegistry.Entry ENTRY = new NamedWriteableRegistry.Entry(
         Expression.class,
         "ToDateRange",
         ToDateRange::new
     );
 
-    private static final Map<DataType, BuildFactory> EVALUATORS = Map.ofEntries(
-        // TODO
-    );
+    private static final Map<DataType, BuildFactory> EVALUATORS = Map.ofEntries(Map.entry(DATE_RANGE, (source, fieldEval) -> fieldEval));
 
-    @FunctionInfo(returnType = "DateRange", description = "TBD")
+    @FunctionInfo(
+        returnType = "date_range",
+        preview = true,
+        appliesTo = { @FunctionAppliesTo(lifeCycle = FunctionAppliesToLifecycle.PREVIEW) },
+        description = "Converts an input value to a `date_range` value."
+    )
     public ToDateRange(
         Source source,
         @Param(
             name = "field",
-            type = {},
+            type = { "date_range" },
             description = "Input value. The input can be a single- or multi-valued column or an expression."
         ) Expression field
     ) {