date_trunc small update for readability (#12479)

* Fix date_trunc error message

It was suggesting only nanosecond timestamps are supported, while all
arrow time units are.

* Remove dead code from date_trunc

`process_array::<TimestampNanosecondType>` can succeed only for an array
that can be downcast to array of nanosecond timestamps, but that case is
explicitly handled earlier.

* Simplify date_trunc time unit switch

Make it clear to the compiler (and the reader) that all time units are
covered.

datafusion/functions/src/datetime/date_trunc.rs

@@ -189,36 +189,37 @@ impl ScalarUDFImpl for DateTruncFunc {
             }
             ColumnarValue::Array(array) => {
                 let array_type = array.data_type();
-                match array_type {
-                    Timestamp(Second, tz_opt) => {
-                        process_array::<TimestampSecondType>(array, granularity, tz_opt)?
+                if let Timestamp(unit, tz_opt) = array_type {
+                    match unit {
+                        Second => process_array::<TimestampSecondType>(
+                            array,
+                            granularity,
+                            tz_opt,
+                        )?,
+                        Millisecond => process_array::<TimestampMillisecondType>(
+                            array,
+                            granularity,
+                            tz_opt,
+                        )?,
+                        Microsecond => process_array::<TimestampMicrosecondType>(
+                            array,
+                            granularity,
+                            tz_opt,
+                        )?,
+                        Nanosecond => process_array::<TimestampNanosecondType>(
+                            array,
+                            granularity,
+                            tz_opt,
+                        )?,
                     }
-                    Timestamp(Millisecond, tz_opt) => process_array::<
-                        TimestampMillisecondType,
-                    >(
-                        array, granularity, tz_opt
-                    )?,
-                    Timestamp(Microsecond, tz_opt) => process_array::<
-                        TimestampMicrosecondType,
-                    >(
-                        array, granularity, tz_opt
-                    )?,
-                    Timestamp(Nanosecond, tz_opt) => process_array::<
-                        TimestampNanosecondType,
-                    >(
-                        array, granularity, tz_opt
-                    )?,
-                    _ => process_array::<TimestampNanosecondType>(
-                        array,
-                        granularity,
-                        &None,
-                    )?,
+                } else {
+                    return exec_err!("second argument of `date_trunc` is an unsupported array type: {array_type}");
                 }
             }
             _ => {
                 return exec_err!(
-            "second argument of `date_trunc` must be nanosecond timestamp scalar or array"
-        );
+                    "second argument of `date_trunc` must be timestamp scalar or array"
+                );
             }
         })
     }