fix clippy error

Author: onlyjackfrost

datafusion/sql/src/expr/unary_op.rs

@@ -49,7 +49,7 @@ impl<S: ContextProvider> SqlToRel<'_, S> {
                         let spans = operand.spans();
                         let span = if let Some(s) = spans.as_ref() { s.first() } else { None };
                         let diagnostic =
-                        Diagnostic::new_error(format!("Unary operator '+' only supports numeric, interval and timestamp types"), span);
+                        Diagnostic::new_error("Unary operator '+' only supports numeric, interval and timestamp types".to_string(), span);
                         e.with_diagnostic(diagnostic)
                     })
                 }

datafusion/sql/tests/cases/diagnostic.rs

@@ -291,7 +291,6 @@ fn test_invalid_function() -> Result<()> {
 fn test_unary_op_plus_with_non_column() -> Result<()> {
     // create a table with a column of type varchar
     let query = "SELECT /*whole*/+'a'/*whole*/ ";
-    let spans = get_spans(query);
     let diag = do_query(query);
     assert_eq!(
         diag.message,