Add tests

src/date.rs

@@ -643,4 +643,10 @@ mod tests {
         date_sub -= TimeDelta::days(5);
         assert_eq!(date_sub, date - TimeDelta::days(5));
     }
+
+    #[test]
+    fn test_invalid_format() {
+        let result = Utc::today().format("%");
+        assert!(result.is_err(), "Invalid format string should be format error")
+    }
 }

src/datetime/mod.rs

@@ -1341,3 +1341,12 @@ fn test_decodable_json<FUtc, FFixed, FLocal, E>(
 fn test_invalid_format() {
     assert!(Utc::now().format("%").is_err());
 }
+
+#[test]
+#[cfg(feature = "unstable-locales")]
+fn test_invalid_format_localized() {
+    assert!(
+        Utc::now().format_localized("%", Locale::de_DE).is_err(),
+        "Invalid format string should be format error"
+    )
+}

src/format/strftime.rs

@@ -811,3 +811,12 @@ fn test_strftime_docs_localized() {
     assert_eq!(nd.format_localized("%F", Locale::de_DE).unwrap().to_string(), "2001-07-08");
     assert_eq!(nd.format_localized("%v", Locale::de_DE).unwrap().to_string(), " 8-Jul-2001");
 }
+
+#[cfg(feature = "unstable-locales")]
+#[test]
+fn test_strftime_with_locale() {
+    assert!(
+        StrftimeItems::new_with_locale("%", Locale::de_DE).is_err(),
+        "Invalid format string should be format error"
+    )
+}

src/naive/date.rs

@@ -3019,4 +3019,12 @@ mod tests {
             }
         }
     }
+
+    #[test]
+    fn test_format() {
+        assert!(
+            NaiveDate::from_ymd_opt(2023, 4, 9).unwrap().format("%").is_err(),
+            "Invalid format string should be format error"
+        )
+    }
 }

src/naive/datetime/mod.rs

@@ -1921,3 +1921,16 @@ where
     assert!(from_str(r#"{"date":{"ymdf":20},"time":{"secs":0,"frac":0}}"#).is_err());
     assert!(from_str(r#"null"#).is_err());
 }
+
+#[test]
+fn test_format() {
+    assert!(
+        NaiveDate::from_ymd_opt(-1, 12, 31)
+            .unwrap()
+            .and_hms_nano_opt(23, 59, 59, 7)
+            .unwrap()
+            .format("%")
+            .is_err(),
+        "Invalid format string should be format error"
+    )
+}

src/naive/time/mod.rs

@@ -1381,3 +1381,11 @@ where
     assert!(from_str(r#"{"secs":0,"frac":0}"#).is_err());
     assert!(from_str(r#"null"#).is_err());
 }
+
+#[test]
+fn test_format() {
+    assert!(
+        NaiveTime::from_hms_opt(1, 2, 3).unwrap().format("%").is_err(),
+        "Invalid format string should be format error"
+    )
+}