handle old bool YAML keywords in serializer for backwards compatibili…

…ty instead of the deserializer

src/de.rs

@@ -931,8 +931,8 @@ fn parse_null(scalar: &[u8]) -> Option<()> {
 
 fn parse_bool(scalar: &str) -> Option<bool> {
     match scalar {
-        "true" | "True" | "TRUE" | "on" | "ON" | "yes" | "YES" => Some(true),
-        "false" | "False" | "FALSE" | "off" | "OFF" | "no" | "NO" => Some(false),
+        "true" | "True" | "TRUE" => Some(true),
+        "false" | "False" | "FALSE" => Some(false),
         _ => None,
     }
 }

src/ser.rs

@@ -352,16 +352,19 @@ where
             }
         }
 
-        let style = if value.contains('\n') {
-            ScalarStyle::Literal
-        } else {
-            let result = crate::de::visit_untagged_scalar(
-                InferScalarStyle,
-                value,
-                None,
-                libyaml::parser::ScalarStyle::Plain,
-            );
-            result.unwrap_or(ScalarStyle::Any)
+        let style = match value {
+            // Backwards compatibility with pre-YAML 1.2.2 spec for boolean keywords.
+            "on" | "ON" | "yes" | "YES" | "no" | "NO" | "off" | "OFF" => ScalarStyle::SingleQuoted,
+            _ if value.contains('\n') => ScalarStyle::Literal,
+            _ => {
+                let result = crate::de::visit_untagged_scalar(
+                    InferScalarStyle,
+                    value,
+                    None,
+                    libyaml::parser::ScalarStyle::Plain,
+                );
+                result.unwrap_or(ScalarStyle::Any)
+            }
         };
 
         self.emit_scalar(Scalar {

tests/test_serde.rs

@@ -271,22 +271,16 @@ fn test_string_escapes() {
 }
 
 #[test]
-fn test_can_deser_bool_keywords_and_quote_as_strings() {
-    for (thing, yaml) in vec![
-        (true, "on"),
-        (false, "off"),
-        (true, "yes"),
-        (false, "no"),
-        (true, "true"),
-        (false, "false"),
-    ] {
-        // Serializing YAML bool keywords to strings adds quotes.
+fn test_bool_keywords_are_serialized_for_backwards_compatibility() {
+    for yaml in vec!["on", "off", "yes", "no"] {
+        // Serializing YAML bool keywords to strings adds quotes for compatibility with
+        // older YAML specs.
         let formatted: String = serde_yaml::to_string(yaml).unwrap();
         assert_eq!(formatted, format!("'{}'\n", yaml));
 
-        // YAML boolean keywords can be deserialized into bool.
-        let value: bool = serde_yaml::from_str(yaml).unwrap();
-        assert_eq!(thing, value);
+        // Old YAML boolean keywords can NOT be deserialized into bool.
+        let value: Result<bool, _> = serde_yaml::from_str(yaml);
+        assert!(value.is_err());
     }
 }