Quote YAML 1.1 bools during serialization

README.md

@@ -75,7 +75,7 @@ fn main() -> Result<(), serde_yaml::Error> {
     let point = Point { x: 1.0, y: 2.0 };
 
     let yaml = serde_yaml::to_string(&point)?;
-    assert_eq!(yaml, "x: 1.0\ny: 2.0\n");
+    assert_eq!(yaml, "x: 1.0\n'y': 2.0\n");
 
     let deserialized_point: Point = serde_yaml::from_str(&yaml)?;
     assert_eq!(point, deserialized_point);
@@ -100,7 +100,7 @@ fn main() -> Result<(), serde_yaml::Error> {
     let yaml = "
         - !Newtype 1
         - !Tuple [0, 0, 0]
-        - !Struct {x: 1.0, y: 2.0}
+        - !Struct {x: 1.0, 'y': 2.0}
     ";
     let values: Vec<Enum> = serde_yaml::from_str(yaml).unwrap();
     assert_eq!(values[0], Enum::Newtype(1));
@@ -115,7 +115,7 @@ fn main() -> Result<(), serde_yaml::Error> {
           - 0
         - !Struct
           x: 1.0
-          y: 2.0
+          'y': 2.0
     ";
     let values: Vec<Enum> = serde_yaml::from_str(yaml).unwrap();
     assert_eq!(values[0], Enum::Tuple(0, 0, 0));

src/lib.rs

@@ -25,7 +25,7 @@
 //!
 //!     // Serialize it to a YAML string.
 //!     let yaml = serde_yaml::to_string(&map)?;
-//!     assert_eq!(yaml, "x: 1.0\ny: 2.0\n");
+//!     assert_eq!(yaml, "x: 1.0\n'y': 2.0\n");
 //!
 //!     // Deserialize it back to a Rust type.
 //!     let deserialized_map: BTreeMap<String, f64> = serde_yaml::from_str(&yaml)?;
@@ -55,7 +55,7 @@
 //!     let point = Point { x: 1.0, y: 2.0 };
 //!
 //!     let yaml = serde_yaml::to_string(&point)?;
-//!     assert_eq!(yaml, "x: 1.0\ny: 2.0\n");
+//!     assert_eq!(yaml, "x: 1.0\n'y': 2.0\n");
 //!
 //!     let deserialized_point: Point = serde_yaml::from_str(&yaml)?;
 //!     assert_eq!(point, deserialized_point);
@@ -96,7 +96,7 @@
 //!           - 0
 //!         - !Struct
 //!           x: 1.0
-//!           y: 2.0
+//!           'y': 2.0
 //!     ";
 //!     let values: Vec<Enum> = serde_yaml::from_str(yaml).unwrap();
 //!     assert_eq!(values[0], Enum::Tuple(0, 0, 0));

src/ser.rs

@@ -352,16 +352,22 @@ 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 old YAML boolean scalars.
+            // See https://yaml.org/type/bool.html
+            "y" | "Y" | "yes" | "Yes" | "YES" | "n" | "N" | "no" | "No" | "NO" | "true"
+            | "True" | "TRUE" | "false" | "False" | "FALSE" | "on" | "On" | "ON" | "off"
+            | "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_de.rs

@@ -209,10 +209,10 @@ fn test_enum_representations() {
         - !Tuple
           - 0
           - 0
-        - !Struct {x: 0, y: 0}
+        - !Struct {x: 0, 'y': 0}
         - !Struct
           x: 0
-          y: 0
+          'y': 0
         - !String '...'
         - !String ...
         - !Number 0
@@ -390,7 +390,7 @@ fn test_bomb() {
         v: &v [*u,*u,*u,*u,*u,*u,*u,*u,*u]
         w: &w [*v,*v,*v,*v,*v,*v,*v,*v,*v]
         x: &x [*w,*w,*w,*w,*w,*w,*w,*w,*w]
-        y: &y [*x,*x,*x,*x,*x,*x,*x,*x,*x]
+        'y': &y [*x,*x,*x,*x,*x,*x,*x,*x,*x]
         z: &z [*y,*y,*y,*y,*y,*y,*y,*y,*y]
         expected: string
     "};

tests/test_serde.rs

@@ -195,7 +195,7 @@ fn test_map() {
     thing.insert("y".to_owned(), 2);
     let yaml = indoc! {"
         x: 1
-        y: 2
+        'y': 2
     "};
     test_serde(&thing, yaml);
 }
@@ -238,7 +238,7 @@ fn test_basic_struct() {
     };
     let yaml = indoc! {r#"
         x: -4
-        y: "hi\tquoted"
+        'y': "hi\tquoted"
         z: true
     "#};
     test_serde(&thing, yaml);
@@ -270,6 +270,66 @@ fn test_string_escapes() {
     test_serde(&"\u{1f389}".to_owned(), yaml);
 }
 
+#[test]
+fn test_boolish_serialization() {
+    // See https://yaml.org/type/bool.html
+    let thing = vec![
+        Value::String("y".to_owned()),
+        Value::String("Y".to_owned()),
+        Value::String("yes".to_owned()),
+        Value::String("Yes".to_owned()),
+        Value::String("YES".to_owned()),
+        Value::String("n".to_owned()),
+        Value::String("N".to_owned()),
+        Value::String("no".to_owned()),
+        Value::String("No".to_owned()),
+        Value::String("NO".to_owned()),
+        Value::String("true".to_owned()),
+        Value::String("True".to_owned()),
+        Value::String("TRUE".to_owned()),
+        Value::String("false".to_owned()),
+        Value::String("False".to_owned()),
+        Value::String("FALSE".to_owned()),
+        Value::String("on".to_owned()),
+        Value::String("On".to_owned()),
+        Value::String("ON".to_owned()),
+        Value::String("off".to_owned()),
+        Value::String("Off".to_owned()),
+        Value::String("OFF".to_owned()),
+        Value::Bool(true),
+        Value::Bool(false),
+    ];
+
+    let yaml = indoc! {"
+        - 'y'
+        - 'Y'
+        - 'yes'
+        - 'Yes'
+        - 'YES'
+        - 'n'
+        - 'N'
+        - 'no'
+        - 'No'
+        - 'NO'
+        - 'true'
+        - 'True'
+        - 'TRUE'
+        - 'false'
+        - 'False'
+        - 'FALSE'
+        - 'on'
+        - 'On'
+        - 'ON'
+        - 'off'
+        - 'Off'
+        - 'OFF'
+        - true
+        - false
+    "};
+
+    test_serde(&thing, yaml);
+}
+
 #[test]
 fn test_multiline_string() {
     #[derive(Serialize, Deserialize, PartialEq, Debug)]

tests/test_value.rs

@@ -63,16 +63,16 @@ fn test_merge() {
     // From https://yaml.org/type/merge.html.
     let yaml = indoc! {"
         ---
-        - &CENTER { x: 1, y: 2 }
-        - &LEFT { x: 0, y: 2 }
+        - &CENTER { x: 1, 'y': 2 }
+        - &LEFT { x: 0, 'y': 2 }
         - &BIG { r: 10 }
         - &SMALL { r: 1 }
 
         # All the following maps are equal:
 
         - # Explicit keys
           x: 1
-          y: 2
+          'y': 2
           r: 10
           label: center/big