Add some null and boolean representations from YAML 1.2

src/de.rs

@@ -923,20 +923,17 @@ fn parse_borrowed_str<'de>(
 }
 
 fn parse_null(scalar: &[u8]) -> Option<()> {
-    if scalar == b"~" || scalar == b"null" {
-        Some(())
-    } else {
-        None
+    match scalar {
+        b"null" | b"Null" | b"NULL" | b"~" => Some(()),
+        _ => None,
     }
 }
 
 fn parse_bool(scalar: &str) -> Option<bool> {
-    if scalar == "true" {
-        Some(true)
-    } else if scalar == "false" {
-        Some(false)
-    } else {
-        None
+    match scalar {
+        "true" | "True" | "TRUE" => Some(true),
+        "false" | "False" | "FALSE" => Some(false),
+        _ => None,
     }
 }
 

tests/test_de.rs

@@ -594,3 +594,69 @@ fn test_python_safe_dump() {
     let expected = Frob { foo: 7200 };
     test_de(yaml, &expected);
 }
+
+#[test]
+fn test_tag_resolution() {
+    // https://yaml.org/spec/1.2.2/#1032-tag-resolution
+    let yaml = indoc! {"
+        - null
+        - Null
+        - NULL
+        - ~
+        -
+        - true
+        - True
+        - TRUE
+        - false
+        - False
+        - FALSE
+        - y
+        - Y
+        - yes
+        - Yes
+        - YES
+        - n
+        - N
+        - no
+        - No
+        - NO
+        - on
+        - On
+        - ON
+        - off
+        - Off
+        - OFF
+    "};
+
+    let expected = vec![
+        Value::Null,
+        Value::Null,
+        Value::Null,
+        Value::Null,
+        Value::Null,
+        Value::Bool(true),
+        Value::Bool(true),
+        Value::Bool(true),
+        Value::Bool(false),
+        Value::Bool(false),
+        Value::Bool(false),
+        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("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()),
+    ];
+
+    test_de(yaml, &expected);
+}

tests/test_value.rs

@@ -97,7 +97,7 @@ fn test_merge() {
 #[test]
 fn test_debug() {
     let yaml = indoc! {"
-        Null: ~
+        'Null': ~
         Bool: true
         Number: 1
         String: ...