Add char::is_titlecase

Author: Jules-Bertholet

library/core/src/char/methods.rs

@@ -822,6 +822,39 @@ impl char {
         }
     }
 
+    /// Returns `true` if this `char` has the general category for titlecase letters.
+    ///
+    /// Titlecase letters (code points with the general category of `Lt`) are described in Chapter 4
+    /// (Character Properties) of the [Unicode Standard] and specified in the [Unicode Character
+    /// Database][ucd] [`UnicodeData.txt`].
+    ///
+    /// [Unicode Standard]: https://www.unicode.org/versions/latest/
+    /// [ucd]: https://www.unicode.org/reports/tr44/
+    /// [`UnicodeData.txt`]: https://www.unicode.org/Public/UCD/latest/ucd/UnicodeData.txt
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(titlecase)]
+    /// assert!('Dž'.is_titlecase());
+    /// assert!('ᾨ'.is_titlecase());
+    /// assert!(!'D'.is_titlecase());
+    /// assert!(!'z'.is_titlecase());
+    /// assert!(!'中'.is_titlecase());
+    /// assert!(!' '.is_titlecase());
+    /// ```
+    #[must_use]
+    #[unstable(feature = "titlecase", issue = "none")]
+    #[inline]
+    pub fn is_titlecase(self) -> bool {
+        match self {
+            '\0'..='\u{01C4}' => false,
+            _ => self.is_cased() && !self.is_lowercase() && !self.is_uppercase(),
+        }
+    }
+
     /// Returns `true` if this `char` has the `Uppercase` property.
     ///
     /// `Uppercase` is described in Chapter 4 (Character Properties) of the [Unicode Standard] and

library/core/tests/char.rs

@@ -61,6 +61,17 @@ fn test_is_lowercase() {
     assert!(!'P'.is_lowercase());
 }
 
+#[test]
+fn test_is_titlecase() {
+    assert!('Dž'.is_titlecase());
+    assert!('ᾨ'.is_titlecase());
+    assert!(!'h'.is_titlecase());
+    assert!(!'ä'.is_titlecase());
+    assert!(!'ß'.is_titlecase());
+    assert!(!'Ö'.is_titlecase());
+    assert!(!'T'.is_titlecase());
+}
+
 #[test]
 fn test_is_uppercase() {
     assert!(!'h'.is_uppercase());
@@ -70,6 +81,26 @@ fn test_is_uppercase() {
     assert!('T'.is_uppercase());
 }
 
+#[test]
+fn titlecase_fast_path() {
+    for c in '\0'..='\u{01C4}' {
+        assert!(!(c.is_cased() && !c.is_lowercase() && !c.is_uppercase()))
+    }
+}
+
+#[test]
+fn at_most_one_case() {
+    for c in '\0'..='\u{10FFFF}' {
+        assert_eq!(
+            !c.is_cased() as u8
+                + c.is_lowercase() as u8
+                + c.is_uppercase() as u8
+                + c.is_titlecase() as u8,
+            1
+        );
+    }
+}
+
 #[test]
 fn test_is_whitespace() {
     assert!(' '.is_whitespace());