Add char::case

Author: Jules-Bertholet

library/core/src/char/methods.rs

@@ -779,6 +779,39 @@ impl char {
         }
     }
 
+    /// Returns the case of this character:
+    /// [`Some(CharCase::Upper)`][`CharCase::Upper`] if [`self.is_uppercase()`][`char::is_uppercase`],
+    /// [`Some(CharCase::Lower)`][`CharCase::Lower`] if [`self.is_lowercase()`][`char::is_lowercase`],
+    /// [`Some(CharCase::Title)`][`CharCase::Title`] if [`self.is_titlecase()`][`char::is_titlecase`], and
+    /// `None` if [`!self.is_cased()`][`char::is_cased`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(titlecase)]
+    /// use core::char::CharCase;
+    /// assert_eq!('a'.case(), Some(CharCase::Lower));
+    /// assert_eq!('δ'.case(), Some(CharCase::Lower));
+    /// assert_eq!('A'.case(), Some(CharCase::Upper));
+    /// assert_eq!('Δ'.case(), Some(CharCase::Upper));
+    /// assert_eq!('Dž'.case(), Some(CharCase::Title));
+    /// assert_eq!('中'.case(), None);
+    /// ```
+    #[must_use]
+    #[unstable(feature = "titlecase", issue = "none")]
+    #[inline]
+    pub fn case(self) -> Option<CharCase> {
+        match self {
+            'A'..='Z' => Some(CharCase::Upper),
+            'a'..='z' => Some(CharCase::Lower),
+            '\0'..='\u{A9}' => None,
+            _ if !self.is_cased() => None,
+            _ if self.is_lowercase() => Some(CharCase::Lower),
+            _ if self.is_uppercase() => Some(CharCase::Upper),
+            _ => Some(CharCase::Title),
+        }
+    }
+
     /// Returns `true` if this `char` has the `Lowercase` property.
     ///
     /// `Lowercase` is described in Chapter 4 (Character Properties) of the [Unicode Standard] and

library/core/src/char/mod.rs

@@ -560,3 +560,16 @@ impl fmt::Display for TryFromCharError {
 
 #[stable(feature = "u8_from_char", since = "1.59.0")]
 impl Error for TryFromCharError {}
+
+/// The case of a cased character,
+/// as returned by [`char::case`].
+#[unstable(feature = "titlecase", issue = "none")]
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, PartialOrd, Ord)]
+pub enum CharCase {
+    /// Lowercase. Corresponds to the `Lowercase` Unicode property.
+    Lower = 0b00,
+    /// Titlecase. Corresponds to the `Titlecase_Letter` Unicode general category.
+    Title = 0b10,
+    /// Uppercase. Corresponds to the `Uppercase` Unicode property.
+    Upper = 0b11,
+}

library/core/tests/char.rs

@@ -1,5 +1,8 @@
 use std::str::FromStr;
-use std::{char, str};
+use std::{
+    char::{self, CharCase},
+    str,
+};
 
 #[test]
 fn test_convert() {
@@ -52,6 +55,18 @@ fn test_is_cased() {
     assert!(!'攂'.is_cased());
 }
 
+#[test]
+fn test_char_case() {
+    for c in '\0'..='\u{10FFFF}' {
+        match c.case() {
+            None => assert!(!c.is_cased()),
+            Some(CharCase::Lower) => assert!(c.is_lowercase()),
+            Some(CharCase::Upper) => assert!(c.is_uppercase()),
+            Some(CharCase::Title) => assert!(c.is_titlecase()),
+        }
+    }
+}
+
 #[test]
 fn test_is_lowercase() {
     assert!('a'.is_lowercase());