Add APIs for dealing with titlecase

- `char::is_cased`
- `char::is_titlecase`
- `char::case`
- `char::to_titlecase`

Author: Jules-Bertholet

library/alloc/src/lib.rs

@@ -148,6 +148,7 @@
 #![feature(slice_range)]
 #![feature(std_internals)]
 #![feature(temporary_niche_types)]
+#![feature(titlecase)]
 #![feature(transmutability)]
 #![feature(trivial_clone)]
 #![feature(trusted_fused)]

library/core/src/char/methods.rs

@@ -777,12 +777,76 @@ impl char {
     #[inline]
     pub fn is_alphabetic(self) -> bool {
         match self {
-            'A'..='Z' | 'a'..='z' => true,
+            'a'..='z' | 'A'..='Z' => true,
             '\0'..='\u{A9}' => false,
             _ => unicode::Alphabetic(self),
         }
     }
 
+    /// Returns `true` if this `char` has the `Cased` property.
+    /// A character is cased if and only if it is uppercase, lowercase, or titlecase.
+    ///
+    /// `Cased` is described in Chapter 4 (Character Properties) of the [Unicode Standard] and
+    /// specified in the [Unicode Character Database][ucd] [`DerivedCoreProperties.txt`].
+    ///
+    /// [Unicode Standard]: https://www.unicode.org/versions/latest/
+    /// [ucd]: https://www.unicode.org/reports/tr44/
+    /// [`DerivedCoreProperties.txt`]: https://www.unicode.org/Public/UCD/latest/ucd/DerivedCoreProperties.txt
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(titlecase)]
+    /// assert!('A'.is_cased());
+    /// assert!('a'.is_cased());
+    /// assert!(!'京'.is_cased());
+    /// ```
+    #[must_use]
+    #[unstable(feature = "titlecase", issue = "153892")]
+    #[inline]
+    pub fn is_cased(self) -> bool {
+        match self {
+            'a'..='z' | 'A'..='Z' => true,
+            '\0'..='\u{A9}' => false,
+            _ => unicode::Cased(self),
+        }
+    }
+
+    /// 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 = "153892")]
+    #[inline]
+    pub fn case(self) -> Option<CharCase> {
+        match self {
+            'a'..='z' => Some(CharCase::Lower),
+            'A'..='Z' => Some(CharCase::Upper),
+            '\0'..='\u{A9}' => None,
+            _ if !unicode::Cased(self) => None,
+            _ if unicode::Lowercase(self) => Some(CharCase::Lower),
+            _ if unicode::Uppercase(self) => 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
@@ -825,6 +889,40 @@ impl char {
         }
     }
 
+    /// Returns `true` if this `char` has the general category for titlecase letters.
+    /// Conceptually, these characters consist of an uppercase portion followed by a lowercase portion.
+    ///
+    /// 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!('ῼ'.is_titlecase());
+    /// assert!(!'D'.is_titlecase());
+    /// assert!(!'z'.is_titlecase());
+    /// assert!(!'中'.is_titlecase());
+    /// assert!(!' '.is_titlecase());
+    /// ```
+    #[must_use]
+    #[unstable(feature = "titlecase", issue = "153892")]
+    #[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
@@ -925,7 +1023,7 @@ impl char {
     #[inline]
     pub fn is_alphanumeric(self) -> bool {
         match self {
-            '0'..='9' | 'A'..='Z' | 'a'..='z' => true,
+            'a'..='z' | 'A'..='Z' | '0'..='9' => true,
             '\0'..='\u{A9}' => false,
             _ => unicode::Alphabetic(self) || unicode::N(self),
         }
@@ -976,26 +1074,6 @@ impl char {
         self > '\u{02FF}' && unicode::Grapheme_Extend(self)
     }
 
-    /// Returns `true` if this `char` has the `Cased` property.
-    ///
-    /// `Cased` is described in Chapter 4 (Character Properties) of the [Unicode Standard] and
-    /// specified in the [Unicode Character Database][ucd] [`DerivedCoreProperties.txt`].
-    ///
-    /// [Unicode Standard]: https://www.unicode.org/versions/latest/
-    /// [ucd]: https://www.unicode.org/reports/tr44/
-    /// [`DerivedCoreProperties.txt`]: https://www.unicode.org/Public/UCD/latest/ucd/DerivedCoreProperties.txt
-    #[must_use]
-    #[inline]
-    #[doc(hidden)]
-    #[unstable(feature = "char_internals", reason = "exposed only for libstd", issue = "none")]
-    pub fn is_cased(self) -> bool {
-        match self {
-            'A'..='Z' | 'a'..='z' => true,
-            '\0'..='\u{A9}' => false,
-            _ => unicode::Cased(self),
-        }
-    }
-
     /// Returns `true` if this `char` has the `Case_Ignorable` property.
     ///
     /// `Case_Ignorable` is described in Chapter 4 (Character Properties) of the [Unicode Standard] and
@@ -1119,17 +1197,123 @@ impl char {
     /// // convert into themselves.
     /// assert_eq!('山'.to_lowercase().to_string(), "山");
     /// ```
-    #[must_use = "this returns the lowercase character as a new iterator, \
+    #[must_use = "this returns the lowercased character as a new iterator, \
                   without modifying the original"]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn to_lowercase(self) -> ToLowercase {
         ToLowercase(CaseMappingIter::new(conversions::to_lower(self)))
     }
 
+    /// Returns an iterator that yields the titlecase mapping of this `char` as one or more
+    /// `char`s.
+    ///
+    /// This is usually, but not always, equivalent to the uppercase mapping
+    /// returned by [`Self::to_uppercase`]. Prefer this method when seeking to capitalize
+    /// Only The First Letter of a word, but use [`Self::to_uppercase`] for ALL CAPS.
+    ///
+    /// If this `char` does not have an titlecase mapping, the iterator yields the same `char`.
+    ///
+    /// If this `char` has a one-to-one titlecase mapping given by the [Unicode Character
+    /// Database][ucd] [`UnicodeData.txt`], the iterator yields that `char`.
+    ///
+    /// [ucd]: https://www.unicode.org/reports/tr44/
+    /// [`UnicodeData.txt`]: https://www.unicode.org/Public/UCD/latest/ucd/UnicodeData.txt
+    ///
+    /// If this `char` requires special considerations (e.g. multiple `char`s) the iterator yields
+    /// the `char`(s) given by [`SpecialCasing.txt`].
+    ///
+    /// [`SpecialCasing.txt`]: https://www.unicode.org/Public/UCD/latest/ucd/SpecialCasing.txt
+    ///
+    /// This operation performs an unconditional mapping without tailoring. That is, the conversion
+    /// is independent of context and language.
+    ///
+    /// In the [Unicode Standard], Chapter 4 (Character Properties) discusses case mapping in
+    /// general and Chapter 3 (Conformance) discusses the default algorithm for case conversion.
+    ///
+    /// [Unicode Standard]: https://www.unicode.org/versions/latest/
+    ///
+    /// # Examples
+    ///
+    /// As an iterator:
+    ///
+    /// ```
+    /// #![feature(titlecase)]
+    /// for c in 'ß'.to_titlecase() {
+    ///     print!("{c}");
+    /// }
+    /// println!();
+    /// ```
+    ///
+    /// Using `println!` directly:
+    ///
+    /// ```
+    /// #![feature(titlecase)]
+    /// println!("{}", 'ß'.to_titlecase());
+    /// ```
+    ///
+    /// Both are equivalent to:
+    ///
+    /// ```
+    /// println!("Ss");
+    /// ```
+    ///
+    /// Using [`to_string`](../std/string/trait.ToString.html#tymethod.to_string):
+    ///
+    /// ```
+    /// #![feature(titlecase)]
+    /// assert_eq!('c'.to_titlecase().to_string(), "C");
+    /// assert_eq!('dž'.to_titlecase().to_string(), "Dž");
+    /// assert_eq!('ῼ'.to_titlecase().to_string(), "ῼ");
+    ///
+    /// // Sometimes the result is more than one character:
+    /// assert_eq!('ß'.to_titlecase().to_string(), "Ss");
+    ///
+    /// // Characters that do not have separate cased forms
+    /// // convert into themselves.
+    /// assert_eq!('山'.to_titlecase().to_string(), "山");
+    /// ```
+    ///
+    /// # Note on locale
+    ///
+    /// In Turkish and Azeri, the equivalent of 'i' in Latin has five forms instead of two:
+    ///
+    /// * 'Dotless': I / ı, sometimes written ï
+    /// * 'Dotted': İ / i
+    ///
+    /// Note that the lowercase dotted 'i' is the same as the Latin. Therefore:
+    ///
+    /// ```
+    /// #![feature(titlecase)]
+    /// let upper_i = 'i'.to_titlecase().to_string();
+    /// ```
+    ///
+    /// The value of `upper_i` here relies on the language of the text: if we're
+    /// in `en-US`, it should be `"I"`, but if we're in `tr-TR` or `az-AZ`, it should
+    /// be `"İ"`. `to_titlecase()` does not take this into account, and so:
+    ///
+    /// ```
+    /// #![feature(titlecase)]
+    /// let upper_i = 'i'.to_titlecase().to_string();
+    ///
+    /// assert_eq!(upper_i, "I");
+    /// ```
+    ///
+    /// holds across languages.
+    #[must_use = "this returns the titlecased character as a new iterator, \
+                  without modifying the original"]
+    #[unstable(feature = "titlecase", issue = "153892")]
+    #[inline]
+    pub fn to_titlecase(self) -> ToTitlecase {
+        ToTitlecase(CaseMappingIter::new(conversions::to_title(self)))
+    }
+
     /// Returns an iterator that yields the uppercase mapping of this `char` as one or more
     /// `char`s.
     ///
+    /// Prefer this method when converting a word into ALL CAPS, but consider [`Self::to_titlecase`]
+    /// instead if you seek to capitalize Only The First Letter.
+    ///
     /// If this `char` does not have an uppercase mapping, the iterator yields the same `char`.
     ///
     /// If this `char` has a one-to-one uppercase mapping given by the [Unicode Character
@@ -1179,9 +1363,11 @@ impl char {
     ///
     /// ```
     /// assert_eq!('c'.to_uppercase().to_string(), "C");
+    /// assert_eq!('dž'.to_uppercase().to_string(), "DŽ");
     ///
     /// // Sometimes the result is more than one character:
     /// assert_eq!('ſt'.to_uppercase().to_string(), "ST");
+    /// assert_eq!('ῼ'.to_uppercase().to_string(), "ΩΙ");
     ///
     /// // Characters that do not have both uppercase and lowercase
     /// // convert into themselves.
@@ -1190,7 +1376,7 @@ impl char {
     ///
     /// # Note on locale
     ///
-    /// In Turkish, the equivalent of 'i' in Latin has five forms instead of two:
+    /// In Turkish and Azeri, the equivalent of 'i' in Latin has five forms instead of two:
     ///
     /// * 'Dotless': I / ı, sometimes written ï
     /// * 'Dotted': İ / i
@@ -1202,7 +1388,7 @@ impl char {
     /// ```
     ///
     /// The value of `upper_i` here relies on the language of the text: if we're
-    /// in `en-US`, it should be `"I"`, but if we're in `tr_TR`, it should
+    /// in `en-US`, it should be `"I"`, but if we're in `tr-TR` or `az-AZ`, it should
     /// be `"İ"`. `to_uppercase()` does not take this into account, and so:
     ///
     /// ```
@@ -1212,7 +1398,7 @@ impl char {
     /// ```
     ///
     /// holds across languages.
-    #[must_use = "this returns the uppercase character as a new iterator, \
+    #[must_use = "this returns the uppercased character as a new iterator, \
                   without modifying the original"]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
@@ -1455,7 +1641,7 @@ impl char {
     #[rustc_const_stable(feature = "const_ascii_ctype_on_intrinsics", since = "1.47.0")]
     #[inline]
     pub const fn is_ascii_alphabetic(&self) -> bool {
-        matches!(*self, 'A'..='Z' | 'a'..='z')
+        matches!(*self, 'a'..='z' | 'A'..='Z')
     }
 
     /// Checks if the value is an ASCII uppercase character: