Add char::word_to_titlecase() to alloc

Author: Jules-Bertholet

library/alloc/src/str.rs

@@ -185,6 +185,22 @@ where
     result
 }
 
+/// Helper for final sigma lowercase
+fn map_uppercase_sigma(from: &str, i: usize) -> char {
+    fn case_ignorable_then_cased<I: Iterator<Item = char>>(iter: I) -> bool {
+        match iter.skip_while(|&c| c.is_case_ignorable()).next() {
+            Some(c) => c.is_cased(),
+            None => false,
+        }
+    }
+
+    // See https://www.unicode.org/versions/latest/core-spec/chapter-3/#G54277
+    // for the definition of `Final_Sigma`.
+    let is_word_final = case_ignorable_then_cased(from[..i].chars().rev())
+        && !case_ignorable_then_cased(from[i + const { 'Σ'.len_utf8() }..].chars());
+    if is_word_final { 'ς' } else { 'σ' }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Borrow<str> for String {
     #[inline]
@@ -345,7 +361,7 @@ impl str {
     ///
     /// Unlike [`char::to_lowercase()`], this method fully handles the context-dependent
     /// casing of Greek sigma. However, like that method, it does not handle locale-specific
-    /// casing, like Turkish and Azeri I/ı/İ/i. See that method's documentation
+    /// casing, like Turkish and Azeri I/ı/İ/i. See its documentation
     /// for more information.
     ///
     /// # Examples
@@ -415,21 +431,136 @@ impl str {
             }
         }
         return s;
+    }
 
-        fn map_uppercase_sigma(from: &str, i: usize) -> char {
-            // See https://www.unicode.org/versions/Unicode7.0.0/ch03.pdf#G33992
-            // for the definition of `Final_Sigma`.
-            let is_word_final = case_ignorable_then_cased(from[..i].chars().rev())
-                && !case_ignorable_then_cased(from[i + const { 'Σ'.len_utf8() }..].chars());
-            if is_word_final { 'ς' } else { 'σ' }
+    /// Returns the titlecase equivalent of this string slice,
+    /// which is assumed to represent a single word,
+    /// as a new [`String`].
+    ///
+    /// Essentially, this consists of uppercasing the first cased letter
+    /// (with [`char::to_titlecase()`]), and lowercasing everything that follows.
+    ///
+    /// 'Titlecase' is defined according to the terms of
+    /// [Chapter 3 (Conformance)](https://www.unicode.org/versions/latest/core-spec/chapter-3/#G34082)
+    /// of the Unicode standard.
+    ///
+    /// Since some characters can expand into multiple characters when changing
+    /// the case, this function returns a [`String`] instead of modifying the
+    /// parameter in-place.
+    ///
+    /// Unlike [`char::to_lowercase()`], this method fully handles the context-dependent
+    /// casing of Greek sigma. However, like that method, it does not handle locale-specific
+    /// casing, like Turkish and Azeri I/ı/İ/i. See its documentation
+    /// for more information.
+    ///
+    /// This method does not perform any kind of word segmentation.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(titlecase)]
+    /// let s = "HELLO";
+    ///
+    /// assert_eq!("Hello", s.word_to_titlecase());
+    /// ```
+    ///
+    /// The first *cased* letter is uppercased:
+    ///
+    /// ```
+    /// #![feature(titlecase)]
+    /// let the_night_before_christmas = "'twas";
+    ///
+    /// assert_eq!("'Twas", the_night_before_christmas.word_to_titlecase());
+    /// ```
+    ///
+    /// Languages without case are not changed:
+    ///
+    /// ```
+    /// #![feature(titlecase)]
+    /// let new_year = "农历新年";
+    ///
+    /// assert_eq!(new_year, new_year.word_to_titlecase());
+    /// ```
+    ///
+    /// Georgian uppercase ("Mtavruli") letters are not used in titlecase:
+    ///
+    /// ```
+    /// #![feature(titlecase)]
+    /// let georgian = "ერთობაშია";
+    ///
+    /// assert_eq!(georgian, georgian.word_to_titlecase());
+    /// ```
+    ///
+    /// No word segmentation is performed,
+    /// so only the first cased letter in the whole string gets uppercased:
+    ///
+    /// ```
+    /// #![feature(titlecase)]
+    /// let blazingly_fast = "ferris and I";
+    ///
+    /// assert_eq!("Ferris and i", blazingly_fast.word_to_titlecase());
+    /// ```
+    ///
+    /// Tricky examples, with sigma:
+    ///
+    /// ```
+    /// #![feature(titlecase)]
+    /// let odysseus = "ὈΔΥΣΣΕΎΣ";
+    ///
+    /// assert_eq!("Ὀδυσσεύς", odysseus.word_to_titlecase());
+    ///
+    /// let odysseus_king_of_ithaca = "Ο ΟΔΥΣΣΈΑΣ ΒΑΣΙΛΙΆΣ ΤΗΣ ΙΘΆΚΗΣ";
+    ///
+    /// assert_eq!("Ο οδυσσέας βασιλιάς της ιθάκης", odysseus_king_of_ithaca.word_to_titlecase());
+    /// ```
+    #[cfg(not(no_global_oom_handling))]
+    #[rustc_allow_incoherent_impl]
+    #[must_use = "this returns the titlecase word as a new String, \
+                  without modifying the original"]
+    #[unstable(feature = "titlecase", issue = "153892")]
+    pub fn word_to_titlecase(&self) -> String {
+        // FIXME: add ASCII fast path
+
+        let mut s = String::with_capacity(self.len());
+        let mut chars = self.char_indices();
+
+        'until_first_cased_char: for (_, c) in chars.by_ref() {
+            if c.is_cased() {
+                s.extend(c.to_titlecase());
+                break 'until_first_cased_char;
+            } else {
+                s.push(c);
+            }
         }
 
-        fn case_ignorable_then_cased<I: Iterator<Item = char>>(iter: I) -> bool {
-            match iter.skip_while(|&c| c.is_case_ignorable()).next() {
-                Some(c) => c.is_cased(),
-                None => false,
+        for (i, c) in chars {
+            if c == 'Σ' {
+                // Σ maps to σ, except at the end of a word where it maps to ς.
+                // This is the only conditional (contextual) but language-independent mapping
+                // in `SpecialCasing.txt`,
+                // so hard-code it rather than have a generic "condition" mechanism.
+                // See https://github.com/rust-lang/rust/issues/26035
+                let sigma_lowercase = map_uppercase_sigma(self, i);
+                s.push(sigma_lowercase);
+            } else {
+                match conversions::to_lower(c) {
+                    [a, '\0', _] => s.push(a),
+                    [a, b, '\0'] => {
+                        s.push(a);
+                        s.push(b);
+                    }
+                    [a, b, c] => {
+                        s.push(a);
+                        s.push(b);
+                        s.push(c);
+                    }
+                }
             }
         }
+
+        s
     }
 
     /// Returns the uppercase equivalent of this string slice, as a new [`String`].