Fix handling of upper-case sigma (#124714)

Author: jhorstmann

library/alloc/src/str.rs

@@ -369,14 +369,16 @@ impl str {
     pub fn to_lowercase(&self) -> String {
         let (mut s, rest) = convert_while_ascii(self, u8::to_ascii_lowercase);
 
+        let prefix_len = s.len();
+
         for (i, c) in rest[..].char_indices() {
             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
-                map_uppercase_sigma(rest, i, &mut s)
+                map_uppercase_sigma(self, prefix_len + i, &mut s)
             } else {
                 match conversions::to_lower(c) {
                     [a, '\0', _] => s.push(a),

library/alloc/tests/str.rs

@@ -1826,6 +1826,19 @@ fn to_lowercase() {
     assert_eq!("Α'Σ".to_lowercase(), "α'ς");
     assert_eq!("Α''Σ".to_lowercase(), "α''ς");
 
+    assert_eq!("aΣ".to_lowercase(), "aς");
+    assert_eq!("a'Σ".to_lowercase(), "a'ς");
+    assert_eq!("a''Σ".to_lowercase(), "a''ς");
+
+    assert_eq!("ÄΣ".to_lowercase(), "äς");
+    assert_eq!("ä'Σ".to_lowercase(), "ä'ς");
+    assert_eq!("ä''Σ".to_lowercase(), "ä''ς");
+
+    // input lengths around the boundary of the chunk size used by the ascii prefix optimization
+    assert_eq!("abcdefghijklmnoΣ".to_lowercase(), "abcdefghijklmnoς");
+    assert_eq!("abcdefghijklmnopΣ".to_lowercase(), "abcdefghijklmnopς");
+    assert_eq!("abcdefghijklmnopqΣ".to_lowercase(), "abcdefghijklmnopqς");
+
     assert_eq!("ΑΣ Α".to_lowercase(), "ας α");
     assert_eq!("Α'Σ Α".to_lowercase(), "α'ς α");
     assert_eq!("Α''Σ Α".to_lowercase(), "α''ς α");
@@ -1840,6 +1853,10 @@ fn to_lowercase() {
     assert_eq!("Α 'Σ".to_lowercase(), "α 'σ");
     assert_eq!("Α ''Σ".to_lowercase(), "α ''σ");
 
+    assert_eq!("Ä Σ".to_lowercase(), "ä σ");
+    assert_eq!("Ä 'Σ".to_lowercase(), "ä 'σ");
+    assert_eq!("Ä ''Σ".to_lowercase(), "ä ''σ");
+
     assert_eq!("Σ".to_lowercase(), "σ");
     assert_eq!("'Σ".to_lowercase(), "'σ");
     assert_eq!("''Σ".to_lowercase(), "''σ");