Address a review comment and fix a bootstrapping issue

src/libcollections/str.rs

@@ -1856,6 +1856,11 @@ impl str {
         let mut s = String::with_capacity(self.len());
         for (i, c) in self[..].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" mechanim.
+                // See https://github.com/rust-lang/rust/issues/26035
                 map_uppercase_sigma(self, i, &mut s)
             } else {
                 s.extend(c.to_lowercase());
@@ -1863,9 +1868,9 @@ impl str {
         }
         return s;
 
-        #[cold]
-        #[inline(never)]
         fn map_uppercase_sigma(from: &str, i: usize, to: &mut String) {
+            // See http://www.unicode.org/versions/Unicode7.0.0/ch03.pdf#G33992
+            // for the definition of `Final_Sigma`.
             debug_assert!('Σ'.len_utf8() == 2);
             let is_word_final =
                 case_ignoreable_then_cased(from[..i].chars().rev()) &&

src/libcollectionstest/str.rs

@@ -1690,8 +1690,34 @@ fn trim_ws() {
 #[test]
 fn to_lowercase() {
     assert_eq!("".to_lowercase(), "");
+    assert_eq!("AÉDžaé ".to_lowercase(), "aédžaé ");
+
     // https://github.com/rust-lang/rust/issues/26035
-    assert_eq!("'Σ AÉΣ'Σ'' Σ DžΣ".to_lowercase(), "'σ aéσ'ς'' σ džς");
+    assert_eq!("ΑΣ".to_lowercase(), "ας");
+    assert_eq!("Α'Σ".to_lowercase(), "α'ς");
+    assert_eq!("Α''Σ".to_lowercase(), "α''ς");
+
+    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");
+
+    assert_eq!("ΑΣ' Α".to_lowercase(), "ας' α");
+    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");
+
+    assert_eq!("Α Σ".to_lowercase(), "α σ");
+    assert_eq!("Α 'Σ".to_lowercase(), "α 'σ");
+    assert_eq!("Α ''Σ".to_lowercase(), "α ''σ");
+
+    assert_eq!("Σ".to_lowercase(), "σ");
+    assert_eq!("'Σ".to_lowercase(), "'σ");
+    assert_eq!("''Σ".to_lowercase(), "''σ");
+
+    assert_eq!("ΑΣΑ".to_lowercase(), "ασα");
+    assert_eq!("ΑΣ'Α".to_lowercase(), "ασ'α");
+    assert_eq!("ΑΣ''Α".to_lowercase(), "ασ''α");
 }
 
 #[test]

src/librustc_lint/lib.rs

@@ -32,7 +32,7 @@
 
 #![feature(box_patterns)]
 #![feature(box_syntax)]
-#![feature(collections)]
+#![cfg_attr(stage0, feature(collections))]
 #![feature(core)]
 #![feature(quote)]
 #![feature(rustc_diagnostic_macros)]

src/librustc_unicode/char.rs

@@ -70,7 +70,7 @@ impl Iterator for ToUppercase {
 /// An iterator over the titlecase mapping of a given character, returned from
 /// the [`to_titlecase` method](../primitive.char.html#method.to_titlecase) on
 /// characters.
-#[stable(feature = "unicode_case_mapping", since = "1.2.0")]
+#[unstable(feature = "unicode", reason = "recently added")]
 pub struct ToTitlecase(CaseMappingIter);
 
 #[stable(feature = "unicode_case_mapping", since = "1.2.0")]
@@ -481,7 +481,7 @@ impl char {
     /// Returns an iterator which yields the characters corresponding to the
     /// lowercase equivalent of the character. If no conversion is possible then
     /// an iterator with just the input character is returned.
-    #[stable(feature = "unicode_case_mapping", since = "1.2.0")]
+    #[unstable(feature = "unicode", reason = "recently added")]
     #[inline]
     pub fn to_titlecase(self) -> ToTitlecase {
         ToTitlecase(CaseMappingIter::new(conversions::to_title(self)))