Implement DoubleEndedIterator for CaseMappingIter

src/libcore/char/mod.rs

@@ -394,6 +394,13 @@ impl Iterator for ToLowercase {
     }
 }
 
+#[stable(feature = "double_ended_case_mapping_iter", since = "1.36.0")]
+impl DoubleEndedIterator for ToLowercase {
+    fn next_back(&mut self) -> Option<char> {
+        self.0.next_back()
+    }
+}
+
 #[stable(feature = "fused", since = "1.26.0")]
 impl FusedIterator for ToLowercase {}
 
@@ -422,6 +429,13 @@ impl Iterator for ToUppercase {
     }
 }
 
+#[stable(feature = "double_ended_case_mapping_iter", since = "1.36.0")]
+impl DoubleEndedIterator for ToUppercase {
+    fn next_back(&mut self) -> Option<char> {
+        self.0.next_back()
+    }
+}
+
 #[stable(feature = "fused", since = "1.26.0")]
 impl FusedIterator for ToUppercase {}
 
@@ -481,6 +495,26 @@ impl Iterator for CaseMappingIter {
     }
 }
 
+impl DoubleEndedIterator for CaseMappingIter {
+    fn next_back(&mut self) -> Option<char> {
+        match *self {
+            CaseMappingIter::Three(a, b, c) => {
+                *self = CaseMappingIter::Two(a, b);
+                Some(c)
+            }
+            CaseMappingIter::Two(a, b) => {
+                *self = CaseMappingIter::One(a);
+                Some(b)
+            }
+            CaseMappingIter::One(a) => {
+                *self = CaseMappingIter::Zero;
+                Some(a)
+            }
+            CaseMappingIter::Zero => None,
+        }
+    }
+}
+
 impl fmt::Display for CaseMappingIter {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match *self {

src/libcore/tests/char.rs

@@ -81,6 +81,7 @@ fn test_to_lowercase() {
         let iter: String = c.to_lowercase().collect();
         let disp: String = c.to_lowercase().to_string();
         assert_eq!(iter, disp);
+        assert!(iter.chars().rev().eq(c.to_lowercase().rev()));
         iter
     }
     assert_eq!(lower('A'), "a");
@@ -108,6 +109,7 @@ fn test_to_uppercase() {
         let iter: String = c.to_uppercase().collect();
         let disp: String = c.to_uppercase().to_string();
         assert_eq!(iter, disp);
+        assert!(iter.chars().rev().eq(c.to_uppercase().rev()));
         iter
     }
     assert_eq!(upper('a'), "A");