Implement ExactSizeIterator for ToLowercase and ToUppercase

src/libcore/char/mod.rs

@@ -389,11 +389,17 @@ impl Iterator for ToLowercase {
     fn next(&mut self) -> Option<char> {
         self.0.next()
     }
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 #[stable(feature = "fused", since = "1.26.0")]
 impl FusedIterator for ToLowercase {}
 
+#[stable(feature = "exact_size_case_mapping_iter", since = "1.34.0")]
+impl ExactSizeIterator for ToLowercase {}
+
 /// Returns an iterator that yields the uppercase equivalent of a `char`.
 ///
 /// This `struct` is created by the [`to_uppercase`] method on [`char`]. See
@@ -411,11 +417,17 @@ impl Iterator for ToUppercase {
     fn next(&mut self) -> Option<char> {
         self.0.next()
     }
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 #[stable(feature = "fused", since = "1.26.0")]
 impl FusedIterator for ToUppercase {}
 
+#[stable(feature = "exact_size_case_mapping_iter", since = "1.34.0")]
+impl ExactSizeIterator for ToUppercase {}
+
 #[derive(Debug, Clone)]
 enum CaseMappingIter {
     Three(char, char, char),
@@ -457,6 +469,16 @@ impl Iterator for CaseMappingIter {
             CaseMappingIter::Zero => None,
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let size = match self {
+            CaseMappingIter::Three(..) => 3,
+            CaseMappingIter::Two(..) => 2,
+            CaseMappingIter::One(_) => 1,
+            CaseMappingIter::Zero => 0,
+        };
+        (size, Some(size))
+    }
 }
 
 impl fmt::Display for CaseMappingIter {

src/libcore/tests/char.rs

@@ -76,6 +76,8 @@ fn test_to_digit() {
 #[test]
 fn test_to_lowercase() {
     fn lower(c: char) -> String {
+        let to_lowercase = c.to_uppercase();
+        assert_eq!(to_lowercase.len(), to_lowercase.count());
         let iter: String = c.to_lowercase().collect();
         let disp: String = c.to_lowercase().to_string();
         assert_eq!(iter, disp);
@@ -101,6 +103,8 @@ fn test_to_lowercase() {
 #[test]
 fn test_to_uppercase() {
     fn upper(c: char) -> String {
+        let to_uppercase = c.to_uppercase();
+        assert_eq!(to_uppercase.len(), to_uppercase.count());
         let iter: String = c.to_uppercase().collect();
         let disp: String = c.to_uppercase().to_string();
         assert_eq!(iter, disp);