Add a test for char::DecodeUtf16::size_hint

library/core/tests/char.rs

@@ -308,6 +308,31 @@ fn test_decode_utf16() {
     check(&[0xD800, 0], &[Err(0xD800), Ok('\0')]);
 }
 
+#[test]
+fn test_decode_utf16_size_hint() {
+    fn check(s: &[u16]) {
+        let mut iter = char::decode_utf16(s.iter().cloned());
+
+        loop {
+            let count = iter.clone().count();
+            let (lower, upper) = iter.size_hint();
+
+            assert!(
+                lower <= count && count <= upper.unwrap(),
+                "lower = {lower}, upper = {upper:?}"
+            );
+
+            if let None = iter.next() {
+                break;
+            }
+        }
+    }
+
+    check(&[0xD800, 0x41, 0x42]);
+    check(&[0xD800, 0]);
+    check(&[0xD834, 0x006d]);
+}
+
 #[test]
 fn ed_iterator_specializations() {
     // Check counting