Add tests and comments about read_to_string and read_line specializations

library/std/src/io/mod.rs

@@ -2405,6 +2405,9 @@ fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> {
         Ok(read)
     }
 
+    // We don't override `read_to_string` here because an UTF-8 sequence could
+    // be split between the two parts of the chain
+
     fn read_buf(&mut self, mut buf: BorrowedCursor<'_>) -> Result<()> {
         if buf.capacity() == 0 {
             return Ok(());
@@ -2454,6 +2457,9 @@ fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize> {
         read += self.second.read_until(byte, buf)?;
         Ok(read)
     }
+
+    // We don't override `read_line` here because an UTF-8 sequence could be
+    // split between the two parts of the chain
 }
 
 impl<T, U> SizeHint for Chain<T, U> {

library/std/src/io/tests.rs

@@ -231,6 +231,17 @@ fn chain_bufread() {
     cmp_bufread(chain1, chain2, &testdata[..]);
 }
 
+#[test]
+fn chain_splitted_char() {
+    let chain = b"\xc3".chain(b"\xa9".as_slice());
+    assert_eq!(crate::io::read_to_string(chain).unwrap(), "é");
+
+    let mut chain = b"\xc3".chain(b"\xa9\n".as_slice());
+    let mut buf = String::new();
+    assert_eq!(chain.read_line(&mut buf).unwrap(), 3);
+    assert_eq!(buf, "é\n");
+}
+
 #[test]
 fn bufreader_size_hint() {
     let testdata = b"ABCDEFGHIJKL";