diff --git a/tokio/src/io/util/read_line.rs b/tokio/src/io/util/read_line.rs
index c5ee597486f..549314089ed 100644
--- a/tokio/src/io/util/read_line.rs
+++ b/tokio/src/io/util/read_line.rs
@@ -5,7 +5,6 @@ use std::future::Future;
 use std::io;
 use std::mem;
 use std::pin::Pin;
-use std::str;
 use std::task::{Context, Poll};
 
 cfg_io_util! {
@@ -26,7 +25,7 @@ where
 {
     ReadLine {
         reader,
-        bytes: unsafe { mem::replace(buf.as_mut_vec(), Vec::new()) },
+        bytes: mem::replace(buf, String::new()).into_bytes(),
         buf,
         read: 0,
     }
@@ -39,20 +38,20 @@ pub(super) fn read_line_internal<R: AsyncBufRead + ?Sized>(
     bytes: &mut Vec<u8>,
     read: &mut usize,
 ) -> Poll<io::Result<usize>> {
-    let ret = ready!(read_until_internal(reader, cx, b'\n', bytes, read));
-    if str::from_utf8(&bytes).is_err() {
-        Poll::Ready(ret.and_then(|_| {
-            Err(io::Error::new(
+    let ret = ready!(read_until_internal(reader, cx, b'\n', bytes, read))?;
+    match String::from_utf8(mem::replace(bytes, Vec::new())) {
+        Ok(string) => {
+            debug_assert!(buf.is_empty());
+            *buf = string;
+            Poll::Ready(Ok(ret))
+        }
+        Err(e) => {
+            *bytes = e.into_bytes();
+            Poll::Ready(Err(io::Error::new(
                 io::ErrorKind::InvalidData,
                 "stream did not contain valid UTF-8",
-            ))
-        }))
-    } else {
-        debug_assert!(buf.is_empty());
-        debug_assert_eq!(*read, 0);
-        // Safety: `bytes` is a valid UTF-8 because `str::from_utf8` returned `Ok`.
-        mem::swap(unsafe { buf.as_mut_vec() }, bytes);
-        Poll::Ready(ret)
+            )))
+        }
     }
 }
 
@@ -66,7 +65,14 @@ impl<R: AsyncBufRead + ?Sized + Unpin> Future for ReadLine<'_, R> {
             bytes,
             read,
         } = &mut *self;
-        read_line_internal(Pin::new(reader), cx, buf, bytes, read)
+        let ret = ready!(read_line_internal(Pin::new(reader), cx, buf, bytes, read));
+        if ret.is_err() {
+            // If an error occurred, put the data back, but only if it is valid utf-8.
+            if let Ok(string) = String::from_utf8(mem::replace(bytes, Vec::new())) {
+                **buf = string;
+            }
+        }
+        Poll::Ready(ret)
     }
 }
 
diff --git a/tokio/tests/io_read_line.rs b/tokio/tests/io_read_line.rs
index 57ae37cef3e..995aa298721 100644
--- a/tokio/tests/io_read_line.rs
+++ b/tokio/tests/io_read_line.rs
@@ -1,8 +1,9 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "full")]
 
-use tokio::io::AsyncBufReadExt;
-use tokio_test::assert_ok;
+use std::io::ErrorKind;
+use tokio::io::{AsyncBufReadExt, BufReader, Error};
+use tokio_test::{assert_ok, io::Builder};
 
 use std::io::Cursor;
 
@@ -27,3 +28,50 @@ async fn read_line() {
     assert_eq!(n, 0);
     assert_eq!(buf, "");
 }
+
+#[tokio::test]
+async fn read_line_not_all_ready() {
+    let mock = Builder::new()
+        .read(b"Hello Wor")
+        .read(b"ld\nFizzBuz")
+        .read(b"z\n1\n2")
+        .build();
+
+    let mut read = BufReader::new(mock);
+
+    let mut line = "We say ".to_string();
+    let bytes = read.read_line(&mut line).await.unwrap();
+    assert_eq!(bytes, "Hello World\n".len());
+    assert_eq!(line.as_str(), "We say Hello World\n");
+
+    line = "I solve ".to_string();
+    let bytes = read.read_line(&mut line).await.unwrap();
+    assert_eq!(bytes, "FizzBuzz\n".len());
+    assert_eq!(line.as_str(), "I solve FizzBuzz\n");
+
+    line.clear();
+    let bytes = read.read_line(&mut line).await.unwrap();
+    assert_eq!(bytes, 2);
+    assert_eq!(line.as_str(), "1\n");
+
+    line.clear();
+    let bytes = read.read_line(&mut line).await.unwrap();
+    assert_eq!(bytes, 1);
+    assert_eq!(line.as_str(), "2");
+}
+
+#[tokio::test]
+async fn read_line_fail() {
+    let mock = Builder::new()
+        .read(b"Hello Wor")
+        .read_error(Error::new(ErrorKind::Other, "The world has no end"))
+        .build();
+
+    let mut read = BufReader::new(mock);
+
+    let mut line = "Foo".to_string();
+    let err = read.read_line(&mut line).await.expect_err("Should fail");
+    assert_eq!(err.kind(), ErrorKind::Other);
+    assert_eq!(err.to_string(), "The world has no end");
+    assert_eq!(line.as_str(), "FooHello Wor");
+}
diff --git a/tokio/tests/io_read_until.rs b/tokio/tests/io_read_until.rs
index 4e0e0d10d34..99a8990a12b 100644
--- a/tokio/tests/io_read_until.rs
+++ b/tokio/tests/io_read_until.rs
@@ -1,8 +1,9 @@
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "full")]
 
-use tokio::io::AsyncBufReadExt;
-use tokio_test::assert_ok;
+use std::io::ErrorKind;
+use tokio::io::{AsyncBufReadExt, BufReader, Error};
+use tokio_test::{assert_ok, io::Builder};
 
 #[tokio::test]
 async fn read_until() {
@@ -21,3 +22,50 @@ async fn read_until() {
     assert_eq!(n, 0);
     assert_eq!(buf, []);
 }
+
+#[tokio::test]
+async fn read_until_not_all_ready() {
+    let mock = Builder::new()
+        .read(b"Hello Wor")
+        .read(b"ld#FizzBuz")
+        .read(b"z#1#2")
+        .build();
+
+    let mut read = BufReader::new(mock);
+
+    let mut chunk = b"We say ".to_vec();
+    let bytes = read.read_until(b'#', &mut chunk).await.unwrap();
+    assert_eq!(bytes, b"Hello World#".len());
+    assert_eq!(chunk, b"We say Hello World#");
+
+    chunk = b"I solve ".to_vec();
+    let bytes = read.read_until(b'#', &mut chunk).await.unwrap();
+    assert_eq!(bytes, "FizzBuzz\n".len());
+    assert_eq!(chunk, b"I solve FizzBuzz#");
+
+    chunk.clear();
+    let bytes = read.read_until(b'#', &mut chunk).await.unwrap();
+    assert_eq!(bytes, 2);
+    assert_eq!(chunk, b"1#");
+
+    chunk.clear();
+    let bytes = read.read_until(b'#', &mut chunk).await.unwrap();
+    assert_eq!(bytes, 1);
+    assert_eq!(chunk, b"2");
+}
+
+#[tokio::test]
+async fn read_until_fail() {
+    let mock = Builder::new()
+        .read(b"Hello Wor")
+        .read_error(Error::new(ErrorKind::Other, "The world has no end"))
+        .build();
+
+    let mut read = BufReader::new(mock);
+
+    let mut chunk = b"Foo".to_vec();
+    let err = read.read_until(b'#', &mut chunk).await.expect_err("Should fail");
+    assert_eq!(err.kind(), ErrorKind::Other);
+    assert_eq!(err.to_string(), "The world has no end");
+    assert_eq!(chunk, b"FooHello Wor");
+}