diff --git a/tokio/src/io/util/lines.rs b/tokio/src/io/util/lines.rs
index be4e86648d8..ee27400c9de 100644
--- a/tokio/src/io/util/lines.rs
+++ b/tokio/src/io/util/lines.rs
@@ -91,6 +91,7 @@ where
         let me = self.project();
 
         let n = ready!(read_line_internal(me.reader, cx, me.buf, me.bytes, me.read))?;
+        debug_assert_eq!(*me.read, 0);
 
         if n == 0 && me.buf.is_empty() {
             return Poll::Ready(Ok(None));
diff --git a/tokio/src/io/util/read_line.rs b/tokio/src/io/util/read_line.rs
index c5ee597486f..d625a76b80a 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! {
@@ -14,45 +13,72 @@ cfg_io_util! {
     #[must_use = "futures do nothing unless you `.await` or poll them"]
     pub struct ReadLine<'a, R: ?Sized> {
         reader: &'a mut R,
-        buf: &'a mut String,
-        bytes: Vec<u8>,
+        /// This is the buffer we were provided. It will be replaced with an empty string
+        /// while reading to postpone utf-8 handling until after reading.
+        output: &'a mut String,
+        /// The actual allocation of the string is moved into a vector instead.
+        buf: Vec<u8>,
+        /// The number of bytes appended to buf. This can be less than buf.len() if
+        /// the buffer was not empty when the operation was started.
         read: usize,
     }
 }
 
-pub(crate) fn read_line<'a, R>(reader: &'a mut R, buf: &'a mut String) -> ReadLine<'a, R>
+pub(crate) fn read_line<'a, R>(reader: &'a mut R, string: &'a mut String) -> ReadLine<'a, R>
 where
     R: AsyncBufRead + ?Sized + Unpin,
 {
     ReadLine {
         reader,
-        bytes: unsafe { mem::replace(buf.as_mut_vec(), Vec::new()) },
-        buf,
+        buf: mem::replace(string, String::new()).into_bytes(),
+        output: string,
         read: 0,
     }
 }
 
+fn put_back_original_data(output: &mut String, mut vector: Vec<u8>, num_bytes_read: usize) {
+    let original_len = vector.len() - num_bytes_read;
+    vector.truncate(original_len);
+    *output = String::from_utf8(vector).expect("The original data must be valid utf-8.");
+}
+
 pub(super) fn read_line_internal<R: AsyncBufRead + ?Sized>(
     reader: Pin<&mut R>,
     cx: &mut Context<'_>,
-    buf: &mut String,
-    bytes: &mut Vec<u8>,
+    output: &mut String,
+    buf: &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 io_res = ready!(read_until_internal(reader, cx, b'\n', buf, read));
+    let utf8_res = String::from_utf8(mem::replace(buf, Vec::new()));
+
+    // At this point both buf and output are empty. The allocation is in utf8_res.
+
+    debug_assert!(buf.is_empty());
+    match (io_res, utf8_res) {
+        (Ok(num_bytes), Ok(string)) => {
+            debug_assert_eq!(*read, 0);
+            *output = string;
+            Poll::Ready(Ok(num_bytes))
+        }
+        (Err(io_err), Ok(string)) => {
+            *output = string;
+            Poll::Ready(Err(io_err))
+        }
+        (Ok(num_bytes), Err(utf8_err)) => {
+            debug_assert_eq!(*read, 0);
+            put_back_original_data(output, utf8_err.into_bytes(), num_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)
+            )))
+        }
+        (Err(io_err), Err(utf8_err)) => {
+            put_back_original_data(output, utf8_err.into_bytes(), *read);
+
+            Poll::Ready(Err(io_err))
+        }
     }
 }
 
@@ -62,11 +88,12 @@ impl<R: AsyncBufRead + ?Sized + Unpin> Future for ReadLine<'_, R> {
     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
         let Self {
             reader,
+            output,
             buf,
-            bytes,
             read,
         } = &mut *self;
-        read_line_internal(Pin::new(reader), cx, buf, bytes, read)
+
+        read_line_internal(Pin::new(reader), cx, output, buf, read)
     }
 }
 
diff --git a/tokio/src/io/util/read_until.rs b/tokio/src/io/util/read_until.rs
index 1adeda66f05..78dac8c2a14 100644
--- a/tokio/src/io/util/read_until.rs
+++ b/tokio/src/io/util/read_until.rs
@@ -8,19 +8,22 @@ use std::task::{Context, Poll};
 
 cfg_io_util! {
     /// Future for the [`read_until`](crate::io::AsyncBufReadExt::read_until) method.
+    /// The delimeter is included in the resulting vector.
     #[derive(Debug)]
     #[must_use = "futures do nothing unless you `.await` or poll them"]
     pub struct ReadUntil<'a, R: ?Sized> {
         reader: &'a mut R,
-        byte: u8,
+        delimeter: u8,
         buf: &'a mut Vec<u8>,
+        /// The number of bytes appended to buf. This can be less than buf.len() if
+        /// the buffer was not empty when the operation was started.
         read: usize,
     }
 }
 
 pub(crate) fn read_until<'a, R>(
     reader: &'a mut R,
-    byte: u8,
+    delimeter: u8,
     buf: &'a mut Vec<u8>,
 ) -> ReadUntil<'a, R>
 where
@@ -28,7 +31,7 @@ where
 {
     ReadUntil {
         reader,
-        byte,
+        delimeter,
         buf,
         read: 0,
     }
@@ -37,14 +40,14 @@ where
 pub(super) fn read_until_internal<R: AsyncBufRead + ?Sized>(
     mut reader: Pin<&mut R>,
     cx: &mut Context<'_>,
-    byte: u8,
+    delimeter: u8,
     buf: &mut Vec<u8>,
     read: &mut usize,
 ) -> Poll<io::Result<usize>> {
     loop {
         let (done, used) = {
             let available = ready!(reader.as_mut().poll_fill_buf(cx))?;
-            if let Some(i) = memchr::memchr(byte, available) {
+            if let Some(i) = memchr::memchr(delimeter, available) {
                 buf.extend_from_slice(&available[..=i]);
                 (true, i + 1)
             } else {
@@ -66,11 +69,11 @@ impl<R: AsyncBufRead + ?Sized + Unpin> Future for ReadUntil<'_, R> {
     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
         let Self {
             reader,
-            byte,
+            delimeter,
             buf,
             read,
         } = &mut *self;
-        read_until_internal(Pin::new(reader), cx, *byte, buf, read)
+        read_until_internal(Pin::new(reader), cx, *delimeter, buf, read)
     }
 }
 
diff --git a/tokio/src/io/util/split.rs b/tokio/src/io/util/split.rs
index f1ed2fd89d3..f552ed503d1 100644
--- a/tokio/src/io/util/split.rs
+++ b/tokio/src/io/util/split.rs
@@ -75,6 +75,8 @@ where
         let n = ready!(read_until_internal(
             me.reader, cx, *me.delim, me.buf, me.read,
         ))?;
+        // read_until_internal resets me.read to zero once it finds the delimeter
+        debug_assert_eq!(*me.read, 0);
 
         if n == 0 && me.buf.is_empty() {
             return Poll::Ready(Ok(None));
diff --git a/tokio/tests/io_read_line.rs b/tokio/tests/io_read_line.rs
index 57ae37cef3e..15841c9b49d 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 +28,80 @@ 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_invalid_utf8() {
+    let mock = Builder::new().read(b"Hello Wor\xffld.\n").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::InvalidData);
+    assert_eq!(err.to_string(), "stream did not contain valid UTF-8");
+    assert_eq!(line.as_str(), "Foo");
+}
+
+#[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");
+}
+
+#[tokio::test]
+async fn read_line_fail_and_utf8_fail() {
+    let mock = Builder::new()
+        .read(b"Hello Wor")
+        .read(b"\xff\xff\xff")
+        .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(), "Foo");
+}
diff --git a/tokio/tests/io_read_until.rs b/tokio/tests/io_read_until.rs
index 4e0e0d10d34..61800a0d9c1 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 +22,53 @@ 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#Fizz\xffBuz")
+        .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, b"Fizz\xffBuzz\n".len());
+    assert_eq!(chunk, b"I solve Fizz\xffBuzz#");
+
+    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 \xffWor")
+        .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 \xffWor");
+}