diff --git a/tokio/Cargo.toml b/tokio/Cargo.toml
index a08fc942a6a..91a16ad51b4 100644
--- a/tokio/Cargo.toml
+++ b/tokio/Cargo.toml
@@ -121,7 +121,7 @@ default-features = false
 optional = true
 
 [dev-dependencies]
-tokio-test = { version = "0.2.0" }
+tokio-test = { version = "0.2.0", path = "../tokio-test" }
 futures = { version = "0.3.0", features = ["async-await"] }
 proptest = "0.9.4"
 tempfile = "3.1.0"
diff --git a/tokio/src/io/util/read_to_string.rs b/tokio/src/io/util/read_to_string.rs
index e77d836dee9..cab0505ab83 100644
--- a/tokio/src/io/util/read_to_string.rs
+++ b/tokio/src/io/util/read_to_string.rs
@@ -4,7 +4,7 @@ use crate::io::AsyncRead;
 use std::future::Future;
 use std::pin::Pin;
 use std::task::{Context, Poll};
-use std::{io, mem, str};
+use std::{io, mem};
 
 cfg_io_util! {
     /// Future for the [`read_to_string`](super::AsyncReadExt::read_to_string) method.
@@ -25,7 +25,7 @@ where
     let start_len = buf.len();
     ReadToString {
         reader,
-        bytes: unsafe { mem::replace(buf.as_mut_vec(), Vec::new()) },
+        bytes: mem::replace(buf, String::new()).into_bytes(),
         buf,
         start_len,
     }
@@ -38,19 +38,20 @@ fn read_to_string_internal<R: AsyncRead + ?Sized>(
     bytes: &mut Vec<u8>,
     start_len: usize,
 ) -> Poll<io::Result<usize>> {
-    let ret = ready!(read_to_end_internal(reader, cx, bytes, start_len));
-    if str::from_utf8(&bytes).is_err() {
-        Poll::Ready(ret.and_then(|_| {
-            Err(io::Error::new(
+    let ret = ready!(read_to_end_internal(reader, cx, bytes, start_len))?;
+    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());
-        // Safety: `bytes` is a valid UTF-8 because `str::from_utf8` returned `Ok`.
-        mem::swap(unsafe { buf.as_mut_vec() }, bytes);
-        Poll::Ready(ret)
+            )))
+        }
     }
 }
 
@@ -67,7 +68,14 @@ where
             bytes,
             start_len,
         } = &mut *self;
-        read_to_string_internal(Pin::new(reader), cx, buf, bytes, *start_len)
+        let ret = read_to_string_internal(Pin::new(reader), cx, buf, bytes, *start_len);
+        if let Poll::Ready(Err(_)) = ret {
+            // Put back the original string.
+            bytes.truncate(*start_len);
+            **buf = String::from_utf8(mem::replace(bytes, Vec::new()))
+                .expect("original string no longer utf-8");
+        }
+        ret
     }
 }
 
diff --git a/tokio/tests/read_to_string.rs b/tokio/tests/read_to_string.rs
new file mode 100644
index 00000000000..db3fa1bf4bd
--- /dev/null
+++ b/tokio/tests/read_to_string.rs
@@ -0,0 +1,49 @@
+use std::io;
+use tokio::io::AsyncReadExt;
+use tokio_test::io::Builder;
+
+#[tokio::test]
+async fn to_string_does_not_truncate_on_utf8_error() {
+    let data = vec![0xff, 0xff, 0xff];
+
+    let mut s = "abc".to_string();
+
+    match AsyncReadExt::read_to_string(&mut data.as_slice(), &mut s).await {
+        Ok(len) => panic!("Should fail: {} bytes.", len),
+        Err(err) if err.to_string() == "stream did not contain valid UTF-8" => {}
+        Err(err) => panic!("Fail: {}.", err),
+    }
+
+    assert_eq!(s, "abc");
+}
+
+#[tokio::test]
+async fn to_string_does_not_truncate_on_io_error() {
+    let mut mock = Builder::new()
+        .read(b"def")
+        .read_error(io::Error::new(io::ErrorKind::Other, "whoops"))
+        .build();
+    let mut s = "abc".to_string();
+
+    match AsyncReadExt::read_to_string(&mut mock, &mut s).await {
+        Ok(len) => panic!("Should fail: {} bytes.", len),
+        Err(err) if err.to_string() == "whoops" => {}
+        Err(err) => panic!("Fail: {}.", err),
+    }
+
+    assert_eq!(s, "abc");
+}
+
+#[tokio::test]
+async fn to_string_appends() {
+    let data = b"def".to_vec();
+
+    let mut s = "abc".to_string();
+
+    let len = AsyncReadExt::read_to_string(&mut data.as_slice(), &mut s)
+        .await
+        .unwrap();
+
+    assert_eq!(len, 3);
+    assert_eq!(s, "abcdef");
+}