Remove unsafe from ReadToString

We can do everything needed with only safe code.

tokio/src/io/util/read_to_string.rs

@@ -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,
     }
@@ -39,18 +39,17 @@ fn read_to_string_internal<R: AsyncRead + ?Sized>(
     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() {
+    if let Ok(string) = String::from_utf8(mem::take(bytes)) {
+        debug_assert!(buf.is_empty());
+        *bytes = mem::replace(buf, string).into_bytes();
+        Poll::Ready(ret)
+    } else {
         Poll::Ready(ret.and_then(|_| {
             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)
     }
 }