io: use a sealed trait to constrain VecWithInitialized (#3450)

tokio/src/io/util/read_to_end.rs

@@ -1,4 +1,4 @@
-use crate::io::util::vec_with_initialized::{into_read_buf_parts, VecWithInitialized};
+use crate::io::util::vec_with_initialized::{into_read_buf_parts, VecU8, VecWithInitialized};
 use crate::io::AsyncRead;
 
 use pin_project_lite::pin_project;
@@ -28,16 +28,15 @@ pub(crate) fn read_to_end<'a, R>(reader: &'a mut R, buffer: &'a mut Vec<u8>) ->
 where
     R: AsyncRead + Unpin + ?Sized,
 {
-    // SAFETY: The generic type on VecWithInitialized is &mut Vec<u8>.
     ReadToEnd {
         reader,
-        buf: unsafe { VecWithInitialized::new(buffer) },
+        buf: VecWithInitialized::new(buffer),
         read: 0,
         _pin: PhantomPinned,
     }
 }
 
-pub(super) fn read_to_end_internal<V: AsMut<Vec<u8>>, R: AsyncRead + ?Sized>(
+pub(super) fn read_to_end_internal<V: VecU8, R: AsyncRead + ?Sized>(
     buf: &mut VecWithInitialized<V>,
     mut reader: Pin<&mut R>,
     num_read: &mut usize,
@@ -58,7 +57,7 @@ pub(super) fn read_to_end_internal<V: AsMut<Vec<u8>>, R: AsyncRead + ?Sized>(
 /// Tries to read from the provided AsyncRead.
 ///
 /// The length of the buffer is increased by the number of bytes read.
-fn poll_read_to_end<V: AsMut<Vec<u8>>, R: AsyncRead + ?Sized>(
+fn poll_read_to_end<V: VecU8, R: AsyncRead + ?Sized>(
     buf: &mut VecWithInitialized<V>,
     read: Pin<&mut R>,
     cx: &mut Context<'_>,

tokio/src/io/util/read_to_string.rs

@@ -38,10 +38,9 @@ where
     R: AsyncRead + ?Sized + Unpin,
 {
     let buf = mem::replace(string, String::new()).into_bytes();
-    // SAFETY: The generic type of the VecWithInitialized is Vec<u8>.
     ReadToString {
         reader,
-        buf: unsafe { VecWithInitialized::new(buf) },
+        buf: VecWithInitialized::new(buf),
         output: string,
         read: 0,
         _pin: PhantomPinned,

tokio/src/io/util/vec_with_initialized.rs

@@ -1,6 +1,19 @@
 use crate::io::ReadBuf;
 use std::mem::MaybeUninit;
 
+mod private {
+    pub trait Sealed {}
+
+    impl Sealed for Vec<u8> {}
+    impl Sealed for &mut Vec<u8> {}
+}
+
+/// A sealed trait that constrains the generic type parameter in `VecWithInitialized<V>`.  That struct's safety relies
+/// on certain invariants upheld by `Vec<u8>`.
+pub(crate) trait VecU8: AsMut<Vec<u8>> + private::Sealed {}
+
+impl VecU8 for Vec<u8> {}
+impl VecU8 for &mut Vec<u8> {}
 /// This struct wraps a `Vec<u8>` or `&mut Vec<u8>`, combining it with a
 /// `num_initialized`, which keeps track of the number of initialized bytes
 /// in the unused capacity.
@@ -28,10 +41,9 @@ impl VecWithInitialized<Vec<u8>> {
 
 impl<V> VecWithInitialized<V>
 where
-    V: AsMut<Vec<u8>>,
+    V: VecU8,
 {
-    /// Safety: The generic parameter `V` must be either `Vec<u8>` or `&mut Vec<u8>`.
-    pub(crate) unsafe fn new(mut vec: V) -> Self {
+    pub(crate) fn new(mut vec: V) -> Self {
         // SAFETY: The safety invariants of vector guarantee that the bytes up
         // to its length are initialized.
         Self {