Implement Read/Write/Seek for Arc<T>

Added a marker trait `IoHandle` which can be used by the standard library to opt-in types to a blanket implementation of the various IO traits on `Arc<T>` where `&T: IoTrait` for some `IoTrait`.

The marker is required to avoid types like `Arc<[u8]>`  being included, since they don't have interior mutability and would not give expected results.

library/std/src/fs.rs

@@ -45,7 +45,6 @@
 use crate::io::{self, BorrowedCursor, IoSlice, IoSliceMut, Read, Seek, SeekFrom, Write};
 use crate::path::{Path, PathBuf};
 use crate::sealed::Sealed;
-use crate::sync::Arc;
 use crate::sys::{AsInner, AsInnerMut, FromInner, IntoInner, fs as fs_imp};
 use crate::time::SystemTime;
 use crate::{error, fmt};
@@ -1541,58 +1540,7 @@ fn stream_position(&mut self) -> io::Result<u64> {
         (&*self).stream_position()
     }
 }
-
-#[stable(feature = "io_traits_arc", since = "1.73.0")]
-impl Read for Arc<File> {
-    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
-        (&**self).read(buf)
-    }
-    fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
-        (&**self).read_vectored(bufs)
-    }
-    fn read_buf(&mut self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
-        (&**self).read_buf(cursor)
-    }
-    #[inline]
-    fn is_read_vectored(&self) -> bool {
-        (&**self).is_read_vectored()
-    }
-    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
-        (&**self).read_to_end(buf)
-    }
-    fn read_to_string(&mut self, buf: &mut String) -> io::Result<usize> {
-        (&**self).read_to_string(buf)
-    }
-}
-#[stable(feature = "io_traits_arc", since = "1.73.0")]
-impl Write for Arc<File> {
-    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
-        (&**self).write(buf)
-    }
-    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
-        (&**self).write_vectored(bufs)
-    }
-    #[inline]
-    fn is_write_vectored(&self) -> bool {
-        (&**self).is_write_vectored()
-    }
-    #[inline]
-    fn flush(&mut self) -> io::Result<()> {
-        (&**self).flush()
-    }
-}
-#[stable(feature = "io_traits_arc", since = "1.73.0")]
-impl Seek for Arc<File> {
-    fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
-        (&**self).seek(pos)
-    }
-    fn stream_len(&mut self) -> io::Result<u64> {
-        (&**self).stream_len()
-    }
-    fn stream_position(&mut self) -> io::Result<u64> {
-        (&**self).stream_position()
-    }
-}
+impl crate::io::IoHandle for File {}
 
 impl Dir {
     /// Attempts to open a directory at `path` in read-only mode.

library/std/src/io/impls.rs

@@ -4,6 +4,7 @@
 use crate::alloc::Allocator;
 use crate::collections::VecDeque;
 use crate::io::{self, BorrowedCursor, BufRead, IoSlice, IoSliceMut, Read, Seek, SeekFrom, Write};
+use crate::sync::Arc;
 use crate::{cmp, fmt, mem, str};
 
 // =============================================================================
@@ -715,3 +716,122 @@ fn flush(&mut self) -> io::Result<()> {
         Ok(())
     }
 }
+
+#[stable(feature = "io_traits_arc", since = "1.73.0")]
+impl<R: Read + ?Sized> Read for Arc<R>
+where
+    for<'a> &'a R: Read,
+    R: crate::io::IoHandle,
+{
+    #[inline]
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        (&**self).read(buf)
+    }
+
+    #[inline]
+    fn read_buf(&mut self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        (&**self).read_buf(cursor)
+    }
+
+    #[inline]
+    fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
+        (&**self).read_vectored(bufs)
+    }
+
+    #[inline]
+    fn is_read_vectored(&self) -> bool {
+        (&**self).is_read_vectored()
+    }
+
+    #[inline]
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
+        (&**self).read_to_end(buf)
+    }
+
+    #[inline]
+    fn read_to_string(&mut self, buf: &mut String) -> io::Result<usize> {
+        (&**self).read_to_string(buf)
+    }
+
+    #[inline]
+    fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
+        (&**self).read_exact(buf)
+    }
+
+    #[inline]
+    fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> io::Result<()> {
+        (&**self).read_buf_exact(cursor)
+    }
+}
+#[stable(feature = "io_traits_arc", since = "1.73.0")]
+impl<W: Write + ?Sized> Write for Arc<W>
+where
+    for<'a> &'a W: Write,
+    W: crate::io::IoHandle,
+{
+    #[inline]
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        (&**self).write(buf)
+    }
+
+    #[inline]
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        (&**self).write_vectored(bufs)
+    }
+
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        (&**self).is_write_vectored()
+    }
+
+    #[inline]
+    fn flush(&mut self) -> io::Result<()> {
+        (&**self).flush()
+    }
+
+    #[inline]
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        (&**self).write_all(buf)
+    }
+
+    #[inline]
+    fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> io::Result<()> {
+        (&**self).write_all_vectored(bufs)
+    }
+
+    #[inline]
+    fn write_fmt(&mut self, fmt: fmt::Arguments<'_>) -> io::Result<()> {
+        (&**self).write_fmt(fmt)
+    }
+}
+#[stable(feature = "io_traits_arc", since = "1.73.0")]
+impl<S: Seek + ?Sized> Seek for Arc<S>
+where
+    for<'a> &'a S: Seek,
+    S: crate::io::IoHandle,
+{
+    #[inline]
+    fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
+        (&**self).seek(pos)
+    }
+
+    #[inline]
+    fn rewind(&mut self) -> io::Result<()> {
+        (&**self).rewind()
+    }
+
+    #[inline]
+    fn stream_len(&mut self) -> io::Result<u64> {
+        (&**self).stream_len()
+    }
+
+    #[inline]
+    fn stream_position(&mut self) -> io::Result<u64> {
+        (&**self).stream_position()
+    }
+
+    #[inline]
+    fn seek_relative(&mut self, offset: i64) -> io::Result<()> {
+        (&**self).seek_relative(offset)
+    }
+}

library/std/src/io/mod.rs

@@ -2226,6 +2226,21 @@ pub enum SeekFrom {
     Current(#[stable(feature = "rust1", since = "1.0.0")] i64),
 }
 
+/// Marks that a type `T` can have IO traits such as [`Seek`], [`Write`], etc. automatically
+/// implemented for handle types like [`Arc`][arc] as well.
+///
+/// This trait should only be implemented for types where `<&T as Trait>::method(&mut &value, ..)`
+/// would be identical to `<T as Trait>::method(&mut value, ..)`.
+///
+/// [`File`][file] passes this test, as operations on `&File` and `File` both affect
+/// the same underlying file.
+/// `[u8]` fails, because any modification to `&mut &[u8]` would only affect a temporary
+/// and be lost after the method has been called.
+///
+/// [file]: crate::fs::File
+/// [arc]: crate::sync::Arc
+pub(crate) trait IoHandle {}
+
 fn read_until<R: BufRead + ?Sized>(r: &mut R, delim: u8, buf: &mut Vec<u8>) -> Result<usize> {
     let mut read = 0;
     loop {