From 1f98ef7793dde6c3a3c216d55ae5858da7e0c34e Mon Sep 17 00:00:00 2001
From: Dan Gohman <dev@sunfishcode.online>
Date: Thu, 10 Feb 2022 18:26:12 -0800
Subject: [PATCH] Implement `AsFd` for `&T` and `&mut T`.

Add implementations of `AsFd` for `&T` and `&mut T`, so that users can
write code like this:

```rust
pub fn fchown<F: AsFd>(fd: F, uid: Option<u32>, gid: Option<u32>) -> io::Result<()> {
```

with `fd: F` rather than `fd: &F`.

And similar for `AsHandle` and `AsSocket` on Windows.

Also, adjust the `fchown` example to pass the file by reference. The
code can work either way now, but passing by reference is more likely
to be what users will want to do.

This is an alternative to #93869, and is a simpler way to achieve the
same goals: users don't need to pass borrowed-`BorrowedFd` arguments,
and it prevents a pitfall in the case where users write `fd: F` instead
of `fd: &F`.
---
 library/std/src/os/fd/owned.rs          | 16 ++++++++++++++++
 library/std/src/os/unix/fs.rs           |  2 +-
 library/std/src/os/windows/io/handle.rs | 16 ++++++++++++++++
 library/std/src/os/windows/io/socket.rs | 16 ++++++++++++++++
 4 files changed, 49 insertions(+), 1 deletion(-)

diff --git a/library/std/src/os/fd/owned.rs b/library/std/src/os/fd/owned.rs
index b53c3e79b0fe6..71c660e718675 100644
--- a/library/std/src/os/fd/owned.rs
+++ b/library/std/src/os/fd/owned.rs
@@ -200,6 +200,22 @@ pub trait AsFd {
     fn as_fd(&self) -> BorrowedFd<'_>;
 }
 
+#[unstable(feature = "io_safety", issue = "87074")]
+impl<T: AsFd> AsFd for &T {
+    #[inline]
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        T::as_fd(self)
+    }
+}
+
+#[unstable(feature = "io_safety", issue = "87074")]
+impl<T: AsFd> AsFd for &mut T {
+    #[inline]
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        T::as_fd(self)
+    }
+}
+
 #[unstable(feature = "io_safety", issue = "87074")]
 impl AsFd for BorrowedFd<'_> {
     #[inline]
diff --git a/library/std/src/os/unix/fs.rs b/library/std/src/os/unix/fs.rs
index db7edcd057432..75d65e6d5fc03 100644
--- a/library/std/src/os/unix/fs.rs
+++ b/library/std/src/os/unix/fs.rs
@@ -966,7 +966,7 @@ pub fn chown<P: AsRef<Path>>(dir: P, uid: Option<u32>, gid: Option<u32>) -> io::
 ///
 /// fn main() -> std::io::Result<()> {
 ///     let f = std::fs::File::open("/file")?;
-///     fs::fchown(f, Some(0), Some(0))?;
+///     fs::fchown(&f, Some(0), Some(0))?;
 ///     Ok(())
 /// }
 /// ```
diff --git a/library/std/src/os/windows/io/handle.rs b/library/std/src/os/windows/io/handle.rs
index e37ce633a129a..8df6c54a4144e 100644
--- a/library/std/src/os/windows/io/handle.rs
+++ b/library/std/src/os/windows/io/handle.rs
@@ -316,6 +316,22 @@ pub trait AsHandle {
     fn as_handle(&self) -> BorrowedHandle<'_>;
 }
 
+#[unstable(feature = "io_safety", issue = "87074")]
+impl<T: AsHandle> AsHandle for &T {
+    #[inline]
+    fn as_handle(&self) -> BorrowedHandle<'_> {
+        T::as_handle(self)
+    }
+}
+
+#[unstable(feature = "io_safety", issue = "87074")]
+impl<T: AsHandle> AsHandle for &mut T {
+    #[inline]
+    fn as_handle(&self) -> BorrowedHandle<'_> {
+        T::as_handle(self)
+    }
+}
+
 impl AsHandle for BorrowedHandle<'_> {
     #[inline]
     fn as_handle(&self) -> BorrowedHandle<'_> {
diff --git a/library/std/src/os/windows/io/socket.rs b/library/std/src/os/windows/io/socket.rs
index d3a5b6dcc76c6..2f13eb77a1b94 100644
--- a/library/std/src/os/windows/io/socket.rs
+++ b/library/std/src/os/windows/io/socket.rs
@@ -210,6 +210,22 @@ pub trait AsSocket {
     fn as_socket(&self) -> BorrowedSocket<'_>;
 }
 
+#[unstable(feature = "io_safety", issue = "87074")]
+impl<T: AsSocket> AsSocket for &T {
+    #[inline]
+    fn as_socket(&self) -> BorrowedSocket<'_> {
+        T::as_socket(self)
+    }
+}
+
+#[unstable(feature = "io_safety", issue = "87074")]
+impl<T: AsSocket> AsSocket for &mut T {
+    #[inline]
+    fn as_socket(&self) -> BorrowedSocket<'_> {
+        T::as_socket(self)
+    }
+}
+
 impl AsSocket for BorrowedSocket<'_> {
     #[inline]
     fn as_socket(&self) -> BorrowedSocket<'_> {