Convert `Unix{Datagram,Stream}::{set_}passcred()` to per-OS traits

These methods are the pre-stabilized API for obtaining peer credentials
from an `AF_UNIX` socket, part of the `unix_socket_ancillary_data` feature.

Their current behavior is to get/set one of the `SO_PASSCRED` (Linux),
`LOCAL_CREDS_PERSISTENT` (FreeBSD), or `LOCAL_CREDS` (NetBSD) socket
options. On other targets the `{set_}passcred()` methods do not exist.

There are two problems with this approach:

1. Having public methods only exist for certain targets isn't permitted
   in a stable `std` API.

2. These options have generally similar purposes, but they are non-POSIX
   and their details can differ in subtle and surprising ways (such as
   whether they continue to be set after the next call to `recvmsg()`).

Splitting into OS-specific extension traits is the preferred solution to
both problems.

1 files changed, 65 insertions, 0 deletions

diff --git a/library/std/src/os/netbsd/net.rs b/library/std/src/os/netbsd/net.rs
new file mode 100644
index 00000000000..5c82f43077d
--- /dev/null
+++ b/library/std/src/os/netbsd/net.rs
@@ -0,0 +1,65 @@
+//! NetBSD-specific networking functionality.
+
+#![unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+
+use crate::io;
+use crate::os::unix::net;
+use crate::sealed::Sealed;
+use crate::sys_common::AsInner;
+
+/// NetBSD-specific functionality for `AF_UNIX` sockets [`UnixDatagram`]
+/// and [`UnixStream`].
+///
+/// [`UnixDatagram`]: net::UnixDatagram
+/// [`UnixStream`]: net::UnixStream
+#[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+pub trait UnixSocketExt: Sealed {
+    /// Query the current setting of socket option `LOCAL_CREDS`.
+    #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+    fn local_creds(&self) -> io::Result<bool>;
+
+    /// Enable or disable socket option `LOCAL_CREDS`.
+    ///
+    /// This option enables the credentials of the sending process to be
+    /// received as a control message in [`AncillaryData`].
+    ///
+    /// [`AncillaryData`]: net::AncillaryData
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// #![feature(unix_socket_ancillary_data)]
+    /// use std::os::netbsd::net::UnixSocketExt;
+    /// use std::os::unix::net::UnixDatagram;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let sock = UnixDatagram::unbound()?;
+    ///     sock.set_local_creds(true).expect("set_local_creds failed");
+    ///     Ok(())
+    /// }
+    /// ```
+    #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+    fn set_local_creds(&self, local_creds: bool) -> io::Result<()>;
+}
+
+#[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+impl UnixSocketExt for net::UnixDatagram {
+    fn local_creds(&self) -> io::Result<bool> {
+        self.as_inner().local_creds()
+    }
+
+    fn set_local_creds(&self, local_creds: bool) -> io::Result<()> {
+        self.as_inner().set_local_creds(local_creds)
+    }
+}
+
+#[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
+impl UnixSocketExt for net::UnixStream {
+    fn local_creds(&self) -> io::Result<bool> {
+        self.as_inner().local_creds()
+    }
+
+    fn set_local_creds(&self, local_creds: bool) -> io::Result<()> {
+        self.as_inner().set_local_creds(local_creds)
+    }
+}