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, 16 insertions, 10 deletions

diff --git a/library/std/src/sys/pal/unix/net.rs b/library/std/src/sys/pal/unix/net.rs
index 60546a3de45..09750b6ffc8 100644
--- a/library/std/src/sys/pal/unix/net.rs
+++ b/library/std/src/sys/pal/unix/net.rs
@@ -465,25 +465,31 @@ impl Socket {
     }
 
     #[cfg(target_os = "netbsd")]
-    pub fn set_passcred(&self, passcred: bool) -> io::Result<()> {
-        setsockopt(self, 0 as libc::c_int, libc::LOCAL_CREDS, passcred as libc::c_int)
+    pub fn set_local_creds(&self, local_creds: bool) -> io::Result<()> {
+        setsockopt(self, 0 as libc::c_int, libc::LOCAL_CREDS, local_creds as libc::c_int)
     }
 
     #[cfg(target_os = "netbsd")]
-    pub fn passcred(&self) -> io::Result<bool> {
-        let passcred: libc::c_int = getsockopt(self, 0 as libc::c_int, libc::LOCAL_CREDS)?;
-        Ok(passcred != 0)
+    pub fn local_creds(&self) -> io::Result<bool> {
+        let local_creds: libc::c_int = getsockopt(self, 0 as libc::c_int, libc::LOCAL_CREDS)?;
+        Ok(local_creds != 0)
     }
 
     #[cfg(target_os = "freebsd")]
-    pub fn set_passcred(&self, passcred: bool) -> io::Result<()> {
-        setsockopt(self, libc::AF_LOCAL, libc::LOCAL_CREDS_PERSISTENT, passcred as libc::c_int)
+    pub fn set_local_creds_persistent(&self, local_creds_persistent: bool) -> io::Result<()> {
+        setsockopt(
+            self,
+            libc::AF_LOCAL,
+            libc::LOCAL_CREDS_PERSISTENT,
+            local_creds_persistent as libc::c_int,
+        )
     }
 
     #[cfg(target_os = "freebsd")]
-    pub fn passcred(&self) -> io::Result<bool> {
-        let passcred: libc::c_int = getsockopt(self, libc::AF_LOCAL, libc::LOCAL_CREDS_PERSISTENT)?;
-        Ok(passcred != 0)
+    pub fn local_creds_persistent(&self) -> io::Result<bool> {
+        let local_creds_persistent: libc::c_int =
+            getsockopt(self, libc::AF_LOCAL, libc::LOCAL_CREDS_PERSISTENT)?;
+        Ok(local_creds_persistent != 0)
     }
 
     #[cfg(not(any(target_os = "solaris", target_os = "illumos", target_os = "vita")))]