feat(windows): add Unix domain socket support

This commit introduces initial, unstable support for Unix domain sockets
(UDS) on Windows, behind the `windows_unix_domain_sockets` feature gate

Added types:
- `std::os::windows::net::SocketAddr`: represents a UDS address with support
  for pathname addresses (abstract and unnamed are parsed but not yet fully
  supported).
- `std::os::windows::net::UnixListener`: server-side UDS listener.
- `std::os::windows::net::UnixStream`: client/server stream for UDS.

Key features:
- Binding and connecting using filesystem paths.
- Basic I/O via `Read`/`Write`.
- Address querying (`local_addr`, `peer_addr`).
- Non-blocking mode, timeouts, and socket duplication.
- Includes basic test coverage for smoke, echo, path length, and bind reuse.

Author: kouhe3

library/std/src/os/windows/mod.rs

@@ -29,6 +29,8 @@
 pub mod ffi;
 pub mod fs;
 pub mod io;
+#[unstable(feature = "windows_unix_domain_sockets", issue = "150487")]
+pub mod net;
 pub mod process;
 pub mod raw;
 pub mod thread;

library/std/src/os/windows/net/addr.rs

@@ -0,0 +1,173 @@
+#![unstable(feature = "windows_unix_domain_sockets", issue = "150487")]
+use crate::bstr::ByteStr;
+use crate::ffi::OsStr;
+use crate::path::Path;
+#[cfg(not(doc))]
+use crate::sys::c::{AF_UNIX, SOCKADDR, SOCKADDR_UN};
+use crate::sys::cvt_nz;
+use crate::{fmt, io, mem, ptr};
+
+#[cfg(not(doc))]
+pub fn sockaddr_un(path: &Path) -> io::Result<(SOCKADDR_UN, usize)> {
+    // SAFETY: All zeros is a valid representation for `sockaddr_un`.
+    let mut addr: SOCKADDR_UN = unsafe { mem::zeroed() };
+    addr.sun_family = AF_UNIX;
+
+    // path to UTF-8 bytes
+    let bytes = path
+        .to_str()
+        .ok_or(io::const_error!(io::ErrorKind::InvalidInput, "path must be valid UTF-8"))?
+        .as_bytes();
+    if bytes.len() >= addr.sun_path.len() {
+        return Err(io::const_error!(io::ErrorKind::InvalidInput, "path too long"));
+    }
+    // SAFETY: `bytes` and `addr.sun_path` are not overlapping and
+    // both point to valid memory.
+    // NOTE: We zeroed the memory above, so the path is already null
+    // terminated.
+    unsafe {
+        ptr::copy_nonoverlapping(bytes.as_ptr(), addr.sun_path.as_mut_ptr().cast(), bytes.len())
+    };
+
+    let len = SUN_PATH_OFFSET + bytes.len() + 1;
+    Ok((addr, len))
+}
+#[cfg(not(doc))]
+const SUN_PATH_OFFSET: usize = mem::offset_of!(SOCKADDR_UN, sun_path);
+pub struct SocketAddr {
+    #[cfg(not(doc))]
+    pub(super) addr: SOCKADDR_UN,
+    pub(super) len: u32, // Use u32 here as same as libc::socklen_t
+}
+impl fmt::Debug for SocketAddr {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.address() {
+            AddressKind::Unnamed => write!(fmt, "(unnamed)"),
+            AddressKind::Abstract(name) => write!(fmt, "{name:?} (abstract)"),
+            AddressKind::Pathname(path) => write!(fmt, "{path:?} (pathname)"),
+        }
+    }
+}
+
+impl SocketAddr {
+    #[cfg(not(doc))]
+    pub(super) fn new<F>(f: F) -> io::Result<SocketAddr>
+    where
+        F: FnOnce(*mut SOCKADDR, *mut i32) -> i32,
+    {
+        unsafe {
+            let mut addr: SOCKADDR_UN = mem::zeroed();
+            let mut len = mem::size_of::<SOCKADDR_UN>() as i32;
+            cvt_nz(f(&raw mut addr as *mut _, &mut len))?;
+            SocketAddr::from_parts(addr, len)
+        }
+    }
+    #[cfg(not(doc))]
+    pub(super) fn from_parts(addr: SOCKADDR_UN, len: i32) -> io::Result<SocketAddr> {
+        if addr.sun_family != AF_UNIX {
+            Err(io::const_error!(io::ErrorKind::InvalidInput, "invalid address family"))
+        } else if len < SUN_PATH_OFFSET as _ || len > mem::size_of::<SOCKADDR_UN>() as _ {
+            Err(io::const_error!(io::ErrorKind::InvalidInput, "invalid address length"))
+        } else {
+            Ok(SocketAddr { addr, len: len as _ })
+        }
+    }
+
+    /// Returns the contents of this address if it is a `pathname` address.
+    ///
+    /// # Examples
+    ///
+    /// With a pathname:
+    ///
+    /// ```no_run
+    /// #![feature(windows_unix_domain_sockets)]
+    /// use std::os::windows::net::UnixListener;
+    /// use std::path::Path;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let socket = UnixListener::bind("/tmp/sock")?;
+    ///     let addr = socket.local_addr().expect("Couldn't get local address");
+    ///     assert_eq!(addr.as_pathname(), Some(Path::new("/tmp/sock")));
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn as_pathname(&self) -> Option<&Path> {
+        if let AddressKind::Pathname(path) = self.address() { Some(path) } else { None }
+    }
+
+    /// Constructs a `SockAddr` with the family `AF_UNIX` and the provided path.
+    ///
+    /// # Errors
+    ///
+    /// Returns an error if the path is longer than `SUN_LEN` or if it contains
+    /// NULL bytes.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// #![feature(windows_unix_domain_sockets)]
+    /// use std::os::windows::net::SocketAddr;
+    /// use std::path::Path;
+    ///
+    /// # fn main() -> std::io::Result<()> {
+    /// let address = SocketAddr::from_pathname("/path/to/socket")?;
+    /// assert_eq!(address.as_pathname(), Some(Path::new("/path/to/socket")));
+    /// # Ok(())
+    /// # }
+    /// ```
+    ///
+    /// Creating a `SocketAddr` with a NULL byte results in an error.
+    ///
+    /// ```no_run
+    /// #![feature(windows_unix_domain_sockets)]
+    /// use std::os::windows::net::SocketAddr;
+    ///
+    /// assert!(SocketAddr::from_pathname("/path/with/\0/bytes").is_err());
+    /// ```
+    pub fn from_pathname<P>(path: P) -> io::Result<SocketAddr>
+    where
+        P: AsRef<Path>,
+    {
+        sockaddr_un(path.as_ref()).map(|(addr, len)| SocketAddr { addr, len: len as _ })
+    }
+    fn address(&self) -> AddressKind<'_> {
+        let len = self.len as usize - SUN_PATH_OFFSET;
+        let path = unsafe { mem::transmute::<&[i8], &[u8]>(&self.addr.sun_path) };
+
+        if len == 0 {
+            AddressKind::Unnamed
+        } else if self.addr.sun_path[0] == 0 {
+            AddressKind::Abstract(ByteStr::from_bytes(&path[1..len]))
+        } else {
+            AddressKind::Pathname(unsafe {
+                OsStr::from_encoded_bytes_unchecked(&path[..len - 1]).as_ref()
+            })
+        }
+    }
+
+    /// Returns `true` if the address is unnamed.
+    ///
+    /// # Examples
+    ///
+    /// A named address:
+    ///
+    /// ```no_run
+    /// #![feature(windows_unix_domain_sockets)]
+    /// use std::os::windows::net::UnixListener;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let socket = UnixListener::bind("/tmp/sock")?;
+    ///     let addr = socket.local_addr().expect("Couldn't get local address");
+    ///     assert_eq!(addr.is_unnamed(), false);
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn is_unnamed(&self) -> bool {
+        matches!(self.address(), AddressKind::Unnamed)
+    }
+}
+enum AddressKind<'a> {
+    Unnamed,
+    Pathname(&'a Path),
+    Abstract(&'a ByteStr),
+}