Make BorrowedFd::borrow_raw a const fn.

Making `BorrowedFd::borrow_raw` a const fn allows it to be used to
create a constant `BorrowedFd<'static>` holding constants such as
`AT_FDCWD`. This will allow [`rustix::fs::cwd`] to become a const fn.

For consistency, make similar changes to `BorrowedHandle::borrow_raw`
and `BorrowedSocket::borrow_raw`.

[`rustix::fs::cwd`]: https://docs.rs/rustix/latest/rustix/fs/fn.cwd.html

Author: sunfishcode

library/std/src/os/fd/owned.rs

@@ -66,8 +66,8 @@ impl BorrowedFd<'_> {
     /// the returned `BorrowedFd`, and it must not have the value `-1`.
     #[inline]
     #[unstable(feature = "io_safety", issue = "87074")]
-    pub unsafe fn borrow_raw(fd: RawFd) -> Self {
-        assert_ne!(fd, u32::MAX as RawFd);
+    pub const unsafe fn borrow_raw(fd: RawFd) -> Self {
+        assert!(fd != u32::MAX as RawFd);
         // SAFETY: we just asserted that the value is in the valid range and isn't `-1` (the only value bigger than `0xFF_FF_FF_FE` unsigned)
         unsafe { Self { fd, _phantom: PhantomData } }
     }

library/std/src/os/windows/io/handle.rs

@@ -137,7 +137,7 @@ impl BorrowedHandle<'_> {
     /// [here]: https://devblogs.microsoft.com/oldnewthing/20040302-00/?p=40443
     #[inline]
     #[unstable(feature = "io_safety", issue = "87074")]
-    pub unsafe fn borrow_raw(handle: RawHandle) -> Self {
+    pub const unsafe fn borrow_raw(handle: RawHandle) -> Self {
         Self { handle, _phantom: PhantomData }
     }
 }

library/std/src/os/windows/io/socket.rs

@@ -71,8 +71,8 @@ impl BorrowedSocket<'_> {
     /// `INVALID_SOCKET`.
     #[inline]
     #[unstable(feature = "io_safety", issue = "87074")]
-    pub unsafe fn borrow_raw(socket: RawSocket) -> Self {
-        debug_assert_ne!(socket, c::INVALID_SOCKET as RawSocket);
+    pub const unsafe fn borrow_raw(socket: RawSocket) -> Self {
+        assert!(socket != c::INVALID_SOCKET as RawSocket);
         Self { socket, _phantom: PhantomData }
     }
 }