Add Safety comments to the As* for Owned* implementations.

Author: sunfishcode

library/std/src/os/unix/io/fd.rs

@@ -178,6 +178,9 @@ impl AsFd for BorrowedFd<'_> {
 impl AsFd for OwnedFd {
     #[inline]
     fn as_fd(&self) -> BorrowedFd<'_> {
+        // Safety: `OwnedFd` and `BorrowedFd` have the same validity
+        // invariants, and the `BorrowdFd` is bounded by the lifetime
+        // of `&self`.
         unsafe { BorrowedFd::borrow_raw_fd(self.as_raw_fd()) }
     }
 }

library/std/src/os/wasi/io/fd.rs

@@ -177,6 +177,9 @@ impl AsFd for BorrowedFd<'_> {
 impl AsFd for OwnedFd {
     #[inline]
     fn as_fd(&self) -> BorrowedFd<'_> {
+        // Safety: `OwnedFd` and `BorrowedFd` have the same validity
+        // invariants, and the `BorrowdFd` is bounded by the lifetime
+        // of `&self`.
         unsafe { BorrowedFd::borrow_raw_fd(self.as_raw_fd()) }
     }
 }

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

@@ -282,6 +282,9 @@ impl AsHandle for BorrowedHandle<'_> {
 impl AsHandle for OwnedHandle {
     #[inline]
     fn as_handle(&self) -> BorrowedHandle<'_> {
+        // Safety: `OwnedHandle` and `BorrowedHandle` have the same validity
+        // invariants, and the `BorrowdHandle` is bounded by the lifetime
+        // of `&self`.
         unsafe { BorrowedHandle::borrow_raw_handle(self.as_raw_handle()) }
     }
 }

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

@@ -145,6 +145,9 @@ impl AsSocket for BorrowedSocket<'_> {
 impl AsSocket for OwnedSocket {
     #[inline]
     fn as_socket(&self) -> BorrowedSocket<'_> {
+        // Safety: `OwnedSocket` and `BorrowedSocket` have the same validity
+        // invariants, and the `BorrowdSocket` is bounded by the lifetime
+        // of `&self`.
         unsafe { BorrowedSocket::borrow_raw_socket(self.as_raw_socket()) }
     }
 }