Add must_use to IntoRawFd/IntoRawSocket/IntoRawHandle's methods.

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

@@ -138,6 +138,7 @@ pub trait IntoRawFd {
     /// let raw_fd: RawFd = f.into_raw_fd();
     /// # Ok::<(), io::Error>(())
     /// ```
+    #[must_use = "losing the raw file descriptor may leak resources"]
     #[stable(feature = "into_raw_os", since = "1.4.0")]
     fn into_raw_fd(self) -> RawFd;
 }

library/std/src/os/solid/io.rs

@@ -347,6 +347,7 @@ pub trait IntoRawFd {
     /// This function **transfers ownership** of the underlying file descriptor
     /// to the caller. Callers are then the unique owners of the file descriptor
     /// and must close the descriptor once it's no longer needed.
+    #[must_use = "losing the raw file descriptor may leak resources"]
     fn into_raw_fd(self) -> RawFd;
 }
 

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

@@ -89,6 +89,7 @@ pub trait IntoRawHandle {
     /// However, transferring ownership is not strictly required. Use a
     /// `Into<OwnedHandle>::into` implementation for an API which strictly
     /// transfers ownership.
+    #[must_use = "losing the raw handle may leak resources"]
     #[stable(feature = "into_raw_os", since = "1.4.0")]
     fn into_raw_handle(self) -> RawHandle;
 }
@@ -230,6 +231,7 @@ pub trait IntoRawSocket {
     /// However, transferring ownership is not strictly required. Use a
     /// `Into<OwnedSocket>::into` implementation for an API which strictly
     /// transfers ownership.
+    #[must_use = "losing the raw socket may leak resources"]
     #[stable(feature = "into_raw_os", since = "1.4.0")]
     fn into_raw_socket(self) -> RawSocket;
 }