From 890125d73e939cda058cf692c6a3ce927fc0898f Mon Sep 17 00:00:00 2001
From: Dan Gohman <dev@sunfishcode.online>
Date: Wed, 13 Apr 2022 14:32:17 -0700
Subject: [PATCH] Add a comment explaining the `(())` idiom for empty structs.

---
 library/std/src/os/windows/io/handle.rs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/library/std/src/os/windows/io/handle.rs b/library/std/src/os/windows/io/handle.rs
index e48f630f76e7..e4de52612ef6 100644
--- a/library/std/src/os/windows/io/handle.rs
+++ b/library/std/src/os/windows/io/handle.rs
@@ -220,6 +220,7 @@ fn try_from(handle_or_invalid: HandleOrInvalid) -> Result<Self, InvalidHandleErr
 
 /// This is the error type used by [`HandleOrNull`] when attempting to convert
 /// into a handle, to indicate that the value is null.
+// The empty field prevents constructing this, and allows extending it in the future.
 #[unstable(feature = "io_safety", issue = "87074")]
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct NullHandleError(());
@@ -237,6 +238,7 @@ impl crate::error::Error for NullHandleError {}
 /// This is the error type used by [`HandleOrInvalid`] when attempting to
 /// convert into a handle, to indicate that the value is
 /// `INVALID_HANDLE_VALUE`.
+// The empty field prevents constructing this, and allows extending it in the future.
 #[unstable(feature = "io_safety", issue = "87074")]
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct InvalidHandleError(());