From 51b09190ac65069b0248bdbc61d50c8026a42efd Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@golang.org>
Date: Thu, 29 Sep 2011 15:35:01 -0700
Subject: [PATCH] io/ioutil: add a comment on why devNull is a ReaderFrom

... protects this optimization from future well-meaning
Gophers looking to delete unused code. :)

R=gri
CC=golang-dev
https://golang.org/cl/5165041
---
 src/pkg/io/ioutil/ioutil.go | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/pkg/io/ioutil/ioutil.go b/src/pkg/io/ioutil/ioutil.go
index fffa1320f5..dd50d96869 100644
--- a/src/pkg/io/ioutil/ioutil.go
+++ b/src/pkg/io/ioutil/ioutil.go
@@ -104,6 +104,10 @@ func NopCloser(r io.Reader) io.ReadCloser {
 
 type devNull int
 
+// devNull implements ReaderFrom as an optimization so io.Copy to
+// ioutil.Discard can avoid doing unnecessary work.
+var _ io.ReaderFrom = devNull(0)
+
 func (devNull) Write(p []byte) (int, os.Error) {
 	return len(p), nil
 }
-- 
2.48.1