io: export StringWriter

And start using it elsewhere in the standard library, removing the
copies in the process.

While at it, rewrite the io.WriteString godoc to be more clear, since it
can now make reference to the defined interface.

Fixes #27946.

Change-Id: Id5ba223c09c19e5fb49815bd3b1bd3254fc786f3
Reviewed-on: https://go-review.googlesource.com/c/139457
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 3 insertions, 6 deletions

diff --git a/src/net/http/serve_test.go b/src/net/http/serve_test.go
index b12fcf4f9e..a282c4bc17 100644
--- a/src/net/http/serve_test.go
+++ b/src/net/http/serve_test.go
@@ -4028,21 +4028,18 @@ func TestRequestBodyCloseDoesntBlock(t *testing.T) {
 	}
 }
 
-// test that ResponseWriter implements io.stringWriter.
+// test that ResponseWriter implements io.StringWriter.
 func TestResponseWriterWriteString(t *testing.T) {
 	okc := make(chan bool, 1)
 	ht := newHandlerTest(HandlerFunc(func(w ResponseWriter, r *Request) {
-		type stringWriter interface {
-			WriteString(s string) (n int, err error)
-		}
-		_, ok := w.(stringWriter)
+		_, ok := w.(io.StringWriter)
 		okc <- ok
 	}))
 	ht.rawResponse("GET / HTTP/1.0")
 	select {
 	case ok := <-okc:
 		if !ok {
-			t.Error("ResponseWriter did not implement io.stringWriter")
+			t.Error("ResponseWriter did not implement io.StringWriter")
 		}
 	default:
 		t.Error("handler was never called")