os: avoid allocation in File.WriteString

Instead of alloc+copy to convert the string
to a byte slice, do an unsafe conversion.

Rely on the kernel not to scribble on the
buffer during the write.

Fixes #42406

Change-Id: I66f4838b43a11bcc3d67bbfa1706726318d55343
Reviewed-on: https://go-review.googlesource.com/c/go/+/268020
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>

src/os/file.go

@@ -44,11 +44,13 @@ import (
 	"errors"
 	"internal/poll"
 	"internal/testlog"
+	"internal/unsafeheader"
 	"io"
 	"io/fs"
 	"runtime"
 	"syscall"
 	"time"
+	"unsafe"
 )
 
 // Name returns the name of the file as presented to Open.
@@ -246,7 +248,12 @@ func (f *File) Seek(offset int64, whence int) (ret int64, err error) {
 // WriteString is like Write, but writes the contents of string s rather than
 // a slice of bytes.
 func (f *File) WriteString(s string) (n int, err error) {
-	return f.Write([]byte(s))
+	var b []byte
+	hdr := (*unsafeheader.Slice)(unsafe.Pointer(&b))
+	hdr.Data = (*unsafeheader.String)(unsafe.Pointer(&s)).Data
+	hdr.Cap = len(s)
+	hdr.Len = len(s)
+	return f.Write(b)
 }
 
 // Mkdir creates a new directory with the specified name and permission

src/os/os_test.go

@@ -2773,3 +2773,21 @@ func TestReadFileProc(t *testing.T) {
 		t.Fatalf("read %s: not newline-terminated: %q", name, data)
 	}
 }
+
+func TestWriteStringAlloc(t *testing.T) {
+	if runtime.GOOS == "js" {
+		t.Skip("js allocates a lot during File.WriteString")
+	}
+	d := t.TempDir()
+	f, err := Create(filepath.Join(d, "whiteboard.txt"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	allocs := testing.AllocsPerRun(100, func() {
+		f.WriteString("I will not allocate when passed a string longer than 32 bytes.\n")
+	})
+	if allocs != 0 {
+		t.Errorf("expected 0 allocs for File.WriteString, got %v", allocs)
+	}
+}