go/token: delete unused File.set field

This field is only used for a sanity check in a test.

Change-Id: I868ed10131ec33994ebb1b1d88f6740956824bd7
Reviewed-on: https://go-review.googlesource.com/c/go/+/409834
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

src/go/token/position.go

@@ -92,7 +92,6 @@ func (p Pos) IsValid() bool {
 // A File is a handle for a file belonging to a FileSet.
 // A File has a name, size, and line offset table.
 type File struct {
-	set  *FileSet
 	name string // file name as provided to AddFile
 	base int    // Pos value range for this file is [base...base+size]
 	size int    // file size as provided to AddFile
@@ -418,7 +417,7 @@ func (s *FileSet) AddFile(filename string, base, size int) *File {
 		panic(fmt.Sprintf("invalid size %d (should be >= 0)", size))
 	}
 	// base >= s.base && size >= 0
-	f := &File{set: s, name: filename, base: base, size: size, lines: []int{0}}
+	f := &File{name: filename, base: base, size: size, lines: []int{0}}
 	base += size + 1 // +1 because EOF also has a position
 	if base < 0 {
 		panic("token.Pos offset overflow (> 2G of source code in file set)")

src/go/token/serialize.go

@@ -31,7 +31,6 @@ func (s *FileSet) Read(decode func(any) error) error {
 	for i := 0; i < len(ss.Files); i++ {
 		f := &ss.Files[i]
 		files[i] = &File{
-			set:   s,
 			name:  f.Name,
 			base:  f.Base,
 			size:  f.Size,

src/go/token/serialize_test.go

@@ -35,12 +35,6 @@ func equal(p, q *FileSet) error {
 
 	for i, f := range p.files {
 		g := q.files[i]
-		if f.set != p {
-			return fmt.Errorf("wrong fileset for %q", f.name)
-		}
-		if g.set != q {
-			return fmt.Errorf("wrong fileset for %q", g.name)
-		}
 		if f.name != g.name {
 			return fmt.Errorf("different filenames: %q != %q", f.name, g.name)
 		}