go/scanner: clear all fields after Scanner reuse

We were missing s.nlPos = token.NoPos in Init, but while we are here
let's make it less likely to hit this it in future.

Change-Id: Ief4c0ba2cf97bc556d901eabc8e172406a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/738680
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/scanner/scanner.go

@@ -144,18 +144,18 @@ func (s *Scanner) Init(file *token.File, src []byte, err ErrorHandler, mode Mode
 	if file.Size() != len(src) {
 		panic(fmt.Sprintf("file size (%d) does not match src len (%d)", file.Size(), len(src)))
 	}
-	s.file = file
-	s.dir, _ = filepath.Split(file.Name())
-	s.src = src
-	s.err = err
-	s.mode = mode
 
-	s.ch = ' '
-	s.offset = 0
-	s.rdOffset = 0
-	s.lineOffset = 0
-	s.insertSemi = false
-	s.ErrorCount = 0
+	dir, _ := filepath.Split(file.Name())
+
+	*s = Scanner{
+		file: file,
+		dir:  dir,
+		src:  src,
+		err:  err,
+		mode: mode,
+
+		ch: ' ',
+	}
 
 	s.next()
 	if s.ch == bom {

src/go/scanner/scanner_test.go

@@ -1153,3 +1153,29 @@ func TestNumbers(t *testing.T) {
 		}
 	}
 }
+
+func TestScanReuseSemiInNewlineComment(t *testing.T) {
+	fset := token.NewFileSet()
+
+	const src = "identifier /*a\nb*/ + other"
+	var s Scanner
+	s.Init(fset.AddFile("test.go", -1, len(src)), []byte(src), func(pos token.Position, msg string) {
+		t.Fatal(msg)
+	}, ScanComments)
+
+	s.Scan() // IDENT(identifier)
+
+	_, tok, _ := s.Scan() // COMMENT(/*a\nb*/)
+	if tok != token.COMMENT {
+		t.Fatalf("tok = %v; want = token.SEMICOLON", tok)
+	}
+
+	s.Init(fset.AddFile("test.go", -1, len(src)), []byte(src), func(pos token.Position, msg string) {
+		t.Fatal(msg)
+	}, ScanComments)
+
+	_, tok, _ = s.Scan()
+	if tok != token.IDENT {
+		t.Fatalf("tok = %v; want = token.IDENT", tok)
+	}
+}