Imported Upstream version 1.4upstream/1.4

4 files changed, 1707 insertions, 0 deletions

diff --git a/src/go/scanner/errors.go b/src/go/scanner/errors.go
new file mode 100644
index 000000000..22de69c3c
--- /dev/null
+++ b/src/go/scanner/errors.go
@@ -0,0 +1,126 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package scanner
+
+import (
+	"fmt"
+	"go/token"
+	"io"
+	"sort"
+)
+
+// In an ErrorList, an error is represented by an *Error.
+// The position Pos, if valid, points to the beginning of
+// the offending token, and the error condition is described
+// by Msg.
+//
+type Error struct {
+	Pos token.Position
+	Msg string
+}
+
+// Error implements the error interface.
+func (e Error) Error() string {
+	if e.Pos.Filename != "" || e.Pos.IsValid() {
+		// don't print "<unknown position>"
+		// TODO(gri) reconsider the semantics of Position.IsValid
+		return e.Pos.String() + ": " + e.Msg
+	}
+	return e.Msg
+}
+
+// ErrorList is a list of *Errors.
+// The zero value for an ErrorList is an empty ErrorList ready to use.
+//
+type ErrorList []*Error
+
+// Add adds an Error with given position and error message to an ErrorList.
+func (p *ErrorList) Add(pos token.Position, msg string) {
+	*p = append(*p, &Error{pos, msg})
+}
+
+// Reset resets an ErrorList to no errors.
+func (p *ErrorList) Reset() { *p = (*p)[0:0] }
+
+// ErrorList implements the sort Interface.
+func (p ErrorList) Len() int      { return len(p) }
+func (p ErrorList) Swap(i, j int) { p[i], p[j] = p[j], p[i] }
+
+func (p ErrorList) Less(i, j int) bool {
+	e := &p[i].Pos
+	f := &p[j].Pos
+	// Note that it is not sufficient to simply compare file offsets because
+	// the offsets do not reflect modified line information (through //line
+	// comments).
+	if e.Filename < f.Filename {
+		return true
+	}
+	if e.Filename == f.Filename {
+		if e.Line < f.Line {
+			return true
+		}
+		if e.Line == f.Line {
+			return e.Column < f.Column
+		}
+	}
+	return false
+}
+
+// Sort sorts an ErrorList. *Error entries are sorted by position,
+// other errors are sorted by error message, and before any *Error
+// entry.
+//
+func (p ErrorList) Sort() {
+	sort.Sort(p)
+}
+
+// RemoveMultiples sorts an ErrorList and removes all but the first error per line.
+func (p *ErrorList) RemoveMultiples() {
+	sort.Sort(p)
+	var last token.Position // initial last.Line is != any legal error line
+	i := 0
+	for _, e := range *p {
+		if e.Pos.Filename != last.Filename || e.Pos.Line != last.Line {
+			last = e.Pos
+			(*p)[i] = e
+			i++
+		}
+	}
+	(*p) = (*p)[0:i]
+}
+
+// An ErrorList implements the error interface.
+func (p ErrorList) Error() string {
+	switch len(p) {
+	case 0:
+		return "no errors"
+	case 1:
+		return p[0].Error()
+	}
+	return fmt.Sprintf("%s (and %d more errors)", p[0], len(p)-1)
+}
+
+// Err returns an error equivalent to this error list.
+// If the list is empty, Err returns nil.
+func (p ErrorList) Err() error {
+	if len(p) == 0 {
+		return nil
+	}
+	return p
+}
+
+// PrintError is a utility function that prints a list of errors to w,
+// one error per line, if the err parameter is an ErrorList. Otherwise
+// it prints the err string.
+//
+func PrintError(w io.Writer, err error) {
+	if list, ok := err.(ErrorList); ok {
+		for _, e := range list {
+			fmt.Fprintf(w, "%s\n", e)
+		}
+	} else if err != nil {
+		fmt.Fprintf(w, "%s\n", err)
+	}
+}
diff --git a/src/go/scanner/example_test.go b/src/go/scanner/example_test.go
new file mode 100644
index 000000000..9004a4ad3
--- /dev/null
+++ b/src/go/scanner/example_test.go
@@ -0,0 +1,46 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package scanner_test
+
+import (
+	"fmt"
+	"go/scanner"
+	"go/token"
+)
+
+func ExampleScanner_Scan() {
+	// src is the input that we want to tokenize.
+	src := []byte("cos(x) + 1i*sin(x) // Euler")
+
+	// Initialize the scanner.
+	var s scanner.Scanner
+	fset := token.NewFileSet()                      // positions are relative to fset
+	file := fset.AddFile("", fset.Base(), len(src)) // register input "file"
+	s.Init(file, src, nil /* no error handler */, scanner.ScanComments)
+
+	// Repeated calls to Scan yield the token sequence found in the input.
+	for {
+		pos, tok, lit := s.Scan()
+		if tok == token.EOF {
+			break
+		}
+		fmt.Printf("%s\t%s\t%q\n", fset.Position(pos), tok, lit)
+	}
+
+	// output:
+	// 1:1	IDENT	"cos"
+	// 1:4	(	""
+	// 1:5	IDENT	"x"
+	// 1:6	)	""
+	// 1:8	+	""
+	// 1:10	IMAG	"1i"
+	// 1:12	*	""
+	// 1:13	IDENT	"sin"
+	// 1:16	(	""
+	// 1:17	IDENT	"x"
+	// 1:18	)	""
+	// 1:20	;	"\n"
+	// 1:20	COMMENT	"// Euler"
+}
diff --git a/src/go/scanner/scanner.go b/src/go/scanner/scanner.go
new file mode 100644
index 000000000..cec82ea10
--- /dev/null
+++ b/src/go/scanner/scanner.go
@@ -0,0 +1,760 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package scanner implements a scanner for Go source text.
+// It takes a []byte as source which can then be tokenized
+// through repeated calls to the Scan method.
+//
+package scanner
+
+import (
+	"bytes"
+	"fmt"
+	"go/token"
+	"path/filepath"
+	"strconv"
+	"unicode"
+	"unicode/utf8"
+)
+
+// An ErrorHandler may be provided to Scanner.Init. If a syntax error is
+// encountered and a handler was installed, the handler is called with a
+// position and an error message. The position points to the beginning of
+// the offending token.
+//
+type ErrorHandler func(pos token.Position, msg string)
+
+// A Scanner holds the scanner's internal state while processing
+// a given text.  It can be allocated as part of another data
+// structure but must be initialized via Init before use.
+//
+type Scanner struct {
+	// immutable state
+	file *token.File  // source file handle
+	dir  string       // directory portion of file.Name()
+	src  []byte       // source
+	err  ErrorHandler // error reporting; or nil
+	mode Mode         // scanning mode
+
+	// scanning state
+	ch         rune // current character
+	offset     int  // character offset
+	rdOffset   int  // reading offset (position after current character)
+	lineOffset int  // current line offset
+	insertSemi bool // insert a semicolon before next newline
+
+	// public state - ok to modify
+	ErrorCount int // number of errors encountered
+}
+
+const bom = 0xFEFF // byte order mark, only permitted as very first character
+
+// Read the next Unicode char into s.ch.
+// s.ch < 0 means end-of-file.
+//
+func (s *Scanner) next() {
+	if s.rdOffset < len(s.src) {
+		s.offset = s.rdOffset
+		if s.ch == '\n' {
+			s.lineOffset = s.offset
+			s.file.AddLine(s.offset)
+		}
+		r, w := rune(s.src[s.rdOffset]), 1
+		switch {
+		case r == 0:
+			s.error(s.offset, "illegal character NUL")
+		case r >= 0x80:
+			// not ASCII
+			r, w = utf8.DecodeRune(s.src[s.rdOffset:])
+			if r == utf8.RuneError && w == 1 {
+				s.error(s.offset, "illegal UTF-8 encoding")
+			} else if r == bom && s.offset > 0 {
+				s.error(s.offset, "illegal byte order mark")
+			}
+		}
+		s.rdOffset += w
+		s.ch = r
+	} else {
+		s.offset = len(s.src)
+		if s.ch == '\n' {
+			s.lineOffset = s.offset
+			s.file.AddLine(s.offset)
+		}
+		s.ch = -1 // eof
+	}
+}
+
+// A mode value is a set of flags (or 0).
+// They control scanner behavior.
+//
+type Mode uint
+
+const (
+	ScanComments    Mode = 1 << iota // return comments as COMMENT tokens
+	dontInsertSemis                  // do not automatically insert semicolons - for testing only
+)
+
+// Init prepares the scanner s to tokenize the text src by setting the
+// scanner at the beginning of src. The scanner uses the file set file
+// for position information and it adds line information for each line.
+// It is ok to re-use the same file when re-scanning the same file as
+// line information which is already present is ignored. Init causes a
+// panic if the file size does not match the src size.
+//
+// Calls to Scan will invoke the error handler err if they encounter a
+// syntax error and err is not nil. Also, for each error encountered,
+// the Scanner field ErrorCount is incremented by one. The mode parameter
+// determines how comments are handled.
+//
+// Note that Init may call err if there is an error in the first character
+// of the file.
+//
+func (s *Scanner) Init(file *token.File, src []byte, err ErrorHandler, mode Mode) {
+	// Explicitly initialize all fields since a scanner may be reused.
+	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
+
+	s.next()
+	if s.ch == bom {
+		s.next() // ignore BOM at file beginning
+	}
+}
+
+func (s *Scanner) error(offs int, msg string) {
+	if s.err != nil {
+		s.err(s.file.Position(s.file.Pos(offs)), msg)
+	}
+	s.ErrorCount++
+}
+
+var prefix = []byte("//line ")
+
+func (s *Scanner) interpretLineComment(text []byte) {
+	if bytes.HasPrefix(text, prefix) {
+		// get filename and line number, if any
+		if i := bytes.LastIndex(text, []byte{':'}); i > 0 {
+			if line, err := strconv.Atoi(string(text[i+1:])); err == nil && line > 0 {
+				// valid //line filename:line comment
+				filename := string(bytes.TrimSpace(text[len(prefix):i]))
+				if filename != "" {
+					filename = filepath.Clean(filename)
+					if !filepath.IsAbs(filename) {
+						// make filename relative to current directory
+						filename = filepath.Join(s.dir, filename)
+					}
+				}
+				// update scanner position
+				s.file.AddLineInfo(s.lineOffset+len(text)+1, filename, line) // +len(text)+1 since comment applies to next line
+			}
+		}
+	}
+}
+
+func (s *Scanner) scanComment() string {
+	// initial '/' already consumed; s.ch == '/' || s.ch == '*'
+	offs := s.offset - 1 // position of initial '/'
+	hasCR := false
+
+	if s.ch == '/' {
+		//-style comment
+		s.next()
+		for s.ch != '\n' && s.ch >= 0 {
+			if s.ch == '\r' {
+				hasCR = true
+			}
+			s.next()
+		}
+		if offs == s.lineOffset {
+			// comment starts at the beginning of the current line
+			s.interpretLineComment(s.src[offs:s.offset])
+		}
+		goto exit
+	}
+
+	/*-style comment */
+	s.next()
+	for s.ch >= 0 {
+		ch := s.ch
+		if ch == '\r' {
+			hasCR = true
+		}
+		s.next()
+		if ch == '*' && s.ch == '/' {
+			s.next()
+			goto exit
+		}
+	}
+
+	s.error(offs, "comment not terminated")
+
+exit:
+	lit := s.src[offs:s.offset]
+	if hasCR {
+		lit = stripCR(lit)
+	}
+
+	return string(lit)
+}
+
+func (s *Scanner) findLineEnd() bool {
+	// initial '/' already consumed
+
+	defer func(offs int) {
+		// reset scanner state to where it was upon calling findLineEnd
+		s.ch = '/'
+		s.offset = offs
+		s.rdOffset = offs + 1
+		s.next() // consume initial '/' again
+	}(s.offset - 1)
+
+	// read ahead until a newline, EOF, or non-comment token is found
+	for s.ch == '/' || s.ch == '*' {
+		if s.ch == '/' {
+			//-style comment always contains a newline
+			return true
+		}
+		/*-style comment: look for newline */
+		s.next()
+		for s.ch >= 0 {
+			ch := s.ch
+			if ch == '\n' {
+				return true
+			}
+			s.next()
+			if ch == '*' && s.ch == '/' {
+				s.next()
+				break
+			}
+		}
+		s.skipWhitespace() // s.insertSemi is set
+		if s.ch < 0 || s.ch == '\n' {
+			return true
+		}
+		if s.ch != '/' {
+			// non-comment token
+			return false
+		}
+		s.next() // consume '/'
+	}
+
+	return false
+}
+
+func isLetter(ch rune) bool {
+	return 'a' <= ch && ch <= 'z' || 'A' <= ch && ch <= 'Z' || ch == '_' || ch >= 0x80 && unicode.IsLetter(ch)
+}
+
+func isDigit(ch rune) bool {
+	return '0' <= ch && ch <= '9' || ch >= 0x80 && unicode.IsDigit(ch)
+}
+
+func (s *Scanner) scanIdentifier() string {
+	offs := s.offset
+	for isLetter(s.ch) || isDigit(s.ch) {
+		s.next()
+	}
+	return string(s.src[offs:s.offset])
+}
+
+func digitVal(ch rune) int {
+	switch {
+	case '0' <= ch && ch <= '9':
+		return int(ch - '0')
+	case 'a' <= ch && ch <= 'f':
+		return int(ch - 'a' + 10)
+	case 'A' <= ch && ch <= 'F':
+		return int(ch - 'A' + 10)
+	}
+	return 16 // larger than any legal digit val
+}
+
+func (s *Scanner) scanMantissa(base int) {
+	for digitVal(s.ch) < base {
+		s.next()
+	}
+}
+
+func (s *Scanner) scanNumber(seenDecimalPoint bool) (token.Token, string) {
+	// digitVal(s.ch) < 10
+	offs := s.offset
+	tok := token.INT
+
+	if seenDecimalPoint {
+		offs--
+		tok = token.FLOAT
+		s.scanMantissa(10)
+		goto exponent
+	}
+
+	if s.ch == '0' {
+		// int or float
+		offs := s.offset
+		s.next()
+		if s.ch == 'x' || s.ch == 'X' {
+			// hexadecimal int
+			s.next()
+			s.scanMantissa(16)
+			if s.offset-offs <= 2 {
+				// only scanned "0x" or "0X"
+				s.error(offs, "illegal hexadecimal number")
+			}
+		} else {
+			// octal int or float
+			seenDecimalDigit := false
+			s.scanMantissa(8)
+			if s.ch == '8' || s.ch == '9' {
+				// illegal octal int or float
+				seenDecimalDigit = true
+				s.scanMantissa(10)
+			}
+			if s.ch == '.' || s.ch == 'e' || s.ch == 'E' || s.ch == 'i' {
+				goto fraction
+			}
+			// octal int
+			if seenDecimalDigit {
+				s.error(offs, "illegal octal number")
+			}
+		}
+		goto exit
+	}
+
+	// decimal int or float
+	s.scanMantissa(10)
+
+fraction:
+	if s.ch == '.' {
+		tok = token.FLOAT
+		s.next()
+		s.scanMantissa(10)
+	}
+
+exponent:
+	if s.ch == 'e' || s.ch == 'E' {
+		tok = token.FLOAT
+		s.next()
+		if s.ch == '-' || s.ch == '+' {
+			s.next()
+		}
+		s.scanMantissa(10)
+	}
+
+	if s.ch == 'i' {
+		tok = token.IMAG
+		s.next()
+	}
+
+exit:
+	return tok, string(s.src[offs:s.offset])
+}
+
+// scanEscape parses an escape sequence where rune is the accepted
+// escaped quote. In case of a syntax error, it stops at the offending
+// character (without consuming it) and returns false. Otherwise
+// it returns true.
+func (s *Scanner) scanEscape(quote rune) bool {
+	offs := s.offset
+
+	var n int
+	var base, max uint32
+	switch s.ch {
+	case 'a', 'b', 'f', 'n', 'r', 't', 'v', '\\', quote:
+		s.next()
+		return true
+	case '0', '1', '2', '3', '4', '5', '6', '7':
+		n, base, max = 3, 8, 255
+	case 'x':
+		s.next()
+		n, base, max = 2, 16, 255
+	case 'u':
+		s.next()
+		n, base, max = 4, 16, unicode.MaxRune
+	case 'U':
+		s.next()
+		n, base, max = 8, 16, unicode.MaxRune
+	default:
+		msg := "unknown escape sequence"
+		if s.ch < 0 {
+			msg = "escape sequence not terminated"
+		}
+		s.error(offs, msg)
+		return false
+	}
+
+	var x uint32
+	for n > 0 {
+		d := uint32(digitVal(s.ch))
+		if d >= base {
+			msg := fmt.Sprintf("illegal character %#U in escape sequence", s.ch)
+			if s.ch < 0 {
+				msg = "escape sequence not terminated"
+			}
+			s.error(s.offset, msg)
+			return false
+		}
+		x = x*base + d
+		s.next()
+		n--
+	}
+
+	if x > max || 0xD800 <= x && x < 0xE000 {
+		s.error(offs, "escape sequence is invalid Unicode code point")
+		return false
+	}
+
+	return true
+}
+
+func (s *Scanner) scanRune() string {
+	// '\'' opening already consumed
+	offs := s.offset - 1
+
+	valid := true
+	n := 0
+	for {
+		ch := s.ch
+		if ch == '\n' || ch < 0 {
+			// only report error if we don't have one already
+			if valid {
+				s.error(offs, "rune literal not terminated")
+				valid = false
+			}
+			break
+		}
+		s.next()
+		if ch == '\'' {
+			break
+		}
+		n++
+		if ch == '\\' {
+			if !s.scanEscape('\'') {
+				valid = false
+			}
+			// continue to read to closing quote
+		}
+	}
+
+	if valid && n != 1 {
+		s.error(offs, "illegal rune literal")
+	}
+
+	return string(s.src[offs:s.offset])
+}
+
+func (s *Scanner) scanString() string {
+	// '"' opening already consumed
+	offs := s.offset - 1
+
+	for {
+		ch := s.ch
+		if ch == '\n' || ch < 0 {
+			s.error(offs, "string literal not terminated")
+			break
+		}
+		s.next()
+		if ch == '"' {
+			break
+		}
+		if ch == '\\' {
+			s.scanEscape('"')
+		}
+	}
+
+	return string(s.src[offs:s.offset])
+}
+
+func stripCR(b []byte) []byte {
+	c := make([]byte, len(b))
+	i := 0
+	for _, ch := range b {
+		if ch != '\r' {
+			c[i] = ch
+			i++
+		}
+	}
+	return c[:i]
+}
+
+func (s *Scanner) scanRawString() string {
+	// '`' opening already consumed
+	offs := s.offset - 1
+
+	hasCR := false
+	for {
+		ch := s.ch
+		if ch < 0 {
+			s.error(offs, "raw string literal not terminated")
+			break
+		}
+		s.next()
+		if ch == '`' {
+			break
+		}
+		if ch == '\r' {
+			hasCR = true
+		}
+	}
+
+	lit := s.src[offs:s.offset]
+	if hasCR {
+		lit = stripCR(lit)
+	}
+
+	return string(lit)
+}
+
+func (s *Scanner) skipWhitespace() {
+	for s.ch == ' ' || s.ch == '\t' || s.ch == '\n' && !s.insertSemi || s.ch == '\r' {
+		s.next()
+	}
+}
+
+// Helper functions for scanning multi-byte tokens such as >> += >>= .
+// Different routines recognize different length tok_i based on matches
+// of ch_i. If a token ends in '=', the result is tok1 or tok3
+// respectively. Otherwise, the result is tok0 if there was no other
+// matching character, or tok2 if the matching character was ch2.
+
+func (s *Scanner) switch2(tok0, tok1 token.Token) token.Token {
+	if s.ch == '=' {
+		s.next()
+		return tok1
+	}
+	return tok0
+}
+
+func (s *Scanner) switch3(tok0, tok1 token.Token, ch2 rune, tok2 token.Token) token.Token {
+	if s.ch == '=' {
+		s.next()
+		return tok1
+	}
+	if s.ch == ch2 {
+		s.next()
+		return tok2
+	}
+	return tok0
+}
+
+func (s *Scanner) switch4(tok0, tok1 token.Token, ch2 rune, tok2, tok3 token.Token) token.Token {
+	if s.ch == '=' {
+		s.next()
+		return tok1
+	}
+	if s.ch == ch2 {
+		s.next()
+		if s.ch == '=' {
+			s.next()
+			return tok3
+		}
+		return tok2
+	}
+	return tok0
+}
+
+// Scan scans the next token and returns the token position, the token,
+// and its literal string if applicable. The source end is indicated by
+// token.EOF.
+//
+// If the returned token is a literal (token.IDENT, token.INT, token.FLOAT,
+// token.IMAG, token.CHAR, token.STRING) or token.COMMENT, the literal string
+// has the corresponding value.
+//
+// If the returned token is a keyword, the literal string is the keyword.
+//
+// If the returned token is token.SEMICOLON, the corresponding
+// literal string is ";" if the semicolon was present in the source,
+// and "\n" if the semicolon was inserted because of a newline or
+// at EOF.
+//
+// If the returned token is token.ILLEGAL, the literal string is the
+// offending character.
+//
+// In all other cases, Scan returns an empty literal string.
+//
+// For more tolerant parsing, Scan will return a valid token if
+// possible even if a syntax error was encountered. Thus, even
+// if the resulting token sequence contains no illegal tokens,
+// a client may not assume that no error occurred. Instead it
+// must check the scanner's ErrorCount or the number of calls
+// of the error handler, if there was one installed.
+//
+// Scan adds line information to the file added to the file
+// set with Init. Token positions are relative to that file
+// and thus relative to the file set.
+//
+func (s *Scanner) Scan() (pos token.Pos, tok token.Token, lit string) {
+scanAgain:
+	s.skipWhitespace()
+
+	// current token start
+	pos = s.file.Pos(s.offset)
+
+	// determine token value
+	insertSemi := false
+	switch ch := s.ch; {
+	case isLetter(ch):
+		lit = s.scanIdentifier()
+		if len(lit) > 1 {
+			// keywords are longer than one letter - avoid lookup otherwise
+			tok = token.Lookup(lit)
+			switch tok {
+			case token.IDENT, token.BREAK, token.CONTINUE, token.FALLTHROUGH, token.RETURN:
+				insertSemi = true
+			}
+		} else {
+			insertSemi = true
+			tok = token.IDENT
+		}
+	case '0' <= ch && ch <= '9':
+		insertSemi = true
+		tok, lit = s.scanNumber(false)
+	default:
+		s.next() // always make progress
+		switch ch {
+		case -1:
+			if s.insertSemi {
+				s.insertSemi = false // EOF consumed
+				return pos, token.SEMICOLON, "\n"
+			}
+			tok = token.EOF
+		case '\n':
+			// we only reach here if s.insertSemi was
+			// set in the first place and exited early
+			// from s.skipWhitespace()
+			s.insertSemi = false // newline consumed
+			return pos, token.SEMICOLON, "\n"
+		case '"':
+			insertSemi = true
+			tok = token.STRING
+			lit = s.scanString()
+		case '\'':
+			insertSemi = true
+			tok = token.CHAR
+			lit = s.scanRune()
+		case '`':
+			insertSemi = true
+			tok = token.STRING
+			lit = s.scanRawString()
+		case ':':
+			tok = s.switch2(token.COLON, token.DEFINE)
+		case '.':
+			if '0' <= s.ch && s.ch <= '9' {
+				insertSemi = true
+				tok, lit = s.scanNumber(true)
+			} else if s.ch == '.' {
+				s.next()
+				if s.ch == '.' {
+					s.next()
+					tok = token.ELLIPSIS
+				}
+			} else {
+				tok = token.PERIOD
+			}
+		case ',':
+			tok = token.COMMA
+		case ';':
+			tok = token.SEMICOLON
+			lit = ";"
+		case '(':
+			tok = token.LPAREN
+		case ')':
+			insertSemi = true
+			tok = token.RPAREN
+		case '[':
+			tok = token.LBRACK
+		case ']':
+			insertSemi = true
+			tok = token.RBRACK
+		case '{':
+			tok = token.LBRACE
+		case '}':
+			insertSemi = true
+			tok = token.RBRACE
+		case '+':
+			tok = s.switch3(token.ADD, token.ADD_ASSIGN, '+', token.INC)
+			if tok == token.INC {
+				insertSemi = true
+			}
+		case '-':
+			tok = s.switch3(token.SUB, token.SUB_ASSIGN, '-', token.DEC)
+			if tok == token.DEC {
+				insertSemi = true
+			}
+		case '*':
+			tok = s.switch2(token.MUL, token.MUL_ASSIGN)
+		case '/':
+			if s.ch == '/' || s.ch == '*' {
+				// comment
+				if s.insertSemi && s.findLineEnd() {
+					// reset position to the beginning of the comment
+					s.ch = '/'
+					s.offset = s.file.Offset(pos)
+					s.rdOffset = s.offset + 1
+					s.insertSemi = false // newline consumed
+					return pos, token.SEMICOLON, "\n"
+				}
+				lit = s.scanComment()
+				if s.mode&ScanComments == 0 {
+					// skip comment
+					s.insertSemi = false // newline consumed
+					goto scanAgain
+				}
+				tok = token.COMMENT
+			} else {
+				tok = s.switch2(token.QUO, token.QUO_ASSIGN)
+			}
+		case '%':
+			tok = s.switch2(token.REM, token.REM_ASSIGN)
+		case '^':
+			tok = s.switch2(token.XOR, token.XOR_ASSIGN)
+		case '<':
+			if s.ch == '-' {
+				s.next()
+				tok = token.ARROW
+			} else {
+				tok = s.switch4(token.LSS, token.LEQ, '<', token.SHL, token.SHL_ASSIGN)
+			}
+		case '>':
+			tok = s.switch4(token.GTR, token.GEQ, '>', token.SHR, token.SHR_ASSIGN)
+		case '=':
+			tok = s.switch2(token.ASSIGN, token.EQL)
+		case '!':
+			tok = s.switch2(token.NOT, token.NEQ)
+		case '&':
+			if s.ch == '^' {
+				s.next()
+				tok = s.switch2(token.AND_NOT, token.AND_NOT_ASSIGN)
+			} else {
+				tok = s.switch3(token.AND, token.AND_ASSIGN, '&', token.LAND)
+			}
+		case '|':
+			tok = s.switch3(token.OR, token.OR_ASSIGN, '|', token.LOR)
+		default:
+			// next reports unexpected BOMs - don't repeat
+			if ch != bom {
+				s.error(s.file.Offset(pos), fmt.Sprintf("illegal character %#U", ch))
+			}
+			insertSemi = s.insertSemi // preserve insertSemi info
+			tok = token.ILLEGAL
+			lit = string(ch)
+		}
+	}
+	if s.mode&dontInsertSemis == 0 {
+		s.insertSemi = insertSemi
+	}
+
+	return
+}
diff --git a/src/go/scanner/scanner_test.go b/src/go/scanner/scanner_test.go
new file mode 100644
index 000000000..fc450d8a6
--- /dev/null
+++ b/src/go/scanner/scanner_test.go
@@ -0,0 +1,775 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package scanner
+
+import (
+	"go/token"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+)
+
+var fset = token.NewFileSet()
+
+const /* class */ (
+	special = iota
+	literal
+	operator
+	keyword
+)
+
+func tokenclass(tok token.Token) int {
+	switch {
+	case tok.IsLiteral():
+		return literal
+	case tok.IsOperator():
+		return operator
+	case tok.IsKeyword():
+		return keyword
+	}
+	return special
+}
+
+type elt struct {
+	tok   token.Token
+	lit   string
+	class int
+}
+
+var tokens = [...]elt{
+	// Special tokens
+	{token.COMMENT, "/* a comment */", special},
+	{token.COMMENT, "// a comment \n", special},
+	{token.COMMENT, "/*\r*/", special},
+	{token.COMMENT, "//\r\n", special},
+
+	// Identifiers and basic type literals
+	{token.IDENT, "foobar", literal},
+	{token.IDENT, "a۰۱۸", literal},
+	{token.IDENT, "foo६४", literal},
+	{token.IDENT, "bar９８７６", literal},
+	{token.IDENT, "ŝ", literal},    // was bug (issue 4000)
+	{token.IDENT, "ŝfoo", literal}, // was bug (issue 4000)
+	{token.INT, "0", literal},
+	{token.INT, "1", literal},
+	{token.INT, "123456789012345678890", literal},
+	{token.INT, "01234567", literal},
+	{token.INT, "0xcafebabe", literal},
+	{token.FLOAT, "0.", literal},
+	{token.FLOAT, ".0", literal},
+	{token.FLOAT, "3.14159265", literal},
+	{token.FLOAT, "1e0", literal},
+	{token.FLOAT, "1e+100", literal},
+	{token.FLOAT, "1e-100", literal},
+	{token.FLOAT, "2.71828e-1000", literal},
+	{token.IMAG, "0i", literal},
+	{token.IMAG, "1i", literal},
+	{token.IMAG, "012345678901234567889i", literal},
+	{token.IMAG, "123456789012345678890i", literal},
+	{token.IMAG, "0.i", literal},
+	{token.IMAG, ".0i", literal},
+	{token.IMAG, "3.14159265i", literal},
+	{token.IMAG, "1e0i", literal},
+	{token.IMAG, "1e+100i", literal},
+	{token.IMAG, "1e-100i", literal},
+	{token.IMAG, "2.71828e-1000i", literal},
+	{token.CHAR, "'a'", literal},
+	{token.CHAR, "'\\000'", literal},
+	{token.CHAR, "'\\xFF'", literal},
+	{token.CHAR, "'\\uff16'", literal},
+	{token.CHAR, "'\\U0000ff16'", literal},
+	{token.STRING, "`foobar`", literal},
+	{token.STRING, "`" + `foo
+	                        bar` +
+		"`",
+		literal,
+	},
+	{token.STRING, "`\r`", literal},
+	{token.STRING, "`foo\r\nbar`", literal},
+
+	// Operators and delimiters
+	{token.ADD, "+", operator},
+	{token.SUB, "-", operator},
+	{token.MUL, "*", operator},
+	{token.QUO, "/", operator},
+	{token.REM, "%", operator},
+
+	{token.AND, "&", operator},
+	{token.OR, "|", operator},
+	{token.XOR, "^", operator},
+	{token.SHL, "<<", operator},
+	{token.SHR, ">>", operator},
+	{token.AND_NOT, "&^", operator},
+
+	{token.ADD_ASSIGN, "+=", operator},
+	{token.SUB_ASSIGN, "-=", operator},
+	{token.MUL_ASSIGN, "*=", operator},
+	{token.QUO_ASSIGN, "/=", operator},
+	{token.REM_ASSIGN, "%=", operator},
+
+	{token.AND_ASSIGN, "&=", operator},
+	{token.OR_ASSIGN, "|=", operator},
+	{token.XOR_ASSIGN, "^=", operator},
+	{token.SHL_ASSIGN, "<<=", operator},
+	{token.SHR_ASSIGN, ">>=", operator},
+	{token.AND_NOT_ASSIGN, "&^=", operator},
+
+	{token.LAND, "&&", operator},
+	{token.LOR, "||", operator},
+	{token.ARROW, "<-", operator},
+	{token.INC, "++", operator},
+	{token.DEC, "--", operator},
+
+	{token.EQL, "==", operator},
+	{token.LSS, "<", operator},
+	{token.GTR, ">", operator},
+	{token.ASSIGN, "=", operator},
+	{token.NOT, "!", operator},
+
+	{token.NEQ, "!=", operator},
+	{token.LEQ, "<=", operator},
+	{token.GEQ, ">=", operator},
+	{token.DEFINE, ":=", operator},
+	{token.ELLIPSIS, "...", operator},
+
+	{token.LPAREN, "(", operator},
+	{token.LBRACK, "[", operator},
+	{token.LBRACE, "{", operator},
+	{token.COMMA, ",", operator},
+	{token.PERIOD, ".", operator},
+
+	{token.RPAREN, ")", operator},
+	{token.RBRACK, "]", operator},
+	{token.RBRACE, "}", operator},
+	{token.SEMICOLON, ";", operator},
+	{token.COLON, ":", operator},
+
+	// Keywords
+	{token.BREAK, "break", keyword},
+	{token.CASE, "case", keyword},
+	{token.CHAN, "chan", keyword},
+	{token.CONST, "const", keyword},
+	{token.CONTINUE, "continue", keyword},
+
+	{token.DEFAULT, "default", keyword},
+	{token.DEFER, "defer", keyword},
+	{token.ELSE, "else", keyword},
+	{token.FALLTHROUGH, "fallthrough", keyword},
+	{token.FOR, "for", keyword},
+
+	{token.FUNC, "func", keyword},
+	{token.GO, "go", keyword},
+	{token.GOTO, "goto", keyword},
+	{token.IF, "if", keyword},
+	{token.IMPORT, "import", keyword},
+
+	{token.INTERFACE, "interface", keyword},
+	{token.MAP, "map", keyword},
+	{token.PACKAGE, "package", keyword},
+	{token.RANGE, "range", keyword},
+	{token.RETURN, "return", keyword},
+
+	{token.SELECT, "select", keyword},
+	{token.STRUCT, "struct", keyword},
+	{token.SWITCH, "switch", keyword},
+	{token.TYPE, "type", keyword},
+	{token.VAR, "var", keyword},
+}
+
+const whitespace = "  \t  \n\n\n" // to separate tokens
+
+var source = func() []byte {
+	var src []byte
+	for _, t := range tokens {
+		src = append(src, t.lit...)
+		src = append(src, whitespace...)
+	}
+	return src
+}()
+
+func newlineCount(s string) int {
+	n := 0
+	for i := 0; i < len(s); i++ {
+		if s[i] == '\n' {
+			n++
+		}
+	}
+	return n
+}
+
+func checkPos(t *testing.T, lit string, p token.Pos, expected token.Position) {
+	pos := fset.Position(p)
+	if pos.Filename != expected.Filename {
+		t.Errorf("bad filename for %q: got %s, expected %s", lit, pos.Filename, expected.Filename)
+	}
+	if pos.Offset != expected.Offset {
+		t.Errorf("bad position for %q: got %d, expected %d", lit, pos.Offset, expected.Offset)
+	}
+	if pos.Line != expected.Line {
+		t.Errorf("bad line for %q: got %d, expected %d", lit, pos.Line, expected.Line)
+	}
+	if pos.Column != expected.Column {
+		t.Errorf("bad column for %q: got %d, expected %d", lit, pos.Column, expected.Column)
+	}
+}
+
+// Verify that calling Scan() provides the correct results.
+func TestScan(t *testing.T) {
+	whitespace_linecount := newlineCount(whitespace)
+
+	// error handler
+	eh := func(_ token.Position, msg string) {
+		t.Errorf("error handler called (msg = %s)", msg)
+	}
+
+	// verify scan
+	var s Scanner
+	s.Init(fset.AddFile("", fset.Base(), len(source)), source, eh, ScanComments|dontInsertSemis)
+
+	// set up expected position
+	epos := token.Position{
+		Filename: "",
+		Offset:   0,
+		Line:     1,
+		Column:   1,
+	}
+
+	index := 0
+	for {
+		pos, tok, lit := s.Scan()
+
+		// check position
+		if tok == token.EOF {
+			// correction for EOF
+			epos.Line = newlineCount(string(source))
+			epos.Column = 2
+		}
+		checkPos(t, lit, pos, epos)
+
+		// check token
+		e := elt{token.EOF, "", special}
+		if index < len(tokens) {
+			e = tokens[index]
+			index++
+		}
+		if tok != e.tok {
+			t.Errorf("bad token for %q: got %s, expected %s", lit, tok, e.tok)
+		}
+
+		// check token class
+		if tokenclass(tok) != e.class {
+			t.Errorf("bad class for %q: got %d, expected %d", lit, tokenclass(tok), e.class)
+		}
+
+		// check literal
+		elit := ""
+		switch e.tok {
+		case token.COMMENT:
+			// no CRs in comments
+			elit = string(stripCR([]byte(e.lit)))
+			//-style comment literal doesn't contain newline
+			if elit[1] == '/' {
+				elit = elit[0 : len(elit)-1]
+			}
+		case token.IDENT:
+			elit = e.lit
+		case token.SEMICOLON:
+			elit = ";"
+		default:
+			if e.tok.IsLiteral() {
+				// no CRs in raw string literals
+				elit = e.lit
+				if elit[0] == '`' {
+					elit = string(stripCR([]byte(elit)))
+				}
+			} else if e.tok.IsKeyword() {
+				elit = e.lit
+			}
+		}
+		if lit != elit {
+			t.Errorf("bad literal for %q: got %q, expected %q", lit, lit, elit)
+		}
+
+		if tok == token.EOF {
+			break
+		}
+
+		// update position
+		epos.Offset += len(e.lit) + len(whitespace)
+		epos.Line += newlineCount(e.lit) + whitespace_linecount
+
+	}
+
+	if s.ErrorCount != 0 {
+		t.Errorf("found %d errors", s.ErrorCount)
+	}
+}
+
+func checkSemi(t *testing.T, line string, mode Mode) {
+	var S Scanner
+	file := fset.AddFile("TestSemis", fset.Base(), len(line))
+	S.Init(file, []byte(line), nil, mode)
+	pos, tok, lit := S.Scan()
+	for tok != token.EOF {
+		if tok == token.ILLEGAL {
+			// the illegal token literal indicates what
+			// kind of semicolon literal to expect
+			semiLit := "\n"
+			if lit[0] == '#' {
+				semiLit = ";"
+			}
+			// next token must be a semicolon
+			semiPos := file.Position(pos)
+			semiPos.Offset++
+			semiPos.Column++
+			pos, tok, lit = S.Scan()
+			if tok == token.SEMICOLON {
+				if lit != semiLit {
+					t.Errorf(`bad literal for %q: got %q, expected %q`, line, lit, semiLit)
+				}
+				checkPos(t, line, pos, semiPos)
+			} else {
+				t.Errorf("bad token for %q: got %s, expected ;", line, tok)
+			}
+		} else if tok == token.SEMICOLON {
+			t.Errorf("bad token for %q: got ;, expected no ;", line)
+		}
+		pos, tok, lit = S.Scan()
+	}
+}
+
+var lines = []string{
+	// # indicates a semicolon present in the source
+	// $ indicates an automatically inserted semicolon
+	"",
+	"\ufeff#;", // first BOM is ignored
+	"#;",
+	"foo$\n",
+	"123$\n",
+	"1.2$\n",
+	"'x'$\n",
+	`"x"` + "$\n",
+	"`x`$\n",
+
+	"+\n",
+	"-\n",
+	"*\n",
+	"/\n",
+	"%\n",
+
+	"&\n",
+	"|\n",
+	"^\n",
+	"<<\n",
+	">>\n",
+	"&^\n",
+
+	"+=\n",
+	"-=\n",
+	"*=\n",
+	"/=\n",
+	"%=\n",
+
+	"&=\n",
+	"|=\n",
+	"^=\n",
+	"<<=\n",
+	">>=\n",
+	"&^=\n",
+
+	"&&\n",
+	"||\n",
+	"<-\n",
+	"++$\n",
+	"--$\n",
+
+	"==\n",
+	"<\n",
+	">\n",
+	"=\n",
+	"!\n",
+
+	"!=\n",
+	"<=\n",
+	">=\n",
+	":=\n",
+	"...\n",
+
+	"(\n",
+	"[\n",
+	"{\n",
+	",\n",
+	".\n",
+
+	")$\n",
+	"]$\n",
+	"}$\n",
+	"#;\n",
+	":\n",
+
+	"break$\n",
+	"case\n",
+	"chan\n",
+	"const\n",
+	"continue$\n",
+
+	"default\n",
+	"defer\n",
+	"else\n",
+	"fallthrough$\n",
+	"for\n",
+
+	"func\n",
+	"go\n",
+	"goto\n",
+	"if\n",
+	"import\n",
+
+	"interface\n",
+	"map\n",
+	"package\n",
+	"range\n",
+	"return$\n",
+
+	"select\n",
+	"struct\n",
+	"switch\n",
+	"type\n",
+	"var\n",
+
+	"foo$//comment\n",
+	"foo$//comment",
+	"foo$/*comment*/\n",
+	"foo$/*\n*/",
+	"foo$/*comment*/    \n",
+	"foo$/*\n*/    ",
+
+	"foo    $// comment\n",
+	"foo    $// comment",
+	"foo    $/*comment*/\n",
+	"foo    $/*\n*/",
+	"foo    $/*  */ /* \n */ bar$/**/\n",
+	"foo    $/*0*/ /*1*/ /*2*/\n",
+
+	"foo    $/*comment*/    \n",
+	"foo    $/*0*/ /*1*/ /*2*/    \n",
+	"foo	$/**/ /*-------------*/       /*----\n*/bar       $/*  \n*/baa$\n",
+	"foo    $/* an EOF terminates a line */",
+	"foo    $/* an EOF terminates a line */ /*",
+	"foo    $/* an EOF terminates a line */ //",
+
+	"package main$\n\nfunc main() {\n\tif {\n\t\treturn /* */ }$\n}$\n",
+	"package main$",
+}
+
+func TestSemis(t *testing.T) {
+	for _, line := range lines {
+		checkSemi(t, line, 0)
+		checkSemi(t, line, ScanComments)
+
+		// if the input ended in newlines, the input must tokenize the
+		// same with or without those newlines
+		for i := len(line) - 1; i >= 0 && line[i] == '\n'; i-- {
+			checkSemi(t, line[0:i], 0)
+			checkSemi(t, line[0:i], ScanComments)
+		}
+	}
+}
+
+type segment struct {
+	srcline  string // a line of source text
+	filename string // filename for current token
+	line     int    // line number for current token
+}
+
+var segments = []segment{
+	// exactly one token per line since the test consumes one token per segment
+	{"  line1", filepath.Join("dir", "TestLineComments"), 1},
+	{"\nline2", filepath.Join("dir", "TestLineComments"), 2},
+	{"\nline3  //line File1.go:100", filepath.Join("dir", "TestLineComments"), 3}, // bad line comment, ignored
+	{"\nline4", filepath.Join("dir", "TestLineComments"), 4},
+	{"\n//line File1.go:100\n  line100", filepath.Join("dir", "File1.go"), 100},
+	{"\n//line  \t :42\n  line1", "", 42},
+	{"\n//line File2.go:200\n  line200", filepath.Join("dir", "File2.go"), 200},
+	{"\n//line foo\t:42\n  line42", filepath.Join("dir", "foo"), 42},
+	{"\n //line foo:42\n  line44", filepath.Join("dir", "foo"), 44},           // bad line comment, ignored
+	{"\n//line foo 42\n  line46", filepath.Join("dir", "foo"), 46},            // bad line comment, ignored
+	{"\n//line foo:42 extra text\n  line48", filepath.Join("dir", "foo"), 48}, // bad line comment, ignored
+	{"\n//line ./foo:42\n  line42", filepath.Join("dir", "foo"), 42},
+	{"\n//line a/b/c/File1.go:100\n  line100", filepath.Join("dir", "a", "b", "c", "File1.go"), 100},
+}
+
+var unixsegments = []segment{
+	{"\n//line /bar:42\n  line42", "/bar", 42},
+}
+
+var winsegments = []segment{
+	{"\n//line c:\\bar:42\n  line42", "c:\\bar", 42},
+	{"\n//line c:\\dir\\File1.go:100\n  line100", "c:\\dir\\File1.go", 100},
+}
+
+// Verify that comments of the form "//line filename:line" are interpreted correctly.
+func TestLineComments(t *testing.T) {
+	segs := segments
+	if runtime.GOOS == "windows" {
+		segs = append(segs, winsegments...)
+	} else {
+		segs = append(segs, unixsegments...)
+	}
+
+	// make source
+	var src string
+	for _, e := range segs {
+		src += e.srcline
+	}
+
+	// verify scan
+	var S Scanner
+	file := fset.AddFile(filepath.Join("dir", "TestLineComments"), fset.Base(), len(src))
+	S.Init(file, []byte(src), nil, dontInsertSemis)
+	for _, s := range segs {
+		p, _, lit := S.Scan()
+		pos := file.Position(p)
+		checkPos(t, lit, p, token.Position{
+			Filename: s.filename,
+			Offset:   pos.Offset,
+			Line:     s.line,
+			Column:   pos.Column,
+		})
+	}
+
+	if S.ErrorCount != 0 {
+		t.Errorf("found %d errors", S.ErrorCount)
+	}
+}
+
+// Verify that initializing the same scanner more than once works correctly.
+func TestInit(t *testing.T) {
+	var s Scanner
+
+	// 1st init
+	src1 := "if true { }"
+	f1 := fset.AddFile("src1", fset.Base(), len(src1))
+	s.Init(f1, []byte(src1), nil, dontInsertSemis)
+	if f1.Size() != len(src1) {
+		t.Errorf("bad file size: got %d, expected %d", f1.Size(), len(src1))
+	}
+	s.Scan()              // if
+	s.Scan()              // true
+	_, tok, _ := s.Scan() // {
+	if tok != token.LBRACE {
+		t.Errorf("bad token: got %s, expected %s", tok, token.LBRACE)
+	}
+
+	// 2nd init
+	src2 := "go true { ]"
+	f2 := fset.AddFile("src2", fset.Base(), len(src2))
+	s.Init(f2, []byte(src2), nil, dontInsertSemis)
+	if f2.Size() != len(src2) {
+		t.Errorf("bad file size: got %d, expected %d", f2.Size(), len(src2))
+	}
+	_, tok, _ = s.Scan() // go
+	if tok != token.GO {
+		t.Errorf("bad token: got %s, expected %s", tok, token.GO)
+	}
+
+	if s.ErrorCount != 0 {
+		t.Errorf("found %d errors", s.ErrorCount)
+	}
+}
+
+func TestStdErrorHander(t *testing.T) {
+	const src = "@\n" + // illegal character, cause an error
+		"@ @\n" + // two errors on the same line
+		"//line File2:20\n" +
+		"@\n" + // different file, but same line
+		"//line File2:1\n" +
+		"@ @\n" + // same file, decreasing line number
+		"//line File1:1\n" +
+		"@ @ @" // original file, line 1 again
+
+	var list ErrorList
+	eh := func(pos token.Position, msg string) { list.Add(pos, msg) }
+
+	var s Scanner
+	s.Init(fset.AddFile("File1", fset.Base(), len(src)), []byte(src), eh, dontInsertSemis)
+	for {
+		if _, tok, _ := s.Scan(); tok == token.EOF {
+			break
+		}
+	}
+
+	if len(list) != s.ErrorCount {
+		t.Errorf("found %d errors, expected %d", len(list), s.ErrorCount)
+	}
+
+	if len(list) != 9 {
+		t.Errorf("found %d raw errors, expected 9", len(list))
+		PrintError(os.Stderr, list)
+	}
+
+	list.Sort()
+	if len(list) != 9 {
+		t.Errorf("found %d sorted errors, expected 9", len(list))
+		PrintError(os.Stderr, list)
+	}
+
+	list.RemoveMultiples()
+	if len(list) != 4 {
+		t.Errorf("found %d one-per-line errors, expected 4", len(list))
+		PrintError(os.Stderr, list)
+	}
+}
+
+type errorCollector struct {
+	cnt int            // number of errors encountered
+	msg string         // last error message encountered
+	pos token.Position // last error position encountered
+}
+
+func checkError(t *testing.T, src string, tok token.Token, pos int, lit, err string) {
+	var s Scanner
+	var h errorCollector
+	eh := func(pos token.Position, msg string) {
+		h.cnt++
+		h.msg = msg
+		h.pos = pos
+	}
+	s.Init(fset.AddFile("", fset.Base(), len(src)), []byte(src), eh, ScanComments|dontInsertSemis)
+	_, tok0, lit0 := s.Scan()
+	if tok0 != tok {
+		t.Errorf("%q: got %s, expected %s", src, tok0, tok)
+	}
+	if tok0 != token.ILLEGAL && lit0 != lit {
+		t.Errorf("%q: got literal %q, expected %q", src, lit0, lit)
+	}
+	cnt := 0
+	if err != "" {
+		cnt = 1
+	}
+	if h.cnt != cnt {
+		t.Errorf("%q: got cnt %d, expected %d", src, h.cnt, cnt)
+	}
+	if h.msg != err {
+		t.Errorf("%q: got msg %q, expected %q", src, h.msg, err)
+	}
+	if h.pos.Offset != pos {
+		t.Errorf("%q: got offset %d, expected %d", src, h.pos.Offset, pos)
+	}
+}
+
+var errors = []struct {
+	src string
+	tok token.Token
+	pos int
+	lit string
+	err string
+}{
+	{"\a", token.ILLEGAL, 0, "", "illegal character U+0007"},
+	{`#`, token.ILLEGAL, 0, "", "illegal character U+0023 '#'"},
+	{`…`, token.ILLEGAL, 0, "", "illegal character U+2026 '…'"},
+	{`' '`, token.CHAR, 0, `' '`, ""},
+	{`''`, token.CHAR, 0, `''`, "illegal rune literal"},
+	{`'12'`, token.CHAR, 0, `'12'`, "illegal rune literal"},
+	{`'123'`, token.CHAR, 0, `'123'`, "illegal rune literal"},
+	{`'\0'`, token.CHAR, 3, `'\0'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\07'`, token.CHAR, 4, `'\07'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\8'`, token.CHAR, 2, `'\8'`, "unknown escape sequence"},
+	{`'\08'`, token.CHAR, 3, `'\08'`, "illegal character U+0038 '8' in escape sequence"},
+	{`'\x'`, token.CHAR, 3, `'\x'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\x0'`, token.CHAR, 4, `'\x0'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\x0g'`, token.CHAR, 4, `'\x0g'`, "illegal character U+0067 'g' in escape sequence"},
+	{`'\u'`, token.CHAR, 3, `'\u'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\u0'`, token.CHAR, 4, `'\u0'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\u00'`, token.CHAR, 5, `'\u00'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\u000'`, token.CHAR, 6, `'\u000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\u000`, token.CHAR, 6, `'\u000`, "escape sequence not terminated"},
+	{`'\u0000'`, token.CHAR, 0, `'\u0000'`, ""},
+	{`'\U'`, token.CHAR, 3, `'\U'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U0'`, token.CHAR, 4, `'\U0'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U00'`, token.CHAR, 5, `'\U00'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U000'`, token.CHAR, 6, `'\U000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U0000'`, token.CHAR, 7, `'\U0000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U00000'`, token.CHAR, 8, `'\U00000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U000000'`, token.CHAR, 9, `'\U000000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U0000000'`, token.CHAR, 10, `'\U0000000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U0000000`, token.CHAR, 10, `'\U0000000`, "escape sequence not terminated"},
+	{`'\U00000000'`, token.CHAR, 0, `'\U00000000'`, ""},
+	{`'\Uffffffff'`, token.CHAR, 2, `'\Uffffffff'`, "escape sequence is invalid Unicode code point"},
+	{`'`, token.CHAR, 0, `'`, "rune literal not terminated"},
+	{`'\`, token.CHAR, 2, `'\`, "escape sequence not terminated"},
+	{"'\n", token.CHAR, 0, "'", "rune literal not terminated"},
+	{"'\n   ", token.CHAR, 0, "'", "rune literal not terminated"},
+	{`""`, token.STRING, 0, `""`, ""},
+	{`"abc`, token.STRING, 0, `"abc`, "string literal not terminated"},
+	{"\"abc\n", token.STRING, 0, `"abc`, "string literal not terminated"},
+	{"\"abc\n   ", token.STRING, 0, `"abc`, "string literal not terminated"},
+	{"``", token.STRING, 0, "``", ""},
+	{"`", token.STRING, 0, "`", "raw string literal not terminated"},
+	{"/**/", token.COMMENT, 0, "/**/", ""},
+	{"/*", token.COMMENT, 0, "/*", "comment not terminated"},
+	{"077", token.INT, 0, "077", ""},
+	{"078.", token.FLOAT, 0, "078.", ""},
+	{"07801234567.", token.FLOAT, 0, "07801234567.", ""},
+	{"078e0", token.FLOAT, 0, "078e0", ""},
+	{"078", token.INT, 0, "078", "illegal octal number"},
+	{"07800000009", token.INT, 0, "07800000009", "illegal octal number"},
+	{"0x", token.INT, 0, "0x", "illegal hexadecimal number"},
+	{"0X", token.INT, 0, "0X", "illegal hexadecimal number"},
+	{"\"abc\x00def\"", token.STRING, 4, "\"abc\x00def\"", "illegal character NUL"},
+	{"\"abc\x80def\"", token.STRING, 4, "\"abc\x80def\"", "illegal UTF-8 encoding"},
+	{"\ufeff\ufeff", token.ILLEGAL, 3, "\ufeff\ufeff", "illegal byte order mark"},                        // only first BOM is ignored
+	{"//\ufeff", token.COMMENT, 2, "//\ufeff", "illegal byte order mark"},                                // only first BOM is ignored
+	{"'\ufeff" + `'`, token.CHAR, 1, "'\ufeff" + `'`, "illegal byte order mark"},                         // only first BOM is ignored
+	{`"` + "abc\ufeffdef" + `"`, token.STRING, 4, `"` + "abc\ufeffdef" + `"`, "illegal byte order mark"}, // only first BOM is ignored
+}
+
+func TestScanErrors(t *testing.T) {
+	for _, e := range errors {
+		checkError(t, e.src, e.tok, e.pos, e.lit, e.err)
+	}
+}
+
+func BenchmarkScan(b *testing.B) {
+	b.StopTimer()
+	fset := token.NewFileSet()
+	file := fset.AddFile("", fset.Base(), len(source))
+	var s Scanner
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		s.Init(file, source, nil, ScanComments)
+		for {
+			_, tok, _ := s.Scan()
+			if tok == token.EOF {
+				break
+			}
+		}
+	}
+}
+
+func BenchmarkScanFile(b *testing.B) {
+	b.StopTimer()
+	const filename = "scanner.go"
+	src, err := ioutil.ReadFile(filename)
+	if err != nil {
+		panic(err)
+	}
+	fset := token.NewFileSet()
+	file := fset.AddFile(filename, fset.Base(), len(src))
+	b.SetBytes(int64(len(src)))
+	var s Scanner
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		s.Init(file, src, nil, ScanComments)
+		for {
+			_, tok, _ := s.Scan()
+			if tok == token.EOF {
+				break
+			}
+		}
+	}
+}