Add support for all charsets

Compared to the trimmed-down list of charsets we had before, this adds
about 100KiB to binaries size. This isn't enough to justify having a
separate package with all charsets.

Closes: #64

README.md

@@ -14,7 +14,8 @@ A Go library for the Internet Message Format. It implements:
 ## Features
 
 * Streaming API
-* Automatic encoding and charset handling
+* Automatic encoding and charset handling (to decode all charsets, add
+  `import _ "github.com/emersion/go-message/charset"` to your application)
 * A [`mail`](https://godoc.org/github.com/emersion/go-message/mail) subpackage
   to read and write mail messages
 * DKIM-friendly

charset/charset.go

@@ -1,4 +1,7 @@
 // Package charset provides functions to decode and encode charsets.
+//
+// It imports all supported charsets, which adds about 1MiB to binaries size.
+// Importing the package automatically sets message.CharsetReader.
 package charset
 
 import (
@@ -9,37 +12,29 @@ import (
 	"github.com/emersion/go-message"
 	"golang.org/x/text/encoding"
 	"golang.org/x/text/encoding/charmap"
-	"golang.org/x/text/encoding/japanese"
+	"golang.org/x/text/encoding/ianaindex"
 	"golang.org/x/text/encoding/simplifiedchinese"
-	"golang.org/x/text/encoding/traditionalchinese"
 )
 
+// Quirks table for charsets not handled by ianaindex
+//
+// For aliases, see
+// https://www.iana.org/assignments/character-sets/character-sets.xhtml
 var charsets = map[string]encoding.Encoding{
-	"big5":             traditionalchinese.Big5,
-	"euc-jp":           japanese.EUCJP,
-	"gbk":              simplifiedchinese.GBK,
-	"gb2312":           simplifiedchinese.GBK,     // as GBK is a superset of HZGB2312, so just use GBK
-	"gb18030":          simplifiedchinese.GB18030, // GB18030 Use for parse QQ business mail message
-	"iso-2022-jp":      japanese.ISO2022JP,
-	"iso-8859-1":       charmap.ISO8859_1,
-	"iso-8859-2":       charmap.ISO8859_2,
-	"iso-8859-3":       charmap.ISO8859_3,
-	"iso-8859-4":       charmap.ISO8859_4,
-	"iso-8859-9":       charmap.ISO8859_9,
-	"iso-8859-10":      charmap.ISO8859_10,
-	"iso-8859-13":      charmap.ISO8859_13,
-	"iso-8859-14":      charmap.ISO8859_14,
-	"iso-8859-15":      charmap.ISO8859_15,
-	"iso-8859-16":      charmap.ISO8859_16,
-	"koi8-r":           charmap.KOI8R,
-	"shift_jis":        japanese.ShiftJIS,
-	"windows-1250":     charmap.Windows1250,
-	"windows-1251":     charmap.Windows1251,
-	"windows-1252":     charmap.Windows1252,
-	"cp1250":           charmap.Windows1250,
-	"cp1251":           charmap.Windows1251,
-	"cp1252":           charmap.Windows1252,
-	"ansi_x3.110-1983": charmap.ISO8859_1,
+	// us-ascii not handled by ianaindex
+	"us-ascii":         encoding.Nop,
+	"iso-ir-6":         encoding.Nop,
+	"ansi_x3.4-1968":   encoding.Nop,
+	"ansi_x3.4-1986":   encoding.Nop,
+	"iso_646.irv:1991": encoding.Nop,
+	"iso646-us":        encoding.Nop,
+	"us":               encoding.Nop,
+	"ibm367":           encoding.Nop,
+	"cp367":            encoding.Nop,
+	"ascii":            encoding.Nop, // non-standard
+
+	"ansi_x3.110-1983": charmap.ISO8859_1,     // see RFC 1345 page 62, mostly superset of ISO 8859-1
+	"gb2312":           simplifiedchinese.GBK, // GBK is a superset of HZGB2312
 }
 
 func init() {
@@ -48,16 +43,22 @@ func init() {
 
 // Reader returns an io.Reader that converts the provided charset to UTF-8.
 func Reader(charset string, input io.Reader) (io.Reader, error) {
-	charset = strings.ToLower(charset)
-	// QUIRK: "ascii" and "utf8" are not in the spec but are common. The
-	// names ANSI_X3.4-{1968,1986} are historical and recognized as aliases
-	if charset == "utf-8" || charset == "utf8" || charset == "us-ascii" || charset == "ascii" || strings.HasPrefix(charset, "ansi_x3.4-") {
-		return input, nil
+	var err error
+	enc, ok := charsets[strings.ToLower(charset)]
+	if !ok {
+		enc, err = ianaindex.MIME.Encoding(charset)
+	}
+	if err != nil {
+		enc, err = ianaindex.MIME.Encoding("cs" + charset)
+	}
+	if err != nil {
+		return nil, fmt.Errorf("charset %q: %v", charset, err)
 	}
-	if enc, ok := charsets[charset]; ok {
-		return enc.NewDecoder().Reader(input), nil
+	// See https://github.com/golang/go/issues/19421
+	if enc == nil {
+		return nil, fmt.Errorf("charset %q: unsupported charset", charset)
 	}
-	return nil, fmt.Errorf("unhandled charset %q", charset)
+	return enc.NewDecoder().Reader(input), nil
 }
 
 // RegisterEncoding registers an encoding. This is intended to be called from