Escape field tags (#379)

Fixes #266

Author: klauspost

_generated/def.go

@@ -31,6 +31,7 @@ type X struct {
 	Others    [][32]int32 // should compile to len(x.Others)*32*msgp.Int32Size
 	Matrix    [][]int32   // should not optimize
 	ManyFixed []Fixed
+	WeirdTag  string `msg:"\x0b"`
 }
 
 // test fixed-size struct

gen/decode.go

@@ -128,7 +128,7 @@ func (d *decodeGen) structAsMap(s *Struct) {
 	d.p.print("\nswitch msgp.UnsafeString(field) {")
 	for i := range s.Fields {
 		d.ctx.PushString(s.Fields[i].FieldName)
-		d.p.printf("\ncase \"%s\":", s.Fields[i].FieldTag)
+		d.p.printf("\ncase %q:", s.Fields[i].FieldTag)
 		fieldElem := s.Fields[i].FieldElem
 		anField := s.Fields[i].HasTagPart("allownil") && fieldElem.AllowNil()
 		if anField {

gen/unmarshal.go

@@ -127,7 +127,7 @@ func (u *unmarshalGen) mapstruct(s *Struct) {
 		if !u.p.ok() {
 			return
 		}
-		u.p.printf("\ncase \"%s\":", s.Fields[i].FieldTag)
+		u.p.printf("\ncase %q:", s.Fields[i].FieldTag)
 		u.ctx.PushString(s.Fields[i].FieldName)
 
 		fieldElem := s.Fields[i].FieldElem