don't garble exported struct fields

They might reasonably affect the behavior of the code, such as when
encoding/json is used without tags.

README.md

@@ -52,5 +52,5 @@ binary doesn't include paths from the current filesystem.
 * Deciding what method names to garble is always going to be difficult, due to
   interfaces that could be implemented up or down the package import tree.
 
-* Some uses of the `reflect` package may break, such as accessing a struct's
-  field, whose name has been garbled.
+* Similarly to methods, exported struct fields are difficult to garble, as the
+  names might be relevant for reflection work like `encoding/json`

main.go

@@ -353,6 +353,11 @@ func transformGo(node ast.Node, info *types.Info) ast.Node {
 			// log.Printf("%#v %T", node, obj)
 			switch x := obj.(type) {
 			case *types.Var:
+				if x.IsField() && x.Exported() {
+					// might be used for reflection, e.g.
+					// encoding/json without struct tags
+					return true
+				}
 				if x.Embedded() {
 					obj = objOf(obj.Type())
 				}

testdata/scripts/syntax.txt

@@ -5,15 +5,22 @@ cmp stderr main.stderr
 -- main.go --
 package main
 
-import "go/ast"
+import (
+	"encoding/json"
+	"go/ast"
+)
 
 var V interface{}
 
-type T struct{
+type T struct {
 	ast.Node
 	*ast.Ident
 }
 
+type EncodingT struct {
+	Foo int
+}
+
 func main() {
 	switch V := V.(type) {
 	case int:
@@ -21,6 +28,10 @@ func main() {
 	case nil:
 		println("nil case")
 	}
+
+	enc, _ := json.Marshal(EncodingT{Foo: 3})
+	println(string(enc))
 }
 -- main.stderr --
 nil case
+{"Foo":3}