eval: allow setting unexported struct fields

This requires the implementation of reflect.StructOf be relaxed
to allow creating structs with unexported fields.
That is https://golang.org/cl/85661

For #170

eval/eval.go

@@ -15,6 +15,7 @@ import (
 	"runtime/debug"
 	"strings"
 	"sync"
+	"unsafe"
 
 	"neugram.io/ng/eval/environ"
 	"neugram.io/ng/eval/gowrap"
@@ -1483,12 +1484,12 @@ func (p *Program) evalExpr(e expr.Expr) []reflect.Value {
 				for i, elem := range e.Values {
 					name := e.Keys[i].(*expr.Ident).Name
 					v := p.evalExprOne(elem)
-					st.FieldByName(name).Set(v)
+					setField(st.FieldByName(name), v)
 				}
 			} else {
 				for i, expr := range e.Values {
 					v := p.evalExprOne(expr)
-					st.Field(i).Set(v)
+					setField(st.Field(i), v)
 				}
 			}
 			return []reflect.Value{st}
@@ -1718,6 +1719,13 @@ func (p *Program) evalExpr(e expr.Expr) []reflect.Value {
 	panic(interpPanic{fmt.Errorf("TODO evalExpr(%s), %T", format.Expr(e), e)})
 }
 
+// setField sets a struct field to value.
+// It sets the value even if the field is unexported.
+func setField(field, value reflect.Value) {
+	fieldPtr := reflect.NewAt(field.Type(), unsafe.Pointer(field.UnsafeAddr()))
+	fieldPtr.Elem().Set(value)
+}
+
 func (p *Program) evalFuncLiteral(e *expr.FuncLiteral, recvt *tipe.Named) reflect.Value {
 	s := &Scope{
 		Parent: p.Universe,

eval/testdata/unexported.ng

@@ -0,0 +1,21 @@
+// Disabled until https://golang.org/cl/85661 is submitted.
+print("OK")
+
+/*
+type t struct {
+	X string
+	Y int
+}
+
+v := t{
+	X: "string1",
+	Y: 1,
+}
+
+v.X = "string2"
+v.Y = 2
+
+if v == (t{"string2", 2}) {
+	print("OK")
+}
+*/