allow copy of struct containing unexported fields

An experiment: allow structs to be copied even if they
contain unexported fields.  This gives packages the
ability to return opaque values in their APIs, like reflect
does for reflect.Value but without the kludgy hacks reflect
resorts to.

In general, we trust programmers not to do silly things
like *x = *y on a package's struct pointers, just as we trust
programmers not to do unicode.Letter = unicode.Digit,
but packages that want a harder guarantee can introduce
an extra level of indirection, like in the changes to os.File
in this CL or by using an interface type.

All in one CL so that it can be rolled back more easily if
we decide this is a bad idea.

Originally discussed in March 2011.
https://groups.google.com/group/golang-dev/t/3f5d30938c7c45ef

R=golang-dev, adg, dvyukov, r, bradfitz, jan.mercl, gri
CC=golang-dev
https://golang.org/cl/5372095

doc/go_spec.html

 <!-- title The Go Programming Language Specification -->
-<!-- subtitle Version of November 13, 2011 -->
+<!-- subtitle Version of November 14, 2011 -->
 
 <!--
 TODO
@@ -1367,15 +1367,6 @@ by a value of type <code>T</code>.
 </li>
 </ul>
 
-<p>
-If <code>T</code> is a struct type with non-<a href="#Exported_identifiers">exported</a>
-fields, the assignment must be in the same package in which <code>T</code> is declared,
-or <code>x</code> must be the receiver of a method call.
-In other words, a struct value can be assigned to a struct variable only if
-every field of the struct may be legally assigned individually by the program,
-or if the assignment is initializing the receiver of a method of the struct type.
-</p>
-
 <p>
 Any value may be assigned to the <a href="#Blank_identifier">blank identifier</a>.
 </p>

src/cmd/gc/go.h

@@ -1234,7 +1234,6 @@ void	walkswitch(Node *sw);
 /*
  *	typecheck.c
  */
-int	exportassignok(Type *t, char *desc);
 int	islvalue(Node *n);
 Node*	typecheck(Node **np, int top);
 void	typechecklist(NodeList *l, int top);

src/cmd/gc/subr.c

@@ -1018,9 +1018,6 @@ eqtypenoname(Type *t1, Type *t2)
 // Is type src assignment compatible to type dst?
 // If so, return op code to use in conversion.
 // If not, return 0.
-//
-// It is the caller's responsibility to call exportassignok
-// to check for assignments to other packages' unexported fields,
 int
 assignop(Type *src, Type *dst, char **why)
 {
@@ -1225,7 +1222,6 @@ assignconv(Node *n, Type *t, char *context)
 	if(t->etype == TBLANK)
 		return n;
 
-	exportassignok(n->type, context);
 	if(eqtype(n->type, t))
 		return n;
 

src/cmd/gc/typecheck.c

@@ -1045,8 +1045,6 @@ reswitch:
 			yyerror("first argument to append must be slice; have %lT", t);
 			goto error;
 		}
-		if(!exportassignok(t->type, "append"))
-			goto error;
 
 		if(n->isddd) {
 			if(args->next == nil) {
@@ -1114,8 +1112,6 @@ reswitch:
 			yyerror("arguments to copy have different element types: %lT and %lT", n->left->type, n->right->type);
 			goto error;
 		}
-		if(!exportassignok(n->left->type->type, "copy"))
-			goto error;
 		goto ret;
 
 	case OCONV:
@@ -1731,7 +1727,6 @@ typecheckaste(int op, Node *call, int isddd, Type *tstruct, NodeList *nl, char *
 		for(tl=tstruct->type; tl; tl=tl->down) {
 			if(tl->isddd) {
 				for(; tn; tn=tn->down) {
-					exportassignok(tn->type, desc);
 					if(assignop(tn->type, tl->type->type, &why) == 0) {
 						if(call != N)
 							yyerror("cannot use %T as type %T in argument to %N%s", tn->type, tl->type, call, why);
@@ -1743,7 +1738,6 @@ typecheckaste(int op, Node *call, int isddd, Type *tstruct, NodeList *nl, char *
 			}
 			if(tn == T)
 				goto notenough;
-			exportassignok(tn->type, desc);
 			if(assignop(tn->type, tl->type, &why) == 0) {
 				if(call != N)
 					yyerror("cannot use %T as type %T in argument to %N%s", tn->type, tl->type, call, why);
@@ -1815,66 +1809,6 @@ toomany:
 	goto out;
 }
 
-/*
- * do the export rules allow writing to this type?
- * cannot be implicitly assigning to any type with
- * an unavailable field.
- */
-int
-exportassignok(Type *t, char *desc)
-{
-	Type *f;
-	Sym *s;
-
-	if(t == T)
-		return 1;
-	if(t->trecur)
-		return 1;
-	t->trecur = 1;
-
-	switch(t->etype) {
-	default:
-		// most types can't contain others; they're all fine.
-		break;
-	case TSTRUCT:
-		for(f=t->type; f; f=f->down) {
-			if(f->etype != TFIELD)
-				fatal("structas: not field");
-			s = f->sym;
-			// s == nil doesn't happen for embedded fields (they get the type symbol).
-			// it only happens for fields in a ... struct.
-			if(s != nil && !exportname(s->name) && s->pkg != localpkg) {
-				char *prefix;
-
-				prefix = "";
-				if(desc != nil)
-					prefix = " in ";
-				else
-					desc = "";
-				yyerror("implicit assignment of unexported field '%s' of %T%s%s", s->name, t, prefix, desc);
-				goto no;
-			}
-			if(!exportassignok(f->type, desc))
-				goto no;
-		}
-		break;
-
-	case TARRAY:
-		if(t->bound < 0)	// slices are pointers; that's fine
-			break;
-		if(!exportassignok(t->type, desc))
-			goto no;
-		break;
-	}
-	t->trecur = 0;
-	return 1;
-
-no:
-	t->trecur = 0;
-	return 0;
-}
-
-
 /*
  * type check composite
  */
@@ -2310,8 +2244,6 @@ typecheckas(Node *n)
 	if(n->right && n->right->type != T) {
 		if(n->left->type != T)
 			n->right = assignconv(n->right, n->left->type, "assignment");
-		else if(!isblank(n->left))
-			exportassignok(n->right->type, "assignment");
 	}
 	if(n->left->defn == n && n->left->ntype == N) {
 		defaultlit(&n->right, T);
@@ -2335,7 +2267,6 @@ checkassignto(Type *src, Node *dst)
 		yyerror("cannot assign %T to %lN in multiple assignment%s", src, dst, why);
 		return;
 	}
-	exportassignok(dst->type, "multiple assignment");
 }
 
 static void

src/pkg/os/file_plan9.go

@@ -11,6 +11,14 @@ import (
 
 // File represents an open file descriptor.
 type File struct {
+	*file
+}
+
+// file is the real representation of *File.
+// The extra level of indirection ensures that no clients of os
+// can overwrite this data, which could cause the finalizer
+// to close the wrong file descriptor.
+type file struct {
 	fd      int
 	name    string
 	dirinfo *dirInfo // nil unless directory being read
@@ -29,8 +37,8 @@ func NewFile(fd int, name string) *File {
 	if fd < 0 {
 		return nil
 	}
-	f := &File{fd: fd, name: name}
-	runtime.SetFinalizer(f, (*File).Close)
+	f := &File{&file{fd: fd, name: name}}
+	runtime.SetFinalizer(f.file, (*file).close)
 	return f
 }
 
@@ -110,6 +118,10 @@ func OpenFile(name string, flag int, perm uint32) (file *File, err error) {
 // Close closes the File, rendering it unusable for I/O.
 // It returns an error, if any.
 func (file *File) Close() error {
+	return file.file.close()
+}
+
+func (file *file) close() error {
 	if file == nil || file.fd < 0 {
 		return Ebadfd
 	}

src/pkg/os/file_unix.go

@@ -13,6 +13,14 @@ import (
 
 // File represents an open file descriptor.
 type File struct {
+	*file
+}
+
+// file is the real representation of *File.
+// The extra level of indirection ensures that no clients of os
+// can overwrite this data, which could cause the finalizer
+// to close the wrong file descriptor.
+type file struct {
 	fd      int
 	name    string
 	dirinfo *dirInfo // nil unless directory being read
@@ -32,8 +40,8 @@ func NewFile(fd int, name string) *File {
 	if fd < 0 {
 		return nil
 	}
-	f := &File{fd: fd, name: name}
-	runtime.SetFinalizer(f, (*File).Close)
+	f := &File{&file{fd: fd, name: name}}
+	runtime.SetFinalizer(f.file, (*file).close)
 	return f
 }
 
@@ -71,6 +79,10 @@ func OpenFile(name string, flag int, perm uint32) (file *File, err error) {
 // Close closes the File, rendering it unusable for I/O.
 // It returns an error, if any.
 func (file *File) Close() error {
+	return file.file.close()
+}
+
+func (file *file) close() error {
 	if file == nil || file.fd < 0 {
 		return EINVAL
 	}

src/pkg/os/file_windows.go

@@ -13,6 +13,14 @@ import (
 
 // File represents an open file descriptor.
 type File struct {
+	*file
+}
+
+// file is the real representation of *File.
+// The extra level of indirection ensures that no clients of os
+// can overwrite this data, which could cause the finalizer
+// to close the wrong file descriptor.
+type file struct {
 	fd      syscall.Handle
 	name    string
 	dirinfo *dirInfo   // nil unless directory being read
@@ -33,8 +41,8 @@ func NewFile(fd syscall.Handle, name string) *File {
 	if fd < 0 {
 		return nil
 	}
-	f := &File{fd: fd, name: name}
-	runtime.SetFinalizer(f, (*File).Close)
+	f := &File{&file{fd: fd, name: name}}
+	runtime.SetFinalizer(f.file, (*file).close)
 	return f
 }
 
@@ -99,6 +107,10 @@ func OpenFile(name string, flag int, perm uint32) (file *File, err error) {
 // Close closes the File, rendering it unusable for I/O.
 // It returns an error, if any.
 func (file *File) Close() error {
+	return file.file.close()
+}
+
+func (file *file) close() error {
 	if file == nil || file.fd < 0 {
 		return EINVAL
 	}

src/pkg/sync/mutex.go

@@ -6,6 +6,8 @@
 // exclusion locks.  Other than the Once and WaitGroup types, most are intended
 // for use by low-level library routines.  Higher-level synchronization is
 // better done via channels and communication.
+//
+// Values containing the types defined in this package should not be copied.
 package sync
 
 import (

test/assign.go

@@ -16,38 +16,38 @@ type T struct {
 func main() {
 	{
 		var x, y sync.Mutex
-		x = y	// ERROR "assignment.*Mutex"
+		x = y // ok
 		_ = x
 	}
 	{
 		var x, y T
-		x = y	// ERROR "assignment.*Mutex"
+		x = y // ok
 		_ = x
 	}
 	{
 		var x, y [2]sync.Mutex
-		x = y	// ERROR "assignment.*Mutex"
+		x = y // ok
 		_ = x
 	}
 	{
 		var x, y [2]T
-		x = y	// ERROR "assignment.*Mutex"
+		x = y // ok
 		_ = x
 	}
 	{
-		x := sync.Mutex{0, 0}	// ERROR "assignment.*Mutex"
+		x := sync.Mutex{0, 0} // ERROR "assignment.*Mutex"
 		_ = x
 	}
 	{
-		x := sync.Mutex{key: 0}	// ERROR "(unknown|assignment).*Mutex"
+		x := sync.Mutex{key: 0} // ERROR "(unknown|assignment).*Mutex"
 		_ = x
 	}
 	{
-		x := &sync.Mutex{}	// ok
-		var y sync.Mutex	// ok
-		y = *x	// ERROR "assignment.*Mutex"
-		*x = y	// ERROR "assignment.*Mutex"
+		x := &sync.Mutex{} // ok
+		var y sync.Mutex   // ok
+		y = *x             // ok
+		*x = y             // ok
 		_ = x
 		_ = y
-	}		
+	}
 }

test/fixedbugs/bug226.dir/x.go

-// 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 x
-
-type T struct { x, Y int }
-
-func (t T) M()

test/fixedbugs/bug226.dir/y.go

-// 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 y
-
-import "./x"
-
-func f() {
-	ok := new(x.T);
-	var ok1 x.T;
-	ok2 := &ok1;
-	ok3 := &x.T{};
-	ok4 := &x.T{Y:2};
-	_ = x.T{};
-	_ = x.T{Y:2};
-	
-	ok1.M();
-	bad1 := *ok;	// ERROR "assignment.*T"
-	bad2 := ok1;	// ERROR "assignment.*T"
-	*ok4 = ok1;	// ERROR "assignment.*T"
-	*ok4 = *ok2;	// ERROR "assignment.*T"
-	ok1 = *ok4;	// ERROR "assignment.*T"
-	_ = bad1;
-	_ = bad2;
-	_ = ok4;
-	_ = ok3;
-	_ = ok2;
-	_ = ok1;
-	_ = ok;
-}

test/fixedbugs/bug226.go

-// $G $D/$F.dir/x.go && errchk $G $D/$F.dir/y.go
-
-// 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.
-
-ignored

test/fixedbugs/bug310.go

-// errchk $G $D/$F.go
-
-// Copyright 2010 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 p
-
-import (
-	"bytes"
-	"fmt"
-)
-
-type t int
-
-func main() {
-	_ = t.bar	// ERROR "no method"
-	var b bytes.Buffer
-	fmt.Print(b)	// ERROR "implicit assignment"
-}

test/fixedbugs/bug359.go

-// errchk $G $D/$F.go
-
-// Copyright 2011 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.
-
-// issue 1910
-// error on wrong line
-
-package main
-
-import "container/list"
-
-type Painting struct {
-	fragments list.List // private
-}
-
-func (p Painting) Foo() {
-	for e := p.fragments; e.Front() != nil; {  // ERROR "unexported field|hidden field"
-	}
-}
-
-// from comment 4 of issue 1910
-type Foo interface {
-	Run(a int) (a int)  // ERROR "a redeclared|redefinition|previous"
-}

test/fixedbugs/bug378.go

-// errchk $G $D/$F.go
-
-// Copyright 2011 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.
-
-// Issue 1387
-package foo
-
-import "bytes"
-
-func i() {
-	a := make([]bytes.Buffer, 1)
-	b := a[0] // ERROR "unexported field"
-}
-
-func f() {
-	a := make([]bytes.Buffer, 1)
-	a = append(a, a...) // ERROR "unexported field"
-}
-
-
-func g() {
-	a := make([]bytes.Buffer, 1)
-	b := make([]bytes.Buffer, 1)
-	copy(b, a)	// ERROR "unexported field"
-}