go/gcimporter15: implement types.Package.SetName for Go 1.5

This makes gcimporter15 build against Go 1.5.

Change-Id: I14e7ff80b28d99f996abc19a0a74b08e5e1bbd75
Reviewed-on: https://go-review.googlesource.com/18771
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

go/gcimporter15/gcimporter.go

@@ -390,7 +390,7 @@ func (p *parser) getPkg(id, name string) *types.Package {
 		// package exists already and we have an expected package name;
 		// make sure names match or set package name if necessary
 		if pname := pkg.Name(); pname == "" {
-			pkg.SetName(name)
+			setName(pkg, name)
 		} else if pname != name {
 			p.errorf("%s package name mismatch: %s (given) vs %s (expected)", pname, name)
 		}

go/gcimporter15/setname15.go

@@ -0,0 +1,31 @@
+// Copyright 2016 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.
+
+// +build !go1.6
+
+package gcimporter
+
+import (
+	"go/types"
+	"unsafe"
+)
+
+func setName(pkg *types.Package, name string) {
+	(*types_Package)(unsafe.Pointer(pkg)).name = name
+}
+
+// The underlying type of types_Package is identical to
+// the underlying type of types.Package. We use it with
+// package unsafe to set the name field since 1.5 does
+// not have the Package.SetName method.
+// TestSetName verifies that the layout with respect to
+// the name field is correct.
+type types_Package struct {
+	path     string
+	name     string
+	scope    *types.Scope
+	complete bool
+	imports  []*types.Package
+	fake     bool
+}

go/gcimporter15/setname16.go

@@ -0,0 +1,13 @@
+// Copyright 2016 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.
+
+// +build go1.6
+
+package gcimporter
+
+import "go/types"
+
+func setName(pkg *types.Package, name string) {
+	pkg.SetName(name)
+}

go/gcimporter15/setname_test.go

@@ -0,0 +1,28 @@
+// Copyright 2016 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.
+
+// +build go1.5
+
+package gcimporter
+
+import (
+	"go/types"
+	"testing"
+)
+
+func TestSetName(t *testing.T) {
+	pkg := types.NewPackage("path", "foo")
+	scope := pkg.Scope()
+
+	// verify setName
+	setName(pkg, "bar")
+	if name := pkg.Name(); name != "bar" {
+		t.Fatalf(`got package name %q; want "bar"`, name)
+	}
+
+	// verify no other fields are changed
+	if pkg.Path() != "path" || pkg.Scope() != scope || pkg.Complete() || pkg.Imports() != nil {
+		t.Fatalf("setName changed other fields")
+	}
+}