refactor/rename: do not allow a new name to be a keyword

Fixes golang/go#16535

Change-Id: I46f9b9530e25bb286557712a7969715f8dde99b9
Reviewed-on: https://go-review.googlesource.com/25343
Reviewed-by: Alan Donovan <adonovan@google.com>

refactor/rename/rename_test.go

@@ -416,6 +416,45 @@ var _ I = E{}
 	}
 }
 
+func TestInvalidIdentifiers(t *testing.T) {
+	ctxt := fakeContext(map[string][]string{
+		"main": {`
+package main
+
+func f() { }
+`}})
+
+	for _, test := range []struct {
+		from, to string // values of the -offset/-from and -to flags
+		want     string // expected error message
+	}{
+		{
+			from: "main.f", to: "_",
+			want: `-to "_": not a valid identifier`,
+		},
+		{
+			from: "main.f", to: "123",
+			want: `-to "123": not a valid identifier`,
+		},
+		{
+			from: "main.f", to: "for",
+			want: `-to "for": not a valid identifier`,
+		},
+		{
+			from: "switch", to: "v",
+			want: `-from "switch": invalid expression`,
+		},
+	} {
+		err := Main(ctxt, "", test.from, test.to)
+		prefix := fmt.Sprintf("-from %q -to %q", test.from, test.to)
+		if err == nil {
+			t.Errorf("%s: expected error %q", prefix, test.want)
+		} else if err.Error() != test.want {
+			t.Errorf("%s: unexpected error\nwant: %s\n got: %s", prefix, test.want, err.Error())
+		}
+	}
+}
+
 func TestRewrites(t *testing.T) {
 	defer func(savedWriteFile func(string, []byte) error) {
 		writeFile = savedWriteFile

refactor/rename/util.go

@@ -8,6 +8,7 @@ package rename
 
 import (
 	"go/ast"
+	"go/token"
 	"go/types"
 	"os"
 	"path/filepath"
@@ -52,7 +53,7 @@ func isValidIdentifier(id string) bool {
 			return false
 		}
 	}
-	return true
+	return token.Lookup(id) == token.IDENT
 }
 
 // isLocal reports whether obj is local to some function.