diff --git a/go.mod b/go.mod
index dd9aa34..278b8be 100644
--- a/go.mod
+++ b/go.mod
@@ -2,4 +2,7 @@ module mvdan.cc/garble
 
 go 1.14
 
-require github.com/rogpeppe/go-internal v1.5.0
+require (
+	github.com/rogpeppe/go-internal v1.5.0
+	golang.org/x/tools v0.0.0-20191206204035-259af5ff87bd
+)
diff --git a/go.sum b/go.sum
index 72d9f51..0520a94 100644
--- a/go.sum
+++ b/go.sum
@@ -1,8 +1,19 @@
+github.com/kr/pretty v0.1.0 h1:L/CwN0zerZDmRFUapSPitk6f+Q3+0za1rQkzVuMiMFI=
 github.com/kr/pretty v0.1.0/go.mod h1:dAy3ld7l9f0ibDNOQOHHMYYIIbhfbHSm3C4ZsoJORNo=
 github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
+github.com/kr/text v0.1.0 h1:45sCR5RtlFHMR4UwH9sdQ5TC8v0qDQCHnXt+kaKSTVE=
 github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
 github.com/rogpeppe/go-internal v1.5.0 h1:Usqs0/lDK/NqTkvrmKSwA/3XkZAs7ZAW/eLeQ2MVBTw=
 github.com/rogpeppe/go-internal v1.5.0/go.mod h1:xXDCJY+GAPziupqXw64V24skbSoqbTEfhy4qGm1nDQc=
+golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
+golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=
+golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
+golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+golang.org/x/tools v0.0.0-20191206204035-259af5ff87bd h1:Zc7EU2PqpsNeIfOoVA7hvQX4cS3YDJEs5KlfatT3hLo=
+golang.org/x/tools v0.0.0-20191206204035-259af5ff87bd/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
+golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
+gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127 h1:qIbj1fsPNlZgppZ+VLlY7N33q108Sa+fhmuc+sWQYwY=
 gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/errgo.v2 v2.1.0 h1:0vLT13EuvQ0hNvakwLuFZ/jYrLp5F3kcWHXdRggjCE8=
 gopkg.in/errgo.v2 v2.1.0/go.mod h1:hNsd1EY+bozCKY1Ytp96fpM3vjJbqLJn88ws8XvfDNI=
diff --git a/main.go b/main.go
index 17d4b60..3432415 100644
--- a/main.go
+++ b/main.go
@@ -22,6 +22,8 @@ import (
 	"os/exec"
 	"path/filepath"
 	"strings"
+
+	"golang.org/x/tools/go/ast/astutil"
 )
 
 var flagSet = flag.NewFlagSet("garble", flag.ContinueOnError)
@@ -186,8 +188,9 @@ func transformCompile(args []string) ([]string, error) {
 		return nil, fmt.Errorf("typecheck error: %v", err)
 	}
 
-	for i, file := range files {
-		file := transformGoFile(file, info)
+	args = flags
+	for _, file := range files {
+		file := transformGo(file, info)
 		f, err := ioutil.TempFile("", "garble")
 		if err != nil {
 			return nil, err
@@ -203,9 +206,9 @@ func transformCompile(args []string) ([]string, error) {
 		deferred = append(deferred, func() error {
 			return os.Remove(f.Name())
 		})
-		paths[i] = f.Name()
+		args = append(args, f.Name())
 	}
-	return append(flags, paths...), nil
+	return args, nil
 }
 
 func readBuildIDs(flags []string) error {
@@ -284,12 +287,14 @@ func hashWith(salt, value string) string {
 	return "z" + sum[:length]
 }
 
-// transformGoFile creates a garbled copy of the Go file at path, and returns
-// the path to the copy.
-func transformGoFile(file *ast.File, info *types.Info) *ast.File {
-	ast.Inspect(file, func(node ast.Node) bool {
-		switch node := node.(type) {
+// transformGo garbles the provided Go syntax node.
+func transformGo(node ast.Node, info *types.Info) ast.Node {
+	pre := func(cursor *astutil.Cursor) bool {
+		switch node := cursor.Node().(type) {
 		case *ast.Ident:
+			if node.Name == "_" {
+				return true // unnamed remains unnamed
+			}
 			obj := info.ObjectOf(node)
 			switch obj.(type) {
 			case *types.Var:
@@ -300,28 +305,36 @@ func transformGoFile(file *ast.File, info *types.Info) *ast.File {
 				case "main", "init":
 					return true // don't break them
 				}
+			case nil:
+				switch cursor.Parent().(type) {
+				case *ast.AssignStmt:
+					// symbolic var v in v := expr.(type)
+				default:
+					return true
+				}
 			default:
 				return true // we only want to rename the above
 			}
-			pkg := obj.Pkg()
-			if pkg == nil {
-				return true // universe scope
-			}
-			path := pkg.Path()
-			if !strings.Contains(path, ".") {
-				return true // std isn't transformed
-			}
-
 			buildID := buildInfo.buildID
-			if id := buildInfo.imports[path].buildID; id != "" {
-				buildID = id
+			if obj != nil {
+				pkg := obj.Pkg()
+				if pkg == nil {
+					return true // universe scope
+				}
+				path := pkg.Path()
+				if !strings.Contains(path, ".") {
+					return true // std isn't transformed
+				}
+				if id := buildInfo.imports[path].buildID; id != "" {
+					buildID = id
+				}
 			}
 			// log.Printf("%#v\n", node.Obj)
 			node.Name = hashWith(buildID, node.Name)
 		}
 		return true
-	})
-	return file
+	}
+	return astutil.Apply(node, pre, nil)
 }
 
 func transformLink(args []string) ([]string, error) {
diff --git a/testdata/scripts/syntax.txt b/testdata/scripts/syntax.txt
new file mode 100644
index 0000000..8147e88
--- /dev/null
+++ b/testdata/scripts/syntax.txt
@@ -0,0 +1,19 @@
+garble build main.go
+exec ./main
+cmp stderr main.stderr
+
+-- main.go --
+package main
+
+var V interface{}
+
+func main() {
+	switch V := V.(type) {
+	case int:
+		var _ int = V
+	case nil:
+		println("nil case")
+	}
+}
+-- main.stderr --
+nil case