diff --git a/endtoend_test.go b/endtoend_test.go
index 3397c06..ff060d7 100644
--- a/endtoend_test.go
+++ b/endtoend_test.go
@@ -111,8 +111,15 @@ func copy(to, from string) error {
 // run runs a single command and returns an error if it does not succeed.
 // os/exec should have this function, to be honest.
 func run(name string, arg ...string) error {
+	return runInDir(".", name, arg...)
+}
+
+// runInDir runs a single command in directory dir and returns an error if
+// it does not succeed.
+func runInDir(dir, name string, arg ...string) error {
 	cmd := exec.Command(name, arg...)
+	cmd.Dir = dir
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr
 	return cmd.Run()
-}
+}
\ No newline at end of file
diff --git a/go.mod b/go.mod
index c586c74..670d4a2 100644
--- a/go.mod
+++ b/go.mod
@@ -2,4 +2,7 @@ module github.com/alvaroloes/enumer
 
 go 1.12
 
-require github.com/pascaldekloe/name v0.0.0-20180628100202-0fd16699aae1
+require (
+	github.com/pascaldekloe/name v0.0.0-20180628100202-0fd16699aae1
+	golang.org/x/tools v0.0.0-20190524210228-3d17549cdc6b
+)
diff --git a/go.sum b/go.sum
index e21b462..0d37513 100644
--- a/go.sum
+++ b/go.sum
@@ -1,2 +1,9 @@
 github.com/pascaldekloe/name v0.0.0-20180628100202-0fd16699aae1 h1:/I3lTljEEDNYLho3/FUB7iD/oc2cEFgVmbHzV+O0PtU=
 github.com/pascaldekloe/name v0.0.0-20180628100202-0fd16699aae1/go.mod h1:eD5JxqMiuNYyFNmyY9rkJ/slN8y59oEu4Ei7F8OoKWQ=
+golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
+golang.org/x/net v0.0.0-20190311183353-d8887717615a/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
+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 v0.0.0-20190524210228-3d17549cdc6b h1:iEAPfYPbYbxG/2lNN4cMOHkmgKNsCuUwkxlDCK46UlU=
+golang.org/x/tools v0.0.0-20190524210228-3d17549cdc6b/go.mod h1:RgjU9mgBXZiqYHBnxXauZ1Gv1EHHAz9KjViQ78xBX0Q=
diff --git a/golden_test.go b/golden_test.go
index 0a81aad..8144fc6 100644
--- a/golden_test.go
+++ b/golden_test.go
@@ -10,6 +10,9 @@
 package main
 
 import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 )
@@ -1050,7 +1053,24 @@ func runGoldenTest(t *testing.T, test Golden, generateJSON, generateYAML, genera
 	var g Generator
 	input := "package test\n" + test.input
 	file := test.name + ".go"
-	g.parsePackage(".", []string{file}, input)
+
+	dir, err := ioutil.TempDir("", "stringer")
+	if err != nil {
+		t.Error(err)
+	}
+	defer func() {
+		err = os.RemoveAll(dir)
+		if err != nil {
+			t.Error(err)
+		}
+	}()
+
+	absFile := filepath.Join(dir, file)
+	err = ioutil.WriteFile(absFile, []byte(input), 0644)
+	if err != nil {
+		t.Error(err)
+	}
+	g.parsePackage([]string{absFile})
 	// Extract the name and type of the constant from the first line.
 	tokens := strings.SplitN(test.input, " ", 3)
 	if len(tokens) != 3 {
diff --git a/stringer.go b/stringer.go
index 593e071..17c670b 100644
--- a/stringer.go
+++ b/stringer.go
@@ -15,13 +15,12 @@ import (
 	"flag"
 	"fmt"
 	"go/ast"
-	"go/build"
 	exact "go/constant"
 	"go/format"
 	"go/importer"
-	"go/parser"
 	"go/token"
 	"go/types"
+	"golang.org/x/tools/go/packages"
 	"io/ioutil"
 	"log"
 	"os"
@@ -97,12 +96,12 @@ func main() {
 
 	if len(args) == 1 && isDirectory(args[0]) {
 		dir = args[0]
-		g.parsePackageDir(args[0])
 	} else {
 		dir = filepath.Dir(args[0])
-		g.parsePackageFiles(args)
 	}
 
+	g.parsePackage(args)
+
 	// Print the header and package clause.
 	g.Printf("// Code generated by \"enumer %s\"; DO NOT EDIT.\n", strings.Join(os.Args[1:], " "))
 	g.Printf("\n")
@@ -194,70 +193,105 @@ type Package struct {
 	typesPkg *types.Package
 }
 
-// parsePackageDir parses the package residing in the directory.
-func (g *Generator) parsePackageDir(directory string) {
-	pkg, err := build.Default.ImportDir(directory, 0)
+//// parsePackageDir parses the package residing in the directory.
+//func (g *Generator) parsePackageDir(directory string) {
+//	pkg, err := build.Default.ImportDir(directory, 0)
+//	if err != nil {
+//		log.Fatalf("cannot process directory %s: %s", directory, err)
+//	}
+//	var names []string
+//	names = append(names, pkg.GoFiles...)
+//	names = append(names, pkg.CgoFiles...)
+//	// TODO: Need to think about constants in test files. Maybe write type_string_test.go
+//	// in a separate pass? For later.
+//	// names = append(names, pkg.TestGoFiles...) // These are also in the "foo" package.
+//	names = append(names, pkg.SFiles...)
+//	names = prefixDirectory(directory, names)
+//	g.parsePackage(directory, names, nil)
+//}
+//
+//// parsePackageFiles parses the package occupying the named files.
+//func (g *Generator) parsePackageFiles(names []string) {
+//	g.parsePackage(".", names, nil)
+//}
+//
+//// prefixDirectory places the directory name on the beginning of each name in the list.
+//func prefixDirectory(directory string, names []string) []string {
+//	if directory == "." {
+//		return names
+//	}
+//	ret := make([]string, len(names))
+//	for i, name := range names {
+//		ret[i] = filepath.Join(directory, name)
+//	}
+//	return ret
+//}
+
+//// parsePackage analyzes the single package constructed from the named files.
+//// If text is non-nil, it is a string to be used instead of the content of the file,
+//// to be used for testing. parsePackage exits if there is an error.
+//func (g *Generator) parsePackage(directory string, names []string, text interface{}) {
+//	var files []*File
+//	var astFiles []*ast.File
+//	g.pkg = new(Package)
+//	fs := token.NewFileSet()
+//	for _, name := range names {
+//		if !strings.HasSuffix(name, ".go") {
+//			continue
+//		}
+//		parsedFile, err := parser.ParseFile(fs, name, text, 0)
+//		if err != nil {
+//			log.Fatalf("parsing package: %s: %s", name, err)
+//		}
+//		astFiles = append(astFiles, parsedFile)
+//		files = append(files, &File{
+//			file: parsedFile,
+//			pkg:  g.pkg,
+//		})
+//	}
+//	if len(astFiles) == 0 {
+//		log.Fatalf("%s: no buildable Go files", directory)
+//	}
+//	g.pkg.name = astFiles[0].Name.Name
+//	g.pkg.files = files
+//	g.pkg.dir = directory
+//	// Type check the package.
+//	g.pkg.check(fs, astFiles)
+//}
+
+// parsePackage analyzes the single package constructed from the patterns and tags.
+// parsePackage exits if there is an error.
+func (g *Generator) parsePackage(patterns []string) {
+	cfg := &packages.Config{
+		Mode: packages.LoadSyntax,
+		// TODO: Need to think about constants in test files. Maybe write type_string_test.go
+		// in a separate pass? For later.
+		Tests: false,
+	}
+	pkgs, err := packages.Load(cfg, patterns...)
 	if err != nil {
-		log.Fatalf("cannot process directory %s: %s", directory, err)
+		log.Fatal(err)
 	}
-	var names []string
-	names = append(names, pkg.GoFiles...)
-	names = append(names, pkg.CgoFiles...)
-	// TODO: Need to think about constants in test files. Maybe write type_string_test.go
-	// in a separate pass? For later.
-	// names = append(names, pkg.TestGoFiles...) // These are also in the "foo" package.
-	names = append(names, pkg.SFiles...)
-	names = prefixDirectory(directory, names)
-	g.parsePackage(directory, names, nil)
+	if len(pkgs) != 1 {
+		log.Fatalf("error: %d packages found", len(pkgs))
+	}
+	g.addPackage(pkgs[0])
 }
 
-// parsePackageFiles parses the package occupying the named files.
-func (g *Generator) parsePackageFiles(names []string) {
-	g.parsePackage(".", names, nil)
-}
-
-// prefixDirectory places the directory name on the beginning of each name in the list.
-func prefixDirectory(directory string, names []string) []string {
-	if directory == "." {
-		return names
+// addPackage adds a type checked Package and its syntax files to the generator.
+func (g *Generator) addPackage(pkg *packages.Package) {
+	g.pkg = &Package{
+		name:  pkg.Name,
+		defs:  pkg.TypesInfo.Defs,
+		files: make([]*File, len(pkg.Syntax)),
 	}
-	ret := make([]string, len(names))
-	for i, name := range names {
-		ret[i] = filepath.Join(directory, name)
-	}
-	return ret
-}
 
-// parsePackage analyzes the single package constructed from the named files.
-// If text is non-nil, it is a string to be used instead of the content of the file,
-// to be used for testing. parsePackage exits if there is an error.
-func (g *Generator) parsePackage(directory string, names []string, text interface{}) {
-	var files []*File
-	var astFiles []*ast.File
-	g.pkg = new(Package)
-	fs := token.NewFileSet()
-	for _, name := range names {
-		if !strings.HasSuffix(name, ".go") {
-			continue
-		}
-		parsedFile, err := parser.ParseFile(fs, name, text, 0)
-		if err != nil {
-			log.Fatalf("parsing package: %s: %s", name, err)
-		}
-		astFiles = append(astFiles, parsedFile)
-		files = append(files, &File{
-			file: parsedFile,
+	for i, file := range pkg.Syntax {
+		g.pkg.files[i] = &File{
+			file: file,
 			pkg:  g.pkg,
-		})
+		}
 	}
-	if len(astFiles) == 0 {
-		log.Fatalf("%s: no buildable Go files", directory)
-	}
-	g.pkg.name = astFiles[0].Name.Name
-	g.pkg.files = files
-	g.pkg.dir = directory
-	// Type check the package.
-	g.pkg.check(fs, astFiles)
 }
 
 // check type-checks the package. The package must be OK to proceed.