mockgen: package _test prepend package name

mockgen/internal/tests/mock_in_test_package/README.md

@@ -0,0 +1,16 @@
+Test the case where the package has the `_test` suffix.
+
+Prior to patch:
+
+    $ go generate
+    $ go test
+    # github.com/golang/mock/mockgen/internal/tests/mock_in_test_package_test [github.com/golang/mock/mockgen/internal/tests/mock_in_test_package.test]
+    ./mock_test.go:36:44: undefined: User
+    ./mock_test.go:38:21: undefined: User
+    FAIL	github.com/golang/mock/mockgen/internal/tests/mock_in_test_package [build failed]
+
+With this patch applied:
+
+    $ go generate
+    $ go test
+    ok  	github.com/golang/mock/mockgen/internal/tests/mock_in_test_package	0.031s

mockgen/internal/tests/mock_in_test_package/user.go

@@ -0,0 +1,12 @@
+package users
+
+//go:generate mockgen --source=user.go --destination=mock_test.go --package=users_test
+
+type User struct {
+	Name string
+}
+
+type Finder interface {
+	FindUser(name string) User
+	Add(u User)
+}

mockgen/internal/tests/test_package/foo.go

@@ -0,0 +1 @@
+package users

mockgen/internal/tests/test_package/user_test.go

@@ -0,0 +1,12 @@
+package users_test
+
+//go:generate mockgen --source=user_test.go --destination=mock_test.go --package=users_test
+
+type User struct {
+	Name string
+}
+
+type Finder interface {
+	FindUser(name string) User
+	Add(u User)
+}

mockgen/mockgen.go

@@ -134,6 +134,7 @@ func main() {
 		log.Fatalf("Failed writing to destination: %v", err)
 	}
 }
+
 func parseMockNames(names string) map[string]string {
 	mocksMap := make(map[string]string)
 	for _, kv := range strings.Split(names, ",") {
@@ -223,6 +224,10 @@ func sanitize(s string) string {
 }
 
 func (g *generator) Generate(pkg *model.Package, pkgName string, outputPackagePath string) error {
+	if pkgName != pkg.Name {
+		outputPackagePath = ""
+	}
+
 	g.p("// Code generated by MockGen. DO NOT EDIT.")
 	if g.filename != "" {
 		g.p("// Source: %v", g.filename)