test: write stubs on tests run (#23)

musttag_test.go

@@ -3,7 +3,8 @@ package musttag
 import (
 	"go/token"
 	"io"
-	"path"
+	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 
@@ -12,14 +13,11 @@ import (
 )
 
 func TestAnalyzer(t *testing.T) {
-	// NOTE(junk1tm): analysistest isn't aware of the main package's modules
-	// (see https://github.com/golang/go/issues/37054), so to run tests with
-	// external dependencies we have to be creative. Using vendor with symlinks
-	// would work but the paths would contain the `vendor/` prefix, and that's
-	// not what we want because we match full paths. The solution is to write
-	// stubs of the dependencies (we don't need the actual code, only the
-	// functions signatures to match) and to put them exactly at
-	// testdata/src/path/to/pkg (GOPATH?), otherwise it won't work.
+	// NOTE(junk1tm): analysistest does not yet support modules
+	// (see https://github.com/golang/go/issues/37054 for details).
+	// So, to be able to run tests with external dependencies,
+	// we first need to write a GOPATH-like tree of stubs.
+	prepareTestFiles(t)
 
 	testPackages = []string{"tests", "examples"}
 
@@ -86,5 +84,74 @@ func shortName(name string) string {
 	name = strings.ReplaceAll(name, "*", "")
 	name = strings.ReplaceAll(name, "(", "")
 	name = strings.ReplaceAll(name, ")", "")
-	return path.Base(name)
+	return filepath.Base(name)
+}
+
+func prepareTestFiles(t *testing.T) {
+	testdata := analysistest.TestData()
+
+	t.Cleanup(func() {
+		_ = os.RemoveAll(filepath.Join(testdata, "src"))
+	})
+
+	hardlink := func(dir, file string) {
+		target := filepath.Join(testdata, "src", dir, file)
+		if err := os.MkdirAll(filepath.Dir(target), 0o777); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.Link(filepath.Join(testdata, file), target); err != nil {
+			t.Fatal(err)
+		}
+	}
+	hardlink("tests", "tests.go")
+	hardlink("examples", "examples.go")
+
+	for file, data := range stubs {
+		target := filepath.Join(testdata, "src", file)
+		if err := os.MkdirAll(filepath.Dir(target), 0o777); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(target, []byte(data), 0o666); err != nil {
+			t.Fatal(err)
+		}
+	}
+}
+
+var stubs = map[string]string{
+	"gopkg.in/yaml.v3/yaml.go": `package yaml
+import "io"
+func Marshal(_ any) ([]byte, error)   { return nil, nil }
+func Unmarshal(_ []byte, _ any) error { return nil }
+type Encoder struct{}
+func NewEncoder(_ io.Writer) *Encoder { return nil }
+func (*Encoder) Encode(_ any) error   { return nil }
+type Decoder struct{}
+func NewDecoder(_ io.Reader) *Decoder { return nil }
+func (*Decoder) Decode(_ any) error   { return nil }`,
+
+	"github.com/BurntSushi/toml/toml.go": `package toml
+import "io"
+import "io/fs"
+func Unmarshal(_ []byte, _ any) error { return nil }
+type MetaData struct{}
+func Decode(_ string, _ any) (MetaData, error)            { return MetaData{}, nil }
+func DecodeFS(_ fs.FS, _ string, _ any) (MetaData, error) { return MetaData{}, nil }
+func DecodeFile(_ string, _ any) (MetaData, error)        { return MetaData{}, nil }
+type Encoder struct{}
+func NewEncoder(_ io.Writer) *Encoder { return nil }
+func (*Encoder) Encode(_ any) error   { return nil }
+type Decoder struct{}
+func NewDecoder(_ io.Reader) *Decoder { return nil }
+func (*Decoder) Decode(_ any) error   { return nil }`,
+
+	"github.com/mitchellh/mapstructure/mapstructure.go": `package mapstructure
+type Metadata struct{}
+func Decode(_, _ any) error                          { return nil }
+func DecodeMetadata(_, _ any, _ *Metadata) error     { return nil }
+func WeakDecode(_, _ any) error                      { return nil }
+func WeakDecodeMetadata(_, _ any, _ *Metadata) error { return nil }`,
+
+	"example.com/custom/custom.go": `package custom
+func Marshal(_ any) ([]byte, error)   { return nil, nil }
+func Unmarshal(_ []byte, _ any) error { return nil }`,
 }