Prepend line directive to copied source files and add test

mod.go

@@ -3,6 +3,7 @@ package testutil
 import (
 	"bytes"
 	"context"
+	"fmt"
 	"io"
 	"io/ioutil"
 	"os"
@@ -50,6 +51,17 @@ func WithModules(t *testing.T, srcdir string, modfile io.Reader) (dir string) {
 		}
 
 		for _, file := range files {
+			// Prepend line directive to .go files
+			if filepath.Ext(file.Name()) == ".go" {
+				fn := filepath.Join(path, file.Name())
+				rel, err := filepath.Rel(dir, fn)
+				if err != nil {
+					t.Fatal("cannot get relative path:", err)
+				}
+				if err := prependToFile(fn, fmt.Sprintf("//line %s:1\n", rel)); err != nil {
+					t.Fatal("cannot prepend line directive:", err)
+				}
+			}
 			if file.Name() == "go.mod" {
 				if modfile != nil {
 					fn := filepath.Join(path, "go.mod")
@@ -85,6 +97,29 @@ func WithModules(t *testing.T, srcdir string, modfile io.Reader) (dir string) {
 	return dir
 }
 
+func prependToFile(filename string, ld string) error {
+	f, err := os.OpenFile(filename, os.O_RDWR, 0)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	b, err := io.ReadAll(f)
+	if err != nil {
+		return err
+	}
+	if _, err := f.Seek(0, 0); err != nil {
+		return err
+	}
+	if _, err := f.WriteString(ld + "\n"); err != nil {
+		return err
+	}
+	if _, err := f.Write(b); err != nil {
+		return err
+	}
+	return nil
+}
+
 // ModFile opens a mod file with the path and fixes versions by the version fixer.
 // If the path is direcotry, ModFile opens go.mod which is under the path.
 func ModFile(t *testing.T, path string, fix modfile.VersionFixer) io.Reader {

mod_test.go

@@ -0,0 +1,33 @@
+package testutil
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"golang.org/x/tools/go/analysis/analysistest"
+)
+
+func TestWithModules(t *testing.T) {
+	t.Parallel()
+
+	t.Run("The line directive is appended to the go source codes", func(t *testing.T) {
+		testdata := WithModules(t, analysistest.TestData(), nil)
+		tests := []struct {
+			path string
+			want string
+		}{
+			{filepath.Join(testdata, "src", "a", "a.go"), "//line src/a/a.go:1"},
+			{filepath.Join(testdata, "src", "a", "b", "b.go"), "//line src/a/b/b.go:1"},
+		}
+		for _, tt := range tests {
+			b, err := os.ReadFile(tt.path)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if got := string(b[:len(tt.want)]); got != tt.want {
+				t.Errorf("got %q, want %q", got, tt.want)
+			}
+		}
+	})
+}

testdata/src/a/a.go

@@ -0,0 +1,4 @@
+package a
+
+func a() {
+}

testdata/src/a/b/b.go

@@ -0,0 +1,4 @@
+package b
+
+func b() {
+}

testdata/src/a/go.mod

@@ -0,0 +1,3 @@
+module a
+
+go 1.21