go/types: add a test that generated files match types2

Adjust the go/types file generation to run in a test, so that we can
easily reuse the existing logic to verify that the current content of
go/types matches the expected result of generating from types2.

This test will enforce that we don't forget to regenerate go/types when
making changes to types2.

Change-Id: Iee14b1402065f7f0ecbcf28000e07a06c08fa42e
Reviewed-on: https://go-review.googlesource.com/c/go/+/462758
Run-TryBot: Robert Findley <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
Auto-Submit: Robert Findley <[email protected]>

Author: findleyr

src/go/types/array.go

@@ -3,6 +3,6 @@
 // license that can be found in the LICENSE file.
 
 // This file exists only to drive go:generate.
-//go:generate go run generator.go
+//go:generate go test -run=Generate -write=all
 
 package types

src/go/types/basic.go

@@ -2,81 +2,93 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build ignore
-
 // This file implements a custom generator to create various go/types
 // source files from the corresponding types2 files.
 
-package main
+package types_test
 
 import (
 	"bytes"
 	"flag"
-	"fmt"
 	"go/ast"
 	"go/format"
 	"go/parser"
 	"go/token"
-	"log"
+	"internal/diff"
 	"os"
 	"path/filepath"
 	"runtime"
 	"strings"
+	"testing"
 )
 
+var filesToWrite = flag.String("write", "", `go/types files to generate, or "all" for all files`)
+
 const (
-	srcDir = "cmd/compile/internal/types2"
-	dstDir = "go/types"
+	srcDir = "/src/cmd/compile/internal/types2/"
+	dstDir = "/src/go/types/"
 )
 
-var fset = token.NewFileSet()
-
-func main() {
-	flag.Parse()
-
-	// process provided filenames, if any
-	if flag.NArg() > 0 {
-		for _, filename := range flag.Args() {
-			generate(filename, filemap[filename])
+// TestGenerate verifies that generated files in go/types match their types2
+// counterpart. If -write is set, this test actually writes the expected
+// content to go/types; otherwise, it just compares with the existing content.
+func TestGenerate(t *testing.T) {
+	// If filesToWrite is set, write the generated content to disk.
+	// In the special case of "all", write all files in filemap.
+	write := *filesToWrite != ""
+	var files []string // files to process
+	if *filesToWrite != "" && *filesToWrite != "all" {
+		files = strings.Split(*filesToWrite, ",")
+	} else {
+		for file := range filemap {
+			files = append(files, file)
 		}
-		fmt.Printf("generated %d files\n", flag.NArg())
-		return
 	}
 
-	// otherwise process per filemap below
-	for filename, action := range filemap {
-		generate(filename, action)
+	for _, filename := range files {
+		generate(t, filename, write)
 	}
-	fmt.Printf("generated %d files\n", len(filemap))
 }
 
-func generate(filename string, action action) {
+func generate(t *testing.T, filename string, write bool) {
 	// parse src
-	srcFilename := filepath.FromSlash(runtime.GOROOT() + "/src/" + srcDir + "/" + filename)
+	srcFilename := filepath.FromSlash(runtime.GOROOT() + srcDir + filename)
 	file, err := parser.ParseFile(fset, srcFilename, nil, parser.ParseComments)
 	if err != nil {
-		log.Fatal(err)
+		t.Fatal(err)
 	}
 
 	// fix package name
 	file.Name.Name = strings.ReplaceAll(file.Name.Name, "types2", "types")
 
 	// rewrite AST as needed
-	if action != nil {
+	if action := filemap[filename]; action != nil {
 		action(file)
 	}
 
 	// format AST
 	var buf bytes.Buffer
-	buf.WriteString("// Code generated by \"go run generator.go\"; DO NOT EDIT.\n\n")
+	buf.WriteString("// Code generated by \"go test -run=Generate -write=all\"; DO NOT EDIT.\n\n")
 	if err := format.Node(&buf, fset, file); err != nil {
-		log.Fatal(err)
+		t.Fatal(err)
 	}
+	generatedContent := buf.Bytes()
 
-	// write dst
-	dstFilename := filepath.FromSlash(runtime.GOROOT() + "/src/" + dstDir + "/" + filename)
-	if err := os.WriteFile(dstFilename, buf.Bytes(), 0o644); err != nil {
-		log.Fatal(err)
+	dstFilename := filepath.FromSlash(runtime.GOROOT() + dstDir + filename)
+	onDiskContent, err := os.ReadFile(dstFilename)
+	if err != nil {
+		t.Fatalf("reading %q: %v", filename, err)
+	}
+
+	if d := diff.Diff(filename+" (on disk)", onDiskContent, filename+" (generated)", generatedContent); d != nil {
+		if write {
+			t.Logf("applying change:\n%s", d)
+			if err := os.WriteFile(dstFilename, generatedContent, 0o644); err != nil {
+				t.Fatalf("writing %q: %v", filename, err)
+			}
+		} else {
+			t.Errorf("generated file content does not match:\n%s", string(d))
+		}
 	}
 }