.github/workflows/vettools: add a static analysis for image importing

Updates #2336
Closes #2337

.github/workflows/test.yml

@@ -73,7 +73,7 @@ jobs:
         run: |
           go vet -tags=example -v ./...
 
-      - name: go vet (atomic align, error check)
+      - name: go vet (vettool)
         run: |
           (cd .github/workflows/vettools; go install .)
           go vet -vettool=$(which vettools)${{ startsWith(matrix.os, 'windows-') && '.exe' || '' }} -tags=example -v ./...

.github/workflows/vettools/imageimportcheck.go

@@ -0,0 +1,83 @@
+// Copyright 2022 The Ebitengine Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"fmt"
+	"go/token"
+	"reflect"
+	"strconv"
+	"strings"
+
+	"golang.org/x/tools/go/analysis"
+)
+
+// imageImportCheckAnalyzer is an analyzer to check whether unexpected `image/*` packages are imported.
+// Importing `image/gif`, `image/jpeg`, and `image/png` registers their recorders at `init` functions, so
+// have affect the result of `image.Decode`. Ebitengine should not have such side-effects.
+var imageImportCheckAnalyzer = &analysis.Analyzer{
+	Name:       "imageimportcheck",
+	Doc:        "check importing image/gif, image/jpeg, and image/png packages",
+	Run:        runImageImportCheck,
+	ResultType: reflect.TypeOf(imageImportCheckResult{}),
+}
+
+type imageImportCheckResult struct {
+	Errors []imageImportCheckError
+}
+
+type imageImportCheckError struct {
+	Pos    token.Pos
+	Import string
+}
+
+func runImageImportCheck(pass *analysis.Pass) (interface{}, error) {
+	pkgPath := pass.Pkg.Path()
+	if strings.HasPrefix(pkgPath, "github.com/hajimehoshi/ebiten/v2/example") {
+		return nil, nil
+	}
+	if strings.HasSuffix(pkgPath, "_test") {
+		return nil, nil
+	}
+
+	// TODO: Remove this exception after v3 is released (#2336).
+	if strings.HasPrefix(pkgPath, "github.com/hajimehoshi/ebiten/v2/ebitenutil") {
+		return nil, nil
+	}
+
+	var errs []imageImportCheckError
+	for _, f := range pass.Files {
+		for _, i := range f.Imports {
+			path, err := strconv.Unquote(i.Path.Value)
+			if err != nil {
+				return nil, err
+			}
+			if path == "image/gif" || path == "image/jpeg" || path == "image/png" {
+				err := imageImportCheckError{
+					Pos:    pass.Fset.File(f.Pos()).Pos(int(i.Pos())),
+					Import: path,
+				}
+				errs = append(errs, err)
+				pass.Report(analysis.Diagnostic{
+					Pos:     err.Pos,
+					Message: fmt.Sprintf("unexpected import %q", err.Import),
+				})
+			}
+		}
+	}
+	return imageImportCheckResult{
+		Errors: errs,
+	}, nil
+}

.github/workflows/vettools/main.go

@@ -23,9 +23,9 @@ import (
 )
 
 func main() {
-	filename := ".errcheck_excludes"
+	const filename = ".errcheck_excludes"
 	if _, err := os.Stat(filename); err == nil {
 		errcheck.Analyzer.Flags.Set("exclude", filename)
 	}
-	multichecker.Main(atomicalign.Analyzer, errcheck.Analyzer)
+	multichecker.Main(atomicalign.Analyzer, errcheck.Analyzer, imageImportCheckAnalyzer)
 }