go/analysis/internal/checker: make applyFixes work from the first character

Make sure modifying the first character of the file takes effect.

Fixes golang/go#54774

Change-Id: Ib77231b9bd15f35fe50b2c2d6c7ea260c9c3cba5
GitHub-Last-Rev: b58bbdf4c247b22947223b157a1d36ecc856c652
GitHub-Pull-Request: golang/tools#393
Reviewed-on: https://go-review.googlesource.com/c/tools/+/426654
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Tim King <taking@google.com>

go/analysis/internal/checker/checker.go

@@ -408,6 +408,8 @@ func applyFixes(roots []*action) {
 			if edit.start > cur {
 				out.Write(contents[cur:edit.start])
 				out.Write(edit.newText)
+			} else if cur == 0 && edit.start == 0 { // edit starts at first character?
+				out.Write(edit.newText)
 			}
 			cur = edit.end
 

go/analysis/internal/checker/start_test.go

@@ -0,0 +1,85 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package checker_test
+
+import (
+	"go/ast"
+	"io/ioutil"
+	"path/filepath"
+	"testing"
+
+	"golang.org/x/tools/go/analysis"
+	"golang.org/x/tools/go/analysis/analysistest"
+	"golang.org/x/tools/go/analysis/internal/checker"
+	"golang.org/x/tools/go/analysis/passes/inspect"
+	"golang.org/x/tools/go/ast/inspector"
+	"golang.org/x/tools/internal/testenv"
+)
+
+// TestStartFixes make sure modifying the first character
+// of the file takes effect.
+func TestStartFixes(t *testing.T) {
+	testenv.NeedsGoPackages(t)
+
+	files := map[string]string{
+		"comment/doc.go": `/* Package comment */
+package comment
+`}
+
+	want := `// Package comment
+package comment
+`
+
+	testdata, cleanup, err := analysistest.WriteFiles(files)
+	if err != nil {
+		t.Fatal(err)
+	}
+	path := filepath.Join(testdata, "src/comment/doc.go")
+	checker.Fix = true
+	checker.Run([]string{"file=" + path}, []*analysis.Analyzer{commentAnalyzer})
+
+	contents, err := ioutil.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	got := string(contents)
+	if got != want {
+		t.Errorf("contents of rewritten file\ngot: %s\nwant: %s", got, want)
+	}
+
+	defer cleanup()
+}
+
+var commentAnalyzer = &analysis.Analyzer{
+	Name:     "comment",
+	Requires: []*analysis.Analyzer{inspect.Analyzer},
+	Run:      commentRun,
+}
+
+func commentRun(pass *analysis.Pass) (interface{}, error) {
+	const (
+		from = "/* Package comment */"
+		to   = "// Package comment"
+	)
+	inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
+	inspect.Preorder(nil, func(n ast.Node) {
+		if n, ok := n.(*ast.Comment); ok && n.Text == from {
+			pass.Report(analysis.Diagnostic{
+				Pos: n.Pos(),
+				End: n.End(),
+				SuggestedFixes: []analysis.SuggestedFix{{
+					TextEdits: []analysis.TextEdit{{
+						Pos:     n.Pos(),
+						End:     n.End(),
+						NewText: []byte(to),
+					}},
+				}},
+			})
+		}
+	})
+
+	return nil, nil
+}