go/printer: do not treat comments inside a ast.Decl as godoc

This change makes sure that we do not format comments
as doc comments inside of a declaration and makes the
go doc formatter idempotent:

Previously:

	// test comment
	//go:directive2
	// test comment
func main() {
}

was formatted to:

// test comment
//go:directive2
// test comment
func main() {
}

after another formatting, it got formatted with doc rules into:

// test comment
// test comment
//
//go:directive2
func main() {
}

With this change it gets directly to the correct form (last one).

Change-Id: Id7d8f03e43474357cd714e0672e886652c3fce86
GitHub-Last-Rev: 9833b87536
GitHub-Pull-Request: golang/go#69134
Reviewed-on: https://go-review.googlesource.com/c/go/+/609077
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>

src/go/printer/nodes.go

@@ -1737,6 +1737,9 @@ func (p *printer) genDecl(d *ast.GenDecl) {
 	p.setPos(d.Pos())
 	p.print(d.Tok, blank)
 
+	defer func(d bool) { p.inDecl = d }(p.inDecl)
+	p.inDecl = true
+
 	if d.Lparen.IsValid() || len(d.Specs) != 1 {
 		// group of parenthesized declarations
 		p.setPos(d.Lparen)
@@ -1921,6 +1924,10 @@ func (p *printer) funcDecl(d *ast.FuncDecl) {
 	p.setComment(d.Doc)
 	p.setPos(d.Pos())
 	p.print(token.FUNC, blank)
+
+	defer func(d bool) { p.inDecl = d }(p.inDecl)
+	p.inDecl = true
+
 	// We have to save startCol only after emitting FUNC; otherwise it can be on a
 	// different line (all whitespace preceding the FUNC is emitted only when the
 	// FUNC is emitted).

src/go/printer/printer.go

@@ -63,6 +63,7 @@ type printer struct {
 	mode         pmode        // current printer mode
 	endAlignment bool         // if set, terminate alignment immediately
 	impliedSemi  bool         // if set, a linebreak implies a semicolon
+	inDecl       bool         // if set, printer is inside declaration (after first token)
 	lastTok      token.Token  // last token printed (token.ILLEGAL if it's whitespace)
 	prevOpen     token.Token  // previous non-brace "open" token (, [, or token.ILLEGAL
 	wsbuf        []whiteSpace // delayed white space
@@ -739,8 +740,9 @@ func (p *printer) intersperseComments(next token.Position, tok token.Token) (wro
 	for p.commentBefore(next) {
 		list := p.comment.List
 		changed := false
-		if p.lastTok != token.IMPORT && // do not rewrite cgo's import "C" comments
-			p.posFor(p.comment.Pos()).Column == 1 &&
+		if !p.inDecl &&
+			p.lastTok != token.IMPORT && // do not rewrite cgo's import "C" comments
+			p.posFor(p.comment.Pos()).Line != p.last.Line &&
 			p.posFor(p.comment.End()+1) == next {
 			// Unindented comment abutting next token position:
 			// a top-level doc comment.

src/go/printer/printer_test.go

@@ -16,6 +16,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 )
@@ -863,3 +864,192 @@ func TestEmptyDecl(t *testing.T) { // issue 63566
 		}
 	}
 }
+
+func TestDocFormat(t *testing.T) {
+	cases := []struct {
+		src  string
+		want string
+	}{
+		{
+			src: `package main
+
+func main() {
+//
+//go:directive
+// test
+//
+}
+`,
+			want: `package main
+
+func main() {
+	//
+	//go:directive
+	// test
+	//
+}
+`,
+		},
+		{
+			src: `package main
+
+func main() {
+	//go:directive
+	// test
+	type a struct{}
+
+//go:directive
+// test
+test()
+}
+`,
+			want: `package main
+
+func main() {
+	//go:directive
+	// test
+	type a struct{}
+
+	//go:directive
+	// test
+	test()
+}
+`,
+		},
+		{
+			src: `package main
+
+func main() {
+//go:directive
+// test
+type a struct{}
+}
+`,
+			want: `package main
+
+func main() {
+	//go:directive
+	// test
+	type a struct{}
+}
+`,
+		},
+		{
+			src: `package main
+
+func a() {
+//line a:5:1
+	//
+}
+`,
+			want: `package main
+
+func a() {
+//line a:5:1
+	//
+}
+`,
+		},
+
+		{
+			src: `package main
+
+// test comment
+//go:directive2
+// test comment
+func main() {
+}
+`,
+			want: `package main
+
+// test comment
+// test comment
+//
+//go:directive2
+func main() {
+}
+`,
+		},
+		{
+			src: `package main
+
+	// test comment
+	//go:directive2
+	// test comment
+func main() {
+}
+`,
+			want: `package main
+
+// test comment
+// test comment
+//
+//go:directive2
+func main() {
+}
+`,
+		},
+		{
+			src: `package main
+
+/* test
+ */ // test comment
+//go:directive2
+// test comment
+func main() {
+}
+`,
+			want: `package main
+
+/* test
+ */ // test comment
+//go:directive2
+// test comment
+func main() {
+}
+`,
+		},
+
+		{
+			src: `package main  //comment
+var a int = 4 //comment
+func a() {
+}
+`,
+			want: `package main  //comment
+var a int = 4 //comment
+func a() {
+}
+`,
+		},
+
+		// Edge case found by a fuzzer, not a real-world example.
+		{
+			src:  "package A\n\nimport(\"\f\"\n//\n\"\")",
+			want: "package A\n\nimport (\n\t\"\f\" //\n\t\"\"\n)\n",
+		},
+		{
+			src:  "package A\n\nimport(`\f`\n//\n\"\")",
+			want: "package A\n\nimport (\n\t`\f` //\n\t\"\"\n)\n",
+		},
+	}
+
+	for _, tt := range cases {
+		fset := token.NewFileSet()
+		f, err := parser.ParseFile(fset, "test.go", tt.src, parser.ParseComments|parser.SkipObjectResolution)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		var buf strings.Builder
+		cfg := Config{Tabwidth: 8, Mode: UseSpaces | TabIndent}
+		if err := cfg.Fprint(&buf, fset, f); err != nil {
+			t.Fatal(err)
+		}
+
+		got := buf.String()
+		if got != tt.want {
+			t.Errorf("source\n%v\nformatted as:\n%v\nwant formatted as:\n%v", tt.src, got, tt.want)
+		}
+	}
+}