Fix legacy formatter behaviour for bodies with no comments or decls

experimental/ast/printer/decl.go

@@ -338,16 +338,20 @@ func (p *printer) printBody(body ast.DeclBody) {
 		forceBroken := triviaHasComments(trivia) ||
 			len(closeComments) > 0 ||
 			p.scopeHasAttachedComments(body.Braces())
-		if !forceBroken && !p.bodyShouldBreak(openTok, closeTok) {
+		if !forceBroken && !p.bodyShouldBreak(openTok, closeTok, body.Decls().Len()) {
 			decls := body.Decls()
-			p.withGroup(func(p *printer) {
-				p.withIndent(func(indented *printer) {
-					for i := range decls.Len() {
-						indented.printDecl(decls.At(i), gapSoftline)
-					}
+			// Only create the indented [dom.Group] in the case where there are decls
+			// to avoid an indent in the flat case with no decls.
+			if decls.Len() > 0 {
+				p.withGroup(func(p *printer) {
+					p.withIndent(func(indented *printer) {
+						for i := range decls.Len() {
+							indented.printDecl(decls.At(i), gapSoftline)
+						}
+					})
+					p.push(tagSoftlineFlat, tagSoftbreak)
 				})
-				p.push(tagSoftlineFlat, tagSoftbreak)
-			})
+			}
 			p.printToken(closeTok, gapNone)
 			return
 		}

experimental/ast/printer/format.go

@@ -125,20 +125,20 @@ func (p *printer) literalShouldBreak(openTok, closeTok token.Token, count int) b
 // decl-bearing body scope (`{ ... }` on message, enum, service,
 // oneof, extend, or RPC method):
 //
-//   - [LayoutStrict]: always broken. Callers handle the empty-body
+//   - [LayoutStrict]: always broken for >0 decls. Callers handle the empty-body
 //     case (rendered as `{}`) before consulting this helper.
 //
 //   - [LayoutDynamic]: broken if and only if source had a newline between open
 //     and close brace.
 //
 // Callers should OR the result with their own forceBroken signal
 // (e.g. for scope-attached comments that require expansion).
-func (p *printer) bodyShouldBreak(openTok, closeTok token.Token) bool {
+func (p *printer) bodyShouldBreak(openTok, closeTok token.Token, declsLen int) bool {
 	switch p.options.Formatting.BodyLayout {
 	case LayoutDynamic:
 		return !sourceWasFlat(openTok, closeTok)
 	default: // LayoutStrict
-		return true
+		return declsLen > 0
 	}
 }
 

experimental/ast/printer/testdata/format/basic.proto

@@ -22,3 +22,8 @@ import "acme/payment/v1/payment.proto";        /* trailing */
 message  Foo { /* Foo */
   string name = 1;
 }
+
+message Bar {
+
+
+}

experimental/ast/printer/testdata/format/basic.proto.default.txt

@@ -18,3 +18,6 @@ option optimize_for = SPEED;
 message Foo { /* Foo */
   string name = 1;
 }
+
+message Bar {
+}

experimental/ast/printer/testdata/format/basic.proto.legacy.txt

@@ -18,3 +18,5 @@ option optimize_for = SPEED;
 message Foo { /* Foo */
   string name = 1;
 }
+
+message Bar {}