fix(format): never peel a stacked label that names a macro

Peeling a stacked label whose name is a macro moves it onto a line of
its own, where its expansion decides the line's shape: an empty body
leaves a bare colon behind, a line the parser rejects.  The peel loops
now hold such labels back with the rest of the line, and the crashing
input joins the corpus.

Assisted-by: GLM 5.3

format/expansion_test.go

@@ -117,6 +117,16 @@ func TestFormatPreservesExpansionSemantics(t *testing.T) {
 				"\tA: M\n" +
 				"\tRET\n",
 		},
+		{
+			// A stacked label naming a macro: its expansion decides what
+			// the line becomes (an empty body makes it vanish), so it is
+			// never peeled onto a line of its own.
+			name: "stacked label naming a macro",
+			src: "#define M\n" +
+				"TEXT ·f(SB), $0\n" +
+				"\tA: M:\n" +
+				"\tRET\n",
+		},
 		{
 			// A use above its define: the name is not a macro at that point
 			// of the file, so splitting would be safe; the whole-file set

format/format.go

@@ -70,10 +70,11 @@ func Source(src string) string {
 				// instruction after the last one is rendered at the
 				// function's alignment width, so its mnemonic counts here,
 				// unless it names a macro and never reaches a line of its
-				// own.
+				// own.  A stacked label naming a macro is never peeled
+				// either: its expansion decides what the line becomes.
 				rest := line[2:]
 				for len(rest) >= 2 && rest[0].Kind == token.Ident && rest[1].Kind == token.Colon &&
-					!isDirective(rest[0].Text) {
+					!isDirective(rest[0].Text) && !macros[rest[0].Text] {
 					rest = rest[2:]
 				}
 				if len(rest) > 0 && rest[0].Kind == token.Ident && !isDirective(rest[0].Text) &&
@@ -146,8 +147,10 @@ func Source(src string) string {
 			// honest about what it is looking at.
 			outs = append(outs, outLine{kind: kLabel, text: line[0].Text + ":"})
 			rest := line[2:]
+			// A stacked label naming a macro is never peeled: its expansion
+			// decides what the line becomes, exactly as the first label's.
 			for len(rest) >= 2 && rest[0].Kind == token.Ident && rest[1].Kind == token.Colon &&
-				!isDirective(rest[0].Text) {
+				!isDirective(rest[0].Text) && !macros[rest[0].Text] {
 				outs = append(outs, outLine{kind: kLabel, text: rest[0].Text + ":"})
 				rest = rest[2:]
 			}

format/testdata/fuzz/FuzzFormatExpansionRoundTrip/4173fd7be1b3f1a3

@@ -0,0 +1,2 @@
+go test fuzz v1
+string("#define M \nA:M:")