fix(parser): peel stacked labels the way the formatter renders them

Assisted-by: GLM 5.3

format/testdata/fuzz/FuzzFormatIdempotency/75eaf4940c1e9b30

@@ -0,0 +1,2 @@
+go test fuzz v1
+string("A:A:")

parser/parser.go

@@ -160,12 +160,20 @@ func (p *state) parseLine(line []token.Token) {
 			return
 		}
 
-		// Label (ident immediately followed by a colon).
+		// Label (ident immediately followed by a colon).  Stacked labels
+		// share one line ("a: b: MOVQ …"): each peels off as its own Label,
+		// the same statement sequence the canonical layout puts on separate
+		// lines, so a file and its formatted image parse alike.  A directive
+		// never peels: it stays with the instruction parse, as in the
+		// formatter's label handling.
 		if len(line) >= 2 && line[1].Kind == token.Colon {
-			lbl := &ast.Label{Name: line[0], Colon: line[1]}
-			p.addStmt(lbl)
-			// A label may share its line with an instruction: "loop: MOVQ …".
-			if rest := dropColon(line); len(rest) > 0 {
+			rest := line
+			for len(rest) >= 2 && rest[0].Kind == token.Ident && rest[1].Kind == token.Colon &&
+				!isDirective(rest[0].Text) {
+				p.addStmt(&ast.Label{Name: rest[0], Colon: rest[1]})
+				rest = rest[2:]
+			}
+			if len(rest) > 0 {
 				p.parseInstr(rest)
 			}
 			p.pending = nil
@@ -182,12 +190,10 @@ func (p *state) parseLine(line []token.Token) {
 	p.pending = nil
 }
 
-// dropColon removes the leading "ident :" of a label, returning the remainder.
-func dropColon(line []token.Token) []token.Token {
-	if len(line) >= 2 && line[1].Kind == token.Colon {
-		return line[2:]
-	}
-	return nil
+// isDirective reports whether the identifier names a top-level declaration
+// directive rather than an instruction or a label.
+func isDirective(s string) bool {
+	return s == "TEXT" || s == "DATA" || s == "GLOBL"
 }
 
 func (p *state) addStmt(s ast.Stmt) {