From f452b8a995fb8a3a90836061a5f3d033b5d4e6c6 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Petr=20Balv=C3=ADn?= <opensource@petrbalvin.org>
Date: Wed, 7 Oct 2026 13:12:35 +0200
Subject: [PATCH] fix(format): keep a macro-named label line whole

A label whose name is a macro expands into whatever the body is, so the
line's statement structure exists only after expansion; splitting the
label off changed clean input into a different statement sequence.  The
formatter now records the names #define'd above each line and renders
such a label line unsplit, and the crashing input joins the corpus.

Assisted-by: GLM 5.3
---
 format/expansion_test.go                      | 20 +++++++++++
 format/format.go                              | 35 ++++++++++++++++---
 .../3508c784782f0866                          |  2 ++
 3 files changed, 53 insertions(+), 4 deletions(-)
 create mode 100644 format/testdata/fuzz/FuzzFormatExpansionRoundTrip/3508c784782f0866

diff --git a/format/expansion_test.go b/format/expansion_test.go
index 32e2600..3a200c1 100644
--- a/format/expansion_test.go
+++ b/format/expansion_test.go
@@ -95,6 +95,26 @@ func TestFormatPreservesExpansionSemantics(t *testing.T) {
 				"\tM(1, R0)\n" +
 				"\tRET\n",
 		},
+		{
+			// A label whose name is a macro: the expansion decides what the
+			// line becomes (here an instruction with several operands), so
+			// the canonical form keeps the line whole instead of splitting
+			// the label off, which would change the statement structure.
+			name: "label named as a macro",
+			src: "#define M B0, C0\n" +
+				"TEXT ·f(SB), $0\n" +
+				"\tM: A00\n" +
+				"\tRET\n",
+		},
+		{
+			// A use before the define: the name is not a macro yet at that
+			// point of the file, so the label splits as any other does.
+			name: "label before the define of its name",
+			src: "TEXT ·f(SB), $0\n" +
+				"\tM: A00\n" +
+				"\tRET\n" +
+				"#define M B0, C0\n",
+		},
 	}
 	for _, tc := range cases {
 		t.Run(tc.name, func(t *testing.T) {
diff --git a/format/format.go b/format/format.go
index 882292c..91f9675 100644
--- a/format/format.go
+++ b/format/format.go
@@ -24,14 +24,16 @@ func Source(src string) string {
 	// index of the TEXT function it belongs to, so that mnemonic widths can be
 	// aligned per function.
 	type info struct {
-		kind    int
-		mnemLen int
-		funcID  int
+		kind      int
+		mnemLen   int
+		funcID    int
+		macroName bool // the line's label names a macro defined above it
 	}
 
 	infos := make([]info, len(lines))
 	funcID := -1
-	maxWidth := map[int]int{} // funcID -> widest mnemonic
+	maxWidth := map[int]int{}   // funcID -> widest mnemonic
+	macros := map[string]bool{} // names #define'd so far, in file order
 	for i, line := range lines {
 		inf := info{kind: kBlank, funcID: funcID}
 		if len(line) > 0 {
@@ -45,6 +47,10 @@ func Source(src string) string {
 				inf.kind = kComment
 			case line[0].Kind == token.Hash:
 				inf.kind = kPreproc
+				if len(line) >= 3 && line[1].Kind == token.Ident &&
+					line[1].Text == "define" && line[2].Kind == token.Ident {
+					macros[line[2].Text] = true
+				}
 			case line[0].Kind == token.Ident && isDirective(line[0].Text):
 				inf.kind = kDirective
 				if line[0].Text == "TEXT" {
@@ -56,6 +62,14 @@ func Source(src string) string {
 				}
 			case len(line) >= 2 && line[1].Kind == token.Colon:
 				inf.kind = kLabel
+				if macros[line[0].Text] {
+					// The label names a macro, so what the line becomes
+					// exists only after expansion; the line must not be
+					// restructured, and no mnemonic of it aligns.
+					inf.macroName = true
+					infos[i] = inf
+					continue
+				}
 				// Peel stacked labels exactly as the render pass does; the
 				// instruction after the last one is rendered at the
 				// function's alignment width, so its mnemonic counts here.
@@ -115,6 +129,19 @@ func Source(src string) string {
 			}
 			inBody = line[0].Text == "TEXT"
 		case kLabel:
+			if inf.macroName && len(line) > 2 {
+				// A label that names a macro expands into whatever the body
+				// is: after expansion the line may not be a label at all, so
+				// its statement structure exists only as one line.  The label
+				// keeps the canonical colon and everything behind it renders
+				// as one unaligned run, unsplit.
+				out := line[0].Text + ":"
+				if rest := renderOps(line[2:]); rest != "" {
+					out += " " + rest
+				}
+				outs = append(outs, outLine{kind: kLabel, text: trimLineRight(out, last)})
+				continue
+			}
 			// Every label, and a trailing instruction, becomes its own
 			// output line: separate outLines keep the blank-line pass
 			// honest about what it is looking at.
diff --git a/format/testdata/fuzz/FuzzFormatExpansionRoundTrip/3508c784782f0866 b/format/testdata/fuzz/FuzzFormatExpansionRoundTrip/3508c784782f0866
new file mode 100644
index 0000000..155e0da
--- /dev/null
+++ b/format/testdata/fuzz/FuzzFormatExpansionRoundTrip/3508c784782f0866
@@ -0,0 +1,2 @@
+go test fuzz v1
+string("#define A B0\"\nA:A00")