fix(format): keep macro content behind a label on its line

The canonical form splits a label from the instruction that follows it,
but an identifier naming a macro may expand into any token at all: moved
to a line of its own it no longer parses, because the parser accepts a
non-mnemonic first token only behind a label.  The names #define'd in
the file now hold such content back, conservatively across the whole
file, and the crashing input joins the corpus.

Assisted-by: GLM 5.3

format/expansion_test.go

@@ -107,8 +107,22 @@ func TestFormatPreservesExpansionSemantics(t *testing.T) {
 				"\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.
+			// Content after a label that names a macro: the expansion may
+			// start with any token, so the canonical form keeps it on the
+			// label line, where the parser accepts a non-mnemonic first
+			// token behind a label but not at the start of a line.
+			name: "macro behind a label",
+			src: "#define M 0\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
+			// the formatter keeps is deliberately conservative and leaves
+			// the line whole, which preserves the expansion in both
+			// directions alike.
 			name: "label before the define of its name",
 			src: "TEXT ·f(SB), $0\n" +
 				"\tM: A00\n" +

format/format.go

@@ -19,6 +19,7 @@ import (
 // Source returns the canonical formatting of src.
 func Source(src string) string {
 	lines := splitLines(lexer.Tokenize(src))
+	macros := macroNames(lines)
 
 	// First pass: classify each line and record, for every instruction, the
 	// index of the TEXT function it belongs to, so that mnemonic widths can be
@@ -27,13 +28,12 @@ func Source(src string) string {
 		kind      int
 		mnemLen   int
 		funcID    int
-		macroName bool // the line's label names a macro defined above it
+		macroName bool // the line's label names a macro
 	}
 
 	infos := make([]info, len(lines))
 	funcID := -1
-	maxWidth := map[int]int{}   // funcID -> widest mnemonic
-	macros := map[string]bool{} // names #define'd so far, in file order
+	maxWidth := map[int]int{} // funcID -> widest mnemonic
 	for i, line := range lines {
 		inf := info{kind: kBlank, funcID: funcID}
 		if len(line) > 0 {
@@ -47,10 +47,6 @@ 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" {
@@ -72,13 +68,16 @@ func Source(src string) string {
 				}
 				// 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.
+				// function's alignment width, so its mnemonic counts here,
+				// unless it names a macro and never reaches a line of its
+				// own.
 				rest := line[2:]
 				for len(rest) >= 2 && rest[0].Kind == token.Ident && rest[1].Kind == token.Colon &&
 					!isDirective(rest[0].Text) {
 					rest = rest[2:]
 				}
-				if len(rest) > 0 && rest[0].Kind == token.Ident && !isDirective(rest[0].Text) {
+				if len(rest) > 0 && rest[0].Kind == token.Ident && !isDirective(rest[0].Text) &&
+					!macros[rest[0].Text] {
 					inf.mnemLen = len(rest[0].Text)
 					if funcID >= 0 && inf.mnemLen > maxWidth[funcID] {
 						maxWidth[funcID] = inf.mnemLen
@@ -155,14 +154,15 @@ func Source(src string) string {
 			// A label may share its line with an instruction; the canonical
 			// form puts the instruction on the following line.  Trailing
 			// content that does not start an instruction (a stray operand
-			// token) stays on the label line: splitting it off would produce
-			// a line the parser rejects.
+			// token, or an identifier naming a macro whose expansion may
+			// start with any token at all) stays on the label line:
+			// splitting it off would produce a line the parser rejects.
 			if len(rest) > 0 && rest[0].Kind == token.Ident && isDirective(rest[0].Text) {
 				// A bare directive cannot start a line of its own (the
 				// parser wants a symbol per line), so a directive sharing
 				// the label's line stays there.
 				outs[len(outs)-1].text += " " + trimLineRight(renderOps(rest), rest[len(rest)-1])
-			} else if len(rest) > 0 && rest[0].Kind == token.Ident {
+			} else if len(rest) > 0 && rest[0].Kind == token.Ident && !macros[rest[0].Text] {
 				outs = append(outs, outLine{kind: kInstr, text: trimLineRight(renderInstr(rest, maxWidth[inf.funcID]), rest[len(rest)-1])})
 				if strings.EqualFold(rest[0].Text, "RET") {
 					inBody = false
@@ -189,6 +189,22 @@ func Source(src string) string {
 	return normalizeSpacing(outs)
 }
 
+// macroNames collects the names #define'd anywhere in the file.  A label or
+// instruction token that names a macro may expand into anything at all, so
+// the lines carrying it are never restructured; the whole-file set is
+// deliberately conservative, because a use above its define splits in
+// neither direction.
+func macroNames(lines [][]token.Token) map[string]bool {
+	names := map[string]bool{}
+	for _, line := range lines {
+		if len(line) >= 3 && line[0].Kind == token.Hash && line[1].Kind == token.Ident &&
+			line[1].Text == "define" && line[2].Kind == token.Ident {
+			names[line[2].Text] = true
+		}
+	}
+	return names
+}
+
 // trimLineRight removes trailing spaces and tabs from a rendered line, which
 // are layout the canonical form drops.  The trim never eats into the text of
 // the line's final token: a string or rune literal may legitimately end in

format/testdata/fuzz/FuzzFormatExpansionRoundTrip/e6b15df9546d86e6

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