fix(format): treat a selector-folded label as naming its macro

The lexer folds NAME.selector into one identifier, and an object macro
reached through the selector expands with it travelling along, so a
label spelled NAME.selector: restructures exactly like the bare name
would.  The macro-name checks now resolve the prefix before the first
period, and the crashing input joins the corpus.

Assisted-by: GLM 5.3

format/expansion_test.go

@@ -127,6 +127,17 @@ func TestFormatPreservesExpansionSemantics(t *testing.T) {
 				"\tA: M:\n" +
 				"\tRET\n",
 		},
+		{
+			// A label reached through a selector: the lexer folds NAME and
+			// .selector into one identifier, and the object macro behind the
+			// name expands with the selector travelling along, so the folded
+			// spelling restructures exactly like the bare name.
+			name: "label naming a macro through a selector",
+			src: "#define A B\n" +
+				"TEXT ·f(SB), $0\n" +
+				"\tA.: A0\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

@@ -58,7 +58,7 @@ func Source(src string) string {
 				}
 			case len(line) >= 2 && line[1].Kind == token.Colon:
 				inf.kind = kLabel
-				if macros[line[0].Text] {
+				if namesMacro(line[0].Text, macros) {
 					// 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.
@@ -74,11 +74,11 @@ func Source(src string) string {
 				// 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) && !macros[rest[0].Text] {
+					!isDirective(rest[0].Text) && !namesMacro(rest[0].Text, macros) {
 					rest = rest[2:]
 				}
 				if len(rest) > 0 && rest[0].Kind == token.Ident && !isDirective(rest[0].Text) &&
-					!macros[rest[0].Text] {
+					!namesMacro(rest[0].Text, macros) {
 					inf.mnemLen = len(rest[0].Text)
 					if funcID >= 0 && inf.mnemLen > maxWidth[funcID] {
 						maxWidth[funcID] = inf.mnemLen
@@ -150,7 +150,7 @@ func Source(src string) string {
 			// 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) && !macros[rest[0].Text] {
+				!isDirective(rest[0].Text) && !namesMacro(rest[0].Text, macros) {
 				outs = append(outs, outLine{kind: kLabel, text: rest[0].Text + ":"})
 				rest = rest[2:]
 			}
@@ -165,7 +165,7 @@ func Source(src string) string {
 				// 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 && !macros[rest[0].Text] {
+			} else if len(rest) > 0 && rest[0].Kind == token.Ident && !namesMacro(rest[0].Text, macros) {
 				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
@@ -208,6 +208,21 @@ func macroNames(lines [][]token.Token) map[string]bool {
 	return names
 }
 
+// namesMacro reports whether text names a macro from names, directly or
+// through the selector suffix the lexer folds into one identifier: an object
+// macro reached as "NAME.selector" expands with the selector travelling
+// along, so such a label restructures exactly like the bare name.  The check
+// ignores the object-and-parameterised distinction and stays conservative.
+func namesMacro(text string, names map[string]bool) bool {
+	if names[text] {
+		return true
+	}
+	if j := strings.IndexByte(text, '.'); j > 0 {
+		return names[text[:j]]
+	}
+	return false
+}
+
 // 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/ea7fb86b86361801

@@ -0,0 +1,2 @@
+go test fuzz v1
+string("#define A B\"\nA.:A0")