lib: graduate bytes/strings find-funcs to (i32 | void)

Replaces the -1 sentinel return on indexbyte/byteindex/rbyteindex/
index with Hare's optional-shaped tagged union. Callers `match` on
the result and bind the index from the i32 variant.

Two cgen fixes were needed first:

1. resolve_type for N_TTAGGED rounded value payload up to an 8-byte
   multiple. (i32 | void) was sized 12 — tag (8) + payload (4) —
   which made the reg-passing ABI compute size/8 = 1 word and drop
   the value word.

2. The call-arg push path special-cased struct and slice args but
   not tagged-return calls. A nested `f(g())` where g returns a
   tagged union pushed only AX (tag); the matching pop loaded a
   stale DX/SI for the value. Now pushes AX/DX[/CX] in order so
   the pop side drains tag → arg-reg[0], value(s) → arg-reg[1..].

strings.contains rewritten to match on the new tagged result. No
other callers existed in lib/ — bufio/io still use their own
shapes.

cmd/w6c/cgen.c

@@ -1670,6 +1670,15 @@ cgexpr(Cg *c, Node *n, Local *locals)
 			} else if (node_isstr(args[i])) {
 				ins1(c, A_PUSHQ, areg(D_BX));	/* len */
 				ins1(c, A_PUSHQ, areg(D_AX));	/* ptr — top */
+			} else if (node_istaggedarg(args[i])) {
+				/* Tagged-return ABI: AX=tag, DX=val0[, CX=val1].
+				 * Push high-to-low so pop drains tag first (into
+				 * arg-reg[0]), then values into arg-reg[1..]. */
+				int sz = tagged_arg_size(args[i]->type);
+				if (sz > 16)
+					ins1(c, A_PUSHQ, areg(D_CX));
+				ins1(c, A_PUSHQ, areg(D_DX));
+				ins1(c, A_PUSHQ, areg(D_AX));
 			} else {
 				ins1(c, A_PUSHQ, areg(D_AX));
 			}

cmd/wcc/check.c

@@ -192,7 +192,12 @@ resolve_type(Checker *c, Node *n)
 		if (nv == 0) return ty_never;
 		if (nv == 1 && head) return head->type;
 		t->params = head;
-		t->size = 8 + maxsz;
+		/* Round value payload up to an 8-byte multiple so the slot
+		 * layout (tag + N value words) stays word-aligned. The reg-
+		 * passing ABI counts size/8 words; 12-byte unions like
+		 * (i32 | void) would otherwise lose a value register. */
+		u64 vsz = (maxsz + 7) & ~(u64)7;
+		t->size = 8 + vsz;
 		t->align = al;
 		return t;
 	}

lib/bytes/bytes.ww

@@ -10,13 +10,15 @@ export fn equal(a: []u8, b: []u8) bool = {
 	return i == b.len;
 };
 
-export fn indexbyte(s: []u8, c: u8) i32 = {
+// indexbyte — first index of byte `c` in `s`. Hare-shaped optional:
+// (i32 | void). void variant indicates "not found".
+export fn indexbyte(s: []u8, c: u8) (i32 | void) = {
 	let i: i32 = 0;
 	for (i < s.len) {
 		if (s[i] == c) { return i; };
 		i += 1;
 	};
-	return -1;
+	return;
 };
 
 export fn copy(dst: []u8, src: []u8) i32 = {
@@ -30,12 +32,12 @@ export fn copy(dst: []u8, src: []u8) i32 = {
 	return n;
 };
 
-// index — first index of `sub` in `s`, or -1. Mirrors Hare's
+// index — first index of `sub` in `s`. Mirrors Hare's bytes::index
-// bytes::index (the []u8 needle variant; the u8 needle stays as
+// (the []u8 needle variant; the u8 needle stays as indexbyte until we
-// indexbyte until we have union-arg dispatch). Empty `sub` matches at 0.
+// have union-arg dispatch). Empty `sub` matches at 0.
-export fn index(s: []u8, sub: []u8) i32 = {
+export fn index(s: []u8, sub: []u8) (i32 | void) = {
 	if (sub.len == 0) { return 0; };
-	if (sub.len > s.len) { return -1; };
+	if (sub.len > s.len) { return; };
 	let last: i32 = s.len - sub.len;
 	let i: i32 = 0;
 	for (i <= last) {
@@ -48,5 +50,5 @@ export fn index(s: []u8, sub: []u8) i32 = {
 		if (ok) { return i; };
 		i += 1;
 	};
-	return -1;
+	return;
 };

lib/strings/strings.ww

@@ -39,36 +39,34 @@ export fn hassuffix(s: str, suf: str) bool = {
 	return true;
 };
 
-// byteindex — first index of byte `c` in `s`, or -1 if absent. Hare
+// byteindex — first index of byte `c` in `s`. Hare-shaped optional:
-// name (strings::byteindex). Plan 9-style sentinel return; callers that
+// (i32 | void). void variant indicates "not found".
-// prefer a fallible shape can wrap this in their own (i32 | str). No
+export fn byteindex(s: str, c: u8) (i32 | void) = {
-// allocation.
-export fn byteindex(s: str, c: u8) i32 = {
 	let i: i32 = 0;
 	for (i < s.len) {
 		if (s[i] == c) { return i; };
 		i += 1;
 	};
-	return -1;
+	return;
 };
 
-// rbyteindex — last index of byte `c` in `s`, or -1 if absent. Mirrors
+// rbyteindex — last index of byte `c` in `s`. Mirrors Hare's
-// Hare's strings::rbyteindex.
+// strings::rbyteindex.
-export fn rbyteindex(s: str, c: u8) i32 = {
+export fn rbyteindex(s: str, c: u8) (i32 | void) = {
 	let i: i32 = s.len - 1;
 	for (i >= 0) {
 		if (s[i] == c) { return i; };
 		i -= 1;
 	};
-	return -1;
+	return;
 };
 
-// index — first index of `sub` in `s`, or -1. Naive scan; fine for
+// index — first index of `sub` in `s`. Naive scan; fine for short
-// short patterns and small strings, which dominate config and CLI
+// patterns and small strings, which dominate config and CLI parsing.
-// parsing. Empty `sub` matches at 0.
+// Empty `sub` matches at 0.
-export fn index(s: str, sub: str) i32 = {
+export fn index(s: str, sub: str) (i32 | void) = {
 	if (sub.len == 0) { return 0; };
-	if (sub.len > s.len) { return -1; };
+	if (sub.len > s.len) { return; };
 	let last: i32 = s.len - sub.len;
 	let i: i32 = 0;
 	for (i <= last) {
@@ -81,11 +79,16 @@ export fn index(s: str, sub: str) i32 = {
 		if (ok) { return i; };
 		i += 1;
 	};
-	return -1;
+	return;
 };
 
 export fn contains(s: str, sub: str) bool = {
-	return index(s, sub) >= 0;
+	let r: (i32 | void) = index(s, sub);
+	match (r) {
+	case let i: i32 => return true;
+	case void => return false;
+	};
+	return false;
 };
 
 // concat — joins two strings into a fresh str. Caller owns the

test/wcc/700_e2e.c

@@ -784,17 +784,24 @@ static const struct row rows[] = {
 	  "  };\n"
 	  "  return acc;\n"
 	  "};", 143 },	/* 123 + 20 */
-	/* strings.byteindex (Plan 9 -1) and strings.index (substring). */
+	/* strings.byteindex and strings.index: now (i32 | void). */
 	{ "use strings;\n"
+	  "fn pick(r: (i32 | void), miss: i32) i32 = {\n"
+	  "  match (r) {\n"
+	  "  case let i: i32 => return i;\n"
+	  "  case void => return miss;\n"
+	  "  };\n"
+	  "  return 0;\n"
+	  "};\n"
 	  "fn main() i32 = {\n"
 	  "  let s: str = \"hello, world\";\n"
-	  "  let i1: i32 = strings.byteindex(s, 44u8);\n"
+	  "  let i1: i32 = pick(strings.byteindex(s, 44u8), -1);\n"
-	  "  let i2: i32 = strings.byteindex(s, 122u8);\n"
+	  "  let i2: i32 = pick(strings.byteindex(s, 122u8), -1);\n"
-	  "  let i3: i32 = strings.index(s, \"world\");\n"
+	  "  let i3: i32 = pick(strings.index(s, \"world\"), -1);\n"
-	  "  let i4: i32 = strings.index(s, \"nope\");\n"
+	  "  let i4: i32 = pick(strings.index(s, \"nope\"), -1);\n"
 	  "  return i1 + i2 + i3 + i4;\n"
 	  "};", 10 },	/* 5 + (-1) + 7 + (-1) */
-	/* bytes.index: substring search over []u8. */
+	/* bytes.index: substring search over []u8, (i32 | void). */
 	{ "use bytes;\n"
 	  "fn main() i32 = {\n"
 	  "  let buf: [12]u8;\n"
@@ -803,7 +810,12 @@ static const struct row rows[] = {
 	  "  buf[8] = 111u8; buf[9] = 114u8; buf[10] = 108u8; buf[11] = 100u8;\n"
 	  "  let needle: [3]u8;\n"
 	  "  needle[0] = 119u8; needle[1] = 111u8; needle[2] = 114u8;\n"
-	  "  return bytes.index(buf[0:12], needle[0:3]);\n"
+	  "  let r: (i32 | void) = bytes.index(buf[0:12], needle[0:3]);\n"
+	  "  match (r) {\n"
+	  "    case let i: i32 => return i;\n"
+	  "    case void => return -1;\n"
+	  "  };\n"
+	  "  return 0;\n"
 	  "};", 7 },
 	/* errors.equal — sentinel comparison through a (T | error) union.
 	 * Sets up two errors, dispatches each, and confirms the matching