w6c+w6c_ww: store full slice header for struct-literal slice fields (fix #24)

cg_structlit_fill / cgstructlitfill had a TY_STR arm that stored all
three header words (ptr@+0, len@+8, cap@+16) but no TY_SLICE arm, so a
slice field in a struct literal `cl{ items = b, n = .. }` fell through to
the generic scalar tail and stored only the ptr word — the field's .len
and .cap read 0. str fields (the same 24B {ptr,len,cap} shape) worked;
slice fields silently dropped two words.

Both stages emitted IDENTICAL wrong asm, so the 990-997 byte-id gate was
green on both-wrong; runtime readback is the only correctness net. Same
is_str/is_slice discrimination gap as #10 part-b, here in the
struct-literal field-init path.

A slice is the same 24B header shape as str, so widen the str arm's
guard to TY_STR || TY_SLICE (cstage) / isstrtype || isslicetype
(wwstage) and let a slice ride the already-correct 3-word store. The
TAGGED arm stays ordered before it, so a nullable/tagged slice
(TY_TAGGED) still routes to the widener, not the 3-word store.

Test 689 (table-driven, runtime readback + dual-stage asm byte-id):
slice .len/.cap/.ptr, a scalar field beside/before the slice, a slice at
a non-zero field offset, two slice fields, and a str field beside a
slice (str-arm regression pin). 33/33 ok.

Makefile

@@ -246,6 +246,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_slice_str_global_zero \
 	  $(BIN)/test_slice_literal_global \
 	  $(BIN)/test_global_arr_elem_field \
+	  $(BIN)/test_structlit_slice_field \
 	  $(BIN)/test_dot_str_chained_arg \
 	  $(BIN)/test_dot_slice_arg \
 	  $(BIN)/test_dot_tagged_source \
@@ -597,6 +598,12 @@ $(BIN)/test_global_arr_elem_field: test/wcc/688_global_arr_elem_field.c $(BIN)/w
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_structlit_slice_field: test/wcc/689_structlit_slice_field.c $(BIN)/ww \
+		$(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
+		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_dot_str_chained_arg: test/wcc/692_dot_str_chained_arg.c $(BIN)/ww \
 		$(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \

cmd/w6c/cgen.c

@@ -2690,8 +2690,10 @@ cg_structlit_fill(Cg *c, Local **locals_p, Type *lu, Node *lit,
 		/* str IS []u8: 3-word field (ptr,len,cap). cgexpr leaves
 		 * AX/BX/CX; for non-BP modes the dst base goes in DX to dodge
 		 * BX=len / CX=cap (the generic store below reloads BX, which
-		 * would clobber len) (#1/Phase 3). */
+		 * would clobber len) (#1/Phase 3). A slice is the same 24B
-		if (fu && fu->kind == TY_STR) {
+		 * {ptr,len,cap} shape, so it rides this arm; without it the
+		 * generic scalar tail stored only the ptr word (#24). */
+		if (fu && (fu->kind == TY_STR || fu->kind == TY_SLICE)) {
 			cgexpr(c, f->lhs, *locals_p);
 			if (mode == DST_BP) {
 				ins2(c, A_MOVQ, areg(D_AX),

selfhost/cmd/w6c/main.combined.ww

@@ -19519,12 +19519,15 @@ fn cgstructlitfill(c: *cgen, si: *structinfo, lit: *node,
                         fi = nil;
                     } else if (callwhole) {
                         fi = nil;
-                    } else if (isstrtype(c, fi.tnode)) {
+                    } else if (isstrtype(c, fi.tnode) || isslicetype(c, fi.tnode)) {
                         // str IS []u8: 3-word field (ptr,len,cap).
                         // cgexpr leaves AX/BX/CX; for non-BP modes
                         // the dst base goes in DX to dodge BX=len /
                         // CX=cap (the generic store reloads BX, which
-                        // would clobber len) (#1/Phase 3).
+                        // would clobber len) (#1/Phase 3). A slice is
+                        // the same 24B {ptr,len,cap} shape, so it rides
+                        // this arm; without it the generic scalar tail
+                        // stored only the ptr word (#24).
                         cgexpr(c, fieldnode.lhs);
                         if (mode == 0) {
                             emitline("\tMOVQ\tAX, ");

selfhost/cmd/wcc/cgenutil.ww

@@ -3853,12 +3853,15 @@ fn cgstructlitfill(c: *cgen, si: *structinfo, lit: *node,
                         fi = nil;
                     } else if (callwhole) {
                         fi = nil;
-                    } else if (isstrtype(c, fi.tnode)) {
+                    } else if (isstrtype(c, fi.tnode) || isslicetype(c, fi.tnode)) {
                         // str IS []u8: 3-word field (ptr,len,cap).
                         // cgexpr leaves AX/BX/CX; for non-BP modes
                         // the dst base goes in DX to dodge BX=len /
                         // CX=cap (the generic store reloads BX, which
-                        // would clobber len) (#1/Phase 3).
+                        // would clobber len) (#1/Phase 3). A slice is
+                        // the same 24B {ptr,len,cap} shape, so it rides
+                        // this arm; without it the generic scalar tail
+                        // stored only the ptr word (#24).
                         cgexpr(c, fieldnode.lhs);
                         if (mode == 0) {
                             emitline("\tMOVQ\tAX, ");

selfhost/cmd/wwdump/main.combined.ww

@@ -19519,12 +19519,15 @@ fn cgstructlitfill(c: *cgen, si: *structinfo, lit: *node,
                         fi = nil;
                     } else if (callwhole) {
                         fi = nil;
-                    } else if (isstrtype(c, fi.tnode)) {
+                    } else if (isstrtype(c, fi.tnode) || isslicetype(c, fi.tnode)) {
                         // str IS []u8: 3-word field (ptr,len,cap).
                         // cgexpr leaves AX/BX/CX; for non-BP modes
                         // the dst base goes in DX to dodge BX=len /
                         // CX=cap (the generic store reloads BX, which
-                        // would clobber len) (#1/Phase 3).
+                        // would clobber len) (#1/Phase 3). A slice is
+                        // the same 24B {ptr,len,cap} shape, so it rides
+                        // this arm; without it the generic scalar tail
+                        // stored only the ptr word (#24).
                         cgexpr(c, fieldnode.lhs);
                         if (mode == 0) {
                             emitline("\tMOVQ\tAX, ");

test/wcc/689_structlit_slice_field.c

@@ -0,0 +1,348 @@
+/*
+ * 689_structlit_slice_field — cstage and wwstage agree, byte-for-byte
+ * and at runtime, that a struct-LITERAL initializer `cl{ items = b, n =
+ * .. }` with a SLICE field copies the FULL 24B { ptr, len, cap } header
+ * of that field (task #24).
+ *
+ * The bug: cg_structlit_fill / cgstructlitfill had a TY_STR arm that
+ * stored all three header words (ptr@+0, len@+8, cap@+16), but NO
+ * TY_SLICE arm — a slice field fell through to the generic scalar tail
+ * which stored only the ptr word, so the stored .len and .cap read 0.
+ * str fields (same 24B shape) worked; slice fields silently dropped
+ * two words. Both stages emitted IDENTICAL wrong asm (the 990-997
+ * byte-id gate was green on both-wrong), so RUNTIME readback is the
+ * correctness net. Same is_str/is_slice discrimination gap as #10
+ * part-b, here in the struct-literal field-init path.
+ *
+ * The fix (BOTH stages, converged byte-identical): the str arm's guard
+ * widens to `TY_STR || TY_SLICE` (cstage) / `isstrtype || isslicetype`
+ * (wwstage) — a slice rides the existing, already-correct 3-word store.
+ * The TAGGED arm stays ordered BEFORE it, so a nullable/tagged slice
+ * (TY_TAGGED) still routes to the widener, not the 3-word store.
+ *
+ * Mutation-sanity: the .len and .cap rows fail if the store drops
+ * either word (the pre-fix 1-word slice store), and the scalar-field
+ * rows (y.n, y.head) pin that the field beside/before the slice is
+ * untouched. The str_* rows are a regression pin on the untouched str
+ * arm.
+ *
+ *   row            | shape                                    | want
+ *   ---------------+------------------------------------------+------
+ *   slice_len      | cl{items=b,n=9}; b.len=5; y.items.len     | 5
+ *   slice_cap      | cl{items=b,n=9}; b.cap=8; y.items.cap     | 8
+ *   slice_n        | cl{items=b,n=9}; y.n (scalar beside slice)| 9
+ *   slice_ptr      | hb[0]=4; cl{items=b,..}; y.items[0]       | 4
+ *   off_len        | struct{head:i64,s:[]u8}; s @foff 8;       | 7
+ *                  | s.len=7; y.s.len (pins disp+foff+8 hdr)   |
+ *   off_cap        | same; s.cap=8; y.s.cap (disp+foff+16)     | 8
+ *   off_head       | same; head=1; y.head (scalar before slice)| 1
+ *   two_b_len      | struct{a:[]u8,b:[]u8}; b @foff 24;        | 6
+ *                  | b.len=6; y.b.len (2nd slice, non-0 foff)  |
+ *   two_b_cap      | same; b.cap=8; y.b.cap                    | 8
+ *   str_name_len   | struct{s:[]u8,name:str}; name="hello";    | 5
+ *                  | y.name.len (str arm regression pin)       |
+ *   str_s_len      | same; s.len=3; y.s.len (slice beside str) | 3
+ *
+ * The asm-byte-id rows pin the symmetric fix (cstage == wwstage); a
+ * non-empty diff means one stage missed the new slice arm.
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+
+static int
+runwait(const char *cmd)
+{
+	int rc = system(cmd);
+	if (rc == -1) return -1;
+	if (WIFEXITED(rc)) return WEXITSTATUS(rc);
+	return -1;
+}
+
+struct row { const char *label; const char *src; int want; };
+
+static const struct row rows[] = {
+	{ "slice_len",
+	  "package main;\n"
+	  "type cl = struct { items: []u8, n: i64 };\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet hb: [8]u8;\n"
+	  "\tlet b: []u8; b.ptr = &hb[0]; b.len = 5; b.cap = 8;\n"
+	  "\tlet y: cl = cl { items = b, n = 9i64 };\n"
+	  "\treturn y.items.len: i32;\n"
+	  "};\n",
+	  5 },
+
+	{ "slice_cap",
+	  "package main;\n"
+	  "type cl = struct { items: []u8, n: i64 };\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet hb: [8]u8;\n"
+	  "\tlet b: []u8; b.ptr = &hb[0]; b.len = 5; b.cap = 8;\n"
+	  "\tlet y: cl = cl { items = b, n = 9i64 };\n"
+	  "\treturn y.items.cap: i32;\n"
+	  "};\n",
+	  8 },
+
+	{ "slice_n",
+	  "package main;\n"
+	  "type cl = struct { items: []u8, n: i64 };\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet hb: [8]u8;\n"
+	  "\tlet b: []u8; b.ptr = &hb[0]; b.len = 5; b.cap = 8;\n"
+	  "\tlet y: cl = cl { items = b, n = 9i64 };\n"
+	  "\treturn y.n: i32;\n"
+	  "};\n",
+	  9 },
+
+	{ "slice_ptr",
+	  "package main;\n"
+	  "type cl = struct { items: []u8, n: i64 };\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet hb: [8]u8; hb[0] = 4u8;\n"
+	  "\tlet b: []u8; b.ptr = &hb[0]; b.len = 5; b.cap = 8;\n"
+	  "\tlet y: cl = cl { items = b, n = 9i64 };\n"
+	  "\treturn y.items[0]: i32;\n"
+	  "};\n",
+	  4 },
+
+	/* slice field at a NON-ZERO field offset (head: i64 @0, s @8).
+	 * Pins disp+foff+8/+16 in the stored header. */
+	{ "off_len",
+	  "package main;\n"
+	  "type hs = struct { head: i64, s: []u8 };\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet hb: [8]u8;\n"
+	  "\tlet b: []u8; b.ptr = &hb[0]; b.len = 7; b.cap = 8;\n"
+	  "\tlet y: hs = hs { head = 1i64, s = b };\n"
+	  "\treturn y.s.len: i32;\n"
+	  "};\n",
+	  7 },
+
+	{ "off_cap",
+	  "package main;\n"
+	  "type hs = struct { head: i64, s: []u8 };\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet hb: [8]u8;\n"
+	  "\tlet b: []u8; b.ptr = &hb[0]; b.len = 7; b.cap = 8;\n"
+	  "\tlet y: hs = hs { head = 1i64, s = b };\n"
+	  "\treturn y.s.cap: i32;\n"
+	  "};\n",
+	  8 },
+
+	{ "off_head",
+	  "package main;\n"
+	  "type hs = struct { head: i64, s: []u8 };\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet hb: [8]u8;\n"
+	  "\tlet b: []u8; b.ptr = &hb[0]; b.len = 7; b.cap = 8;\n"
+	  "\tlet y: hs = hs { head = 1i64, s = b };\n"
+	  "\treturn y.head: i32;\n"
+	  "};\n",
+	  1 },
+
+	/* two slice fields: a @0, b @24. Pins the 2nd slice's header at a
+	 * non-zero field offset (the 24B stride). */
+	{ "two_b_len",
+	  "package main;\n"
+	  "type tt = struct { a: []u8, b: []u8 };\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet hb: [8]u8;\n"
+	  "\tlet p: []u8; p.ptr = &hb[0]; p.len = 3; p.cap = 8;\n"
+	  "\tlet q: []u8; q.ptr = &hb[0]; q.len = 6; q.cap = 8;\n"
+	  "\tlet y: tt = tt { a = p, b = q };\n"
+	  "\treturn y.b.len: i32;\n"
+	  "};\n",
+	  6 },
+
+	{ "two_b_cap",
+	  "package main;\n"
+	  "type tt = struct { a: []u8, b: []u8 };\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet hb: [8]u8;\n"
+	  "\tlet p: []u8; p.ptr = &hb[0]; p.len = 3; p.cap = 8;\n"
+	  "\tlet q: []u8; q.ptr = &hb[0]; q.len = 6; q.cap = 8;\n"
+	  "\tlet y: tt = tt { a = p, b = q };\n"
+	  "\treturn y.b.cap: i32;\n"
+	  "};\n",
+	  8 },
+
+	/* str field beside a slice field — the untouched str arm must still
+	 * store its full header (regression pin). */
+	{ "str_name_len",
+	  "package main;\n"
+	  "type sn = struct { s: []u8, name: str };\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet hb: [8]u8;\n"
+	  "\tlet b: []u8; b.ptr = &hb[0]; b.len = 3; b.cap = 8;\n"
+	  "\tlet y: sn = sn { s = b, name = \"hello\" };\n"
+	  "\treturn y.name.len: i32;\n"
+	  "};\n",
+	  5 },
+
+	{ "str_s_len",
+	  "package main;\n"
+	  "type sn = struct { s: []u8, name: str };\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet hb: [8]u8;\n"
+	  "\tlet b: []u8; b.ptr = &hb[0]; b.len = 3; b.cap = 8;\n"
+	  "\tlet y: sn = sn { s = b, name = \"hello\" };\n"
+	  "\treturn y.s.len: i32;\n"
+	  "};\n",
+	  3 },
+};
+
+static int
+run_driver(const char *driver, const struct row *r, int i)
+{
+	char src[64], tmpdir[64], cmd[1024];
+	snprintf(src, sizeof src, "/tmp/slf_%d_%d.ww", getpid(), i);
+	snprintf(tmpdir, sizeof tmpdir, "/tmp/slf_%d_d_%d", getpid(), i);
+
+	FILE *f = fopen(src, "wb");
+	if (!f) return -1;
+	fputs(r->src, f);
+	fclose(f);
+
+	mkdir(tmpdir, 0755);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
+	    tmpdir, driver, src);
+	if (runwait(cmd) != 0) {
+		fprintf(stderr, "row[%s]: build via %s failed\n",
+		    r->label, driver);
+		unlink(src); rmdir(tmpdir);
+		return -1;
+	}
+
+	const char *base = strrchr(src, '/');
+	base = base ? base + 1 : src;
+	char outbin[128];
+	snprintf(outbin, sizeof outbin, "%s/%s", tmpdir, base);
+	char *dot = strrchr(outbin, '.');
+	if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
+	int got = runwait(outbin);
+
+	unlink(src); unlink(outbin); rmdir(tmpdir);
+	return got;
+}
+
+/* asm_byte_identical — w6c vs w6c_ww .s for the same source must match.
+ * Pre-fix BOTH stages dropped len+cap identically (byte-id green on
+ * both-wrong); the symmetric fix keeps them byte-identical and correct. */
+static int
+asm_byte_identical(const char *bin, const struct row *r, int i)
+{
+	char src[64], cs[64], ws[64], cmd[1024];
+	snprintf(src, sizeof src, "/tmp/slf_asm_%d_%d.ww", getpid(), i);
+	snprintf(cs,  sizeof cs,  "/tmp/slf_asm_%d_%d_c.s", getpid(), i);
+	snprintf(ws,  sizeof ws,  "/tmp/slf_asm_%d_%d_w.s", getpid(), i);
+
+	FILE *f = fopen(src, "wb");
+	if (!f) return -1;
+	fputs(r->src, f);
+	fclose(f);
+
+	snprintf(cmd, sizeof cmd, "%s/w6c -o %s %s 2>/dev/null", bin, cs, src);
+	if (runwait(cmd) != 0) {
+		fprintf(stderr, "row[%s]: w6c errored\n", r->label);
+		unlink(src);
+		return -1;
+	}
+	snprintf(cmd, sizeof cmd, "%s/w6c_ww -o %s %s 2>/dev/null",
+	    bin, ws, src);
+	if (runwait(cmd) != 0) {
+		fprintf(stderr, "row[%s]: w6c_ww errored\n", r->label);
+		unlink(src); unlink(cs);
+		return -1;
+	}
+
+	FILE *fc = fopen(cs, "rb");
+	FILE *fw = fopen(ws, "rb");
+	int rc = 0;
+	if (!fc || !fw) {
+		rc = -1;
+	} else {
+		for (;;) {
+			int a = fgetc(fc);
+			int b = fgetc(fw);
+			if (a != b) { rc = -1; break; }
+			if (a == EOF) break;
+		}
+	}
+	if (fc) fclose(fc);
+	if (fw) fclose(fw);
+	if (rc != 0)
+		fprintf(stderr, "row[%s]: cstage vs wwstage asm differs\n",
+		    r->label);
+	unlink(src); unlink(cs); unlink(ws);
+	return rc;
+}
+
+int
+main(void)
+{
+	const char *bin = getenv("BIN");
+	if (!bin) bin = "out/bin";
+	char absbin[1024];
+	if (bin[0] != '/') {
+		char cwd[1024];
+		if (getcwd(cwd, sizeof cwd) == NULL) return 1;
+		snprintf(absbin, sizeof absbin, "%s/%s", cwd, bin);
+		bin = absbin;
+	}
+
+	char cdrv[1024];
+	snprintf(cdrv, sizeof cdrv, "%s/ww", bin);
+	char wdrv[1024];
+	snprintf(wdrv, sizeof wdrv, "%s/ww_ww", bin);
+
+	struct { const char *name; const char *path; int gated_on_existence; }
+	drivers[] = {
+		{ "cstage",  cdrv, 0 },
+		{ "wwstage", wdrv, 1 },
+		{ NULL, NULL, 0 },
+	};
+
+	int n = (int)(sizeof rows / sizeof rows[0]);
+	int total = 0, fail = 0;
+
+	for (int d = 0; drivers[d].name; d++) {
+		if (drivers[d].gated_on_existence
+		    && access(drivers[d].path, X_OK) != 0) {
+			fprintf(stderr, "structlit_slice_field: skip %s (no %s)\n",
+			    drivers[d].name, drivers[d].path);
+			continue;
+		}
+		for (int i = 0; i < n; i++) {
+			int got = run_driver(drivers[d].path, &rows[i], i);
+			total++;
+			if (got != rows[i].want) {
+				fprintf(stderr,
+				    "structlit_slice_field[%s][%s]: exit=%d want=%d\n",
+				    drivers[d].name, rows[i].label,
+				    got, rows[i].want);
+				fail++;
+			}
+		}
+	}
+
+	if (access(wdrv, X_OK) == 0) {
+		for (int i = 0; i < n; i++) {
+			total++;
+			if (asm_byte_identical(bin, &rows[i], i) != 0)
+				fail++;
+		}
+	}
+
+	if (fail) {
+		fprintf(stderr,
+		    "structlit_slice_field: %d/%d fixtures failed\n",
+		    fail, total);
+		return 1;
+	}
+	printf("structlit_slice_field: %d/%d ok\n", total, total);
+	return 0;
+}