w6c+w6c_ww: loud-stop non-ident/non-local append spread source (#34 review)

A spread whose source was not a local ident fell PAST the spread arm:
cstage continued into the single-value stores with the N_SPREAD node
(garbage store), wwstage silently SKIPPED the value entirely — a silent,
cs!=ww-divergent miscompile (append(ys, f()...): cs exit 0 / ww exit 144,
want 3), reachable for every element kind and predating #34 for scalars.
Both stages now rule-7 loud-stop the shape (deferred, task #37).

Also pins the widener's already-tagged single-value source path
(tagged_ident_src row, i64 + bool members) and adds the
spread_call_loudstop BUILD_FAIL row — test 800 is now 15 rows / 43
fixtures.

cmd/w6c/cgen.c

@@ -6357,6 +6357,20 @@ cgexpr(Cg *c, Node *n, Local *locals)
 				fatal("#34: append() element kind "
 				    "unsupported (rule-7)");
 			for (Node *vn = sn->next; vn; vn = vn->next) {
+				/* #34 review: a spread whose source is not a
+				 * local ident used to fall PAST the spread arm
+				 * into the single-value stores with the
+				 * N_SPREAD node (cstage garbage store; wwstage
+				 * silently SKIPPED it — divergent). Deferred
+				 * source shape, task #37. */
+				if (vn->kind == N_SPREAD) {
+					if (!vn->lhs || vn->lhs->kind != N_IDENT)
+						fatal("#34: append() spread source "
+						    "shape unsupported (rule-7)");
+					if (localfind(locals, vn->lhs->str) == 0)
+						fatal("#34: append() spread source "
+						    "ident is not a local (rule-7)");
+				}
 				if (vn->kind == N_SPREAD &&
 				    vn->lhs && vn->lhs->kind == N_IDENT) {
 					int it_off = localfind(locals, vn->lhs->str);

selfhost/cmd/w6c/main.combined.ww

@@ -23942,11 +23942,23 @@ fn cgappend(c: *cgen, n: *node) void = {
 	let vn: *node = sn.next;
 	for (vn != nil) {
 		if (vn.kind == nkind.N_SPREAD) {
+			// #34 review: a non-ident/non-local spread source used
+			// to be silently SKIPPED here while cstage fell past
+			// its spread arm into the single-value stores with the
+			// N_SPREAD node (garbage store) — divergent. Deferred
+			// source shape, task #37.
 			let it: *node = vn.lhs;
-			if (it == nil) { vn = vn.next; continue; };
-			if (it.kind != nkind.N_IDENT) { vn = vn.next; continue; };
+			if (it == nil || it.kind != nkind.N_IDENT) {
+				let m34p: str = "#34: append() spread source shape unsupported (rule-7)\n";
+				os.write(2, m34p.ptr, m34p.len: u64);
+				os.exit(1);
+			};
 			let itlocal: *local = localfindnode(c, it.str);
-			if (itlocal == nil) { vn = vn.next; continue; };
+			if (itlocal == nil) {
+				let m34q: str = "#34: append() spread source ident is not a local (rule-7)\n";
+				os.write(2, m34q.ptr, m34q.len: u64);
+				os.exit(1);
+			};
 			let it_off: i32 = itlocal.off;
 			let load_op: str = tnodeloadop(c, etnode, esz);
 			emitline("\tSUBQ\t$8, SP\n");

selfhost/cmd/wcc/cgenexpr.ww

@@ -4149,11 +4149,23 @@ fn cgappend(c: *cgen, n: *node) void = {
 	let vn: *node = sn.next;
 	for (vn != nil) {
 		if (vn.kind == nkind.N_SPREAD) {
+			// #34 review: a non-ident/non-local spread source used
+			// to be silently SKIPPED here while cstage fell past
+			// its spread arm into the single-value stores with the
+			// N_SPREAD node (garbage store) — divergent. Deferred
+			// source shape, task #37.
 			let it: *node = vn.lhs;
-			if (it == nil) { vn = vn.next; continue; };
-			if (it.kind != nkind.N_IDENT) { vn = vn.next; continue; };
+			if (it == nil || it.kind != nkind.N_IDENT) {
+				let m34p: str = "#34: append() spread source shape unsupported (rule-7)\n";
+				os.write(2, m34p.ptr, m34p.len: u64);
+				os.exit(1);
+			};
 			let itlocal: *local = localfindnode(c, it.str);
-			if (itlocal == nil) { vn = vn.next; continue; };
+			if (itlocal == nil) {
+				let m34q: str = "#34: append() spread source ident is not a local (rule-7)\n";
+				os.write(2, m34q.ptr, m34q.len: u64);
+				os.exit(1);
+			};
 			let it_off: i32 = itlocal.off;
 			let load_op: str = tnodeloadop(c, etnode, esz);
 			emitline("\tSUBQ\t$8, SP\n");

selfhost/cmd/wwdump/main.combined.ww

@@ -23942,11 +23942,23 @@ fn cgappend(c: *cgen, n: *node) void = {
 	let vn: *node = sn.next;
 	for (vn != nil) {
 		if (vn.kind == nkind.N_SPREAD) {
+			// #34 review: a non-ident/non-local spread source used
+			// to be silently SKIPPED here while cstage fell past
+			// its spread arm into the single-value stores with the
+			// N_SPREAD node (garbage store) — divergent. Deferred
+			// source shape, task #37.
 			let it: *node = vn.lhs;
-			if (it == nil) { vn = vn.next; continue; };
-			if (it.kind != nkind.N_IDENT) { vn = vn.next; continue; };
+			if (it == nil || it.kind != nkind.N_IDENT) {
+				let m34p: str = "#34: append() spread source shape unsupported (rule-7)\n";
+				os.write(2, m34p.ptr, m34p.len: u64);
+				os.exit(1);
+			};
 			let itlocal: *local = localfindnode(c, it.str);
-			if (itlocal == nil) { vn = vn.next; continue; };
+			if (itlocal == nil) {
+				let m34q: str = "#34: append() spread source ident is not a local (rule-7)\n";
+				os.write(2, m34q.ptr, m34q.len: u64);
+				os.exit(1);
+			};
 			let it_off: i32 = itlocal.off;
 			let load_op: str = tnodeloadop(c, etnode, esz);
 			emitline("\tSUBQ\t$8, SP\n");

test/wcc/800_append_wide_elem.c

@@ -60,10 +60,18 @@
  *                        | scratch slots where wwstage dedups)  |
  *   spread_str           | append(ys, xs...) of []str + single  | 7
  *   spread_tagged        | append(ys, xs...) of [](i64|bool)    | 42
+ *   tagged_ident_src     | append of ALREADY-TAGGED locals      | 42
+ *                        | (i64 + bool members) — the widener's |
+ *                        | already-tagged source path.          |
  *   struct_call_loudstop | append(xs, f()) struct-from-call is  | BUILD_FAIL
  *                        | the deferred source shape — must     |
  *                        | fail LOUD on both stages (rule-7),   |
  *                        | never silently store one word.       |
+ *   spread_call_loudstop | append(ys, f()...) non-ident spread  | BUILD_FAIL
+ *                        | source — pre-review this fell PAST   |
+ *                        | the spread arm (cstage garbage store |
+ *                        | vs wwstage silent skip, divergent);  |
+ *                        | now a rule-7 loud-stop (task #37).   |
  *
  * Every non-BUILD_FAIL row also asserts cstage/wwstage asm byte-id,
  * which subsumes the frame-size canary (TEXT main,$N) — the @tagbase/
@@ -251,6 +259,28 @@ static const struct row rows[] = {
 	  "};\n",
 	  42 },
 
+	{ "tagged_ident_src",
+	  "package main;\n"
+	  "type cell = (i64 | bool);\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet xs: []cell = [];\n"
+	  "\tlet c: cell = 40i64;\n"
+	  "\tlet b: cell = true;\n"
+	  "\tappend(xs, c);\n"
+	  "\tappend(xs, b);\n"
+	  "\tlet r: i32 = 0;\n"
+	  "\tmatch (xs[0]) {\n"
+	  "\tcase let v: i64 => r += v: i32;\n"
+	  "\tcase bool => r = 99;\n"
+	  "\t};\n"
+	  "\tmatch (xs[1]) {\n"
+	  "\tcase i64 => r = 98;\n"
+	  "\tcase let w: bool => { if (w) { r += 2; }; };\n"
+	  "\t};\n"
+	  "\treturn r;\n"
+	  "};\n",
+	  42 },
+
 	{ "struct_call_loudstop",
 	  "package main;\n"
 	  "type pt = struct { x: i32, y: i32, z: i64 };\n"
@@ -263,6 +293,21 @@ static const struct row rows[] = {
 	  "\treturn 0;\n"
 	  "};\n",
 	  BUILD_FAIL },
+
+	{ "spread_call_loudstop",
+	  "package main;\n"
+	  "fn mks() []str = {\n"
+	  "\tlet xs: []str = [];\n"
+	  "\tlet a: str = \"abc\";\n"
+	  "\tappend(xs, a);\n"
+	  "\treturn xs;\n"
+	  "};\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet ys: []str = [];\n"
+	  "\tappend(ys, mks()...);\n"
+	  "\treturn ys[0].len: i32;\n"
+	  "};\n",
+	  BUILD_FAIL },
 };
 
 static int