test: require diagnostic on reject rows, close crash-vacuity (#20)

Reject helpers checked only rc!=0, so a SEGFAULT (exit 139, or the
driver's "w6c failed" exit 1) counted as a clean reject -- a wwstage
crash could pass vacuously (it did, latently, on bodied bare-... pre
#11). Every reject row now captures stderr and requires the actual
diagnostic substring (rc!=0 AND strstr) across all 16 reject tests; a
crash emits no diagnostic, so it now fails. This is the
differential-reject backstop (#15): both stages must cleanly reject
with the expected message.

Two genuine cstage/wwstage diagnostic-body divergences are documented
inline via per-stage substrings, not papered over (845 tuple parse,
catA_f2 tuple arity); catalogued in #21.

test/wcc/845_tuple_trailing_comma_reject.c

@@ -50,30 +50,59 @@ outbin(char *dst, size_t n, const char *tmpdir, const char *src)
 	if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
 }
 
-/* build_should_fail — returns 0 when the build correctly FAILS. */
+static int
+filehas(const char *path, const char *needle)
+{
+	char buf[8192];
+	FILE *f = fopen(path, "rb");
+	if (!f) return 0;
+	size_t n = fread(buf, 1, sizeof buf - 1, f);
+	fclose(f);
+	buf[n] = '\0';
+	return strstr(buf, needle) != NULL;
+}
+
+/* build_should_fail — returns 0 when the build FAILS *and* the stage's
+ * own parse diagnostic appears; a crash wraps as a nonzero "w6c failed"
+ * with no diagnostic, so the substring distinguishes it (#20). The two
+ * stages' parse messages diverge with no common core (cstage "unexpected
+ * token in expression"/"expected ), got ;" vs wwstage "parse: expected
+ * expression"/"parse: expected ')' in tuple"), so `diag` is passed
+ * per-stage by the caller (a real fidelity gap, filed under #20). */
 static int
 build_should_fail(const char *driver, const char *label, const char *src,
-    int i)
+    const char *diag, int i)
 {
-	char s[64], tmpdir[64], cmd[1024], bin[128];
+	char s[64], tmpdir[64], cmd[1024], bin[128], errf[64];
 	snprintf(s, sizeof s, "/tmp/tuptc_%d_%d.ww", getpid(), i);
 	snprintf(tmpdir, sizeof tmpdir, "/tmp/tuptc_%d_d_%d", getpid(), i);
+	snprintf(errf, sizeof errf, "/tmp/tuptc_%d_e_%d.txt", getpid(), i);
 	FILE *f = fopen(s, "wb");
 	if (!f) return -1;
 	fputs(src, f);
 	fclose(f);
 	mkdir(tmpdir, 0755);
-	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
-	    tmpdir, driver, s);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s >/dev/null 2>%s",
+	    tmpdir, driver, s, errf);
 	int rc = runwait(cmd);
+	int hasmsg = filehas(errf, diag);
 	outbin(bin, sizeof bin, tmpdir, s);
 	unlink(s);
 	unlink(bin);
+	unlink(errf);
 	rmdir(tmpdir);
-	if (rc == 0)
+	if (rc == 0) {
 		fprintf(stderr, "tuptc[%s][%s]: built ok, expected a reject\n",
 		    driver, label);
-	return rc == 0 ? -1 : 0;
+		return -1;
+	}
+	if (!hasmsg) {
+		fprintf(stderr, "tuptc[%s][%s]: nonzero exit but missing "
+		    "diagnostic \"%s\" (a crash, not a clean reject)\n",
+		    driver, label, diag);
+		return -1;
+	}
+	return 0;
 }
 
 /* run_build — build a program that MUST compile, run it, return exit. */
@@ -102,12 +131,18 @@ run_build(const char *driver, const char *label, const char *src, int i)
 	return got;
 }
 
-struct rejrow { const char *label; const char *src; };
+struct rejrow {
+	const char *label;
+	const char *src;
+	const char *cdiag;	/* cstage diagnostic body substring */
+	const char *wdiag;	/* wwstage diagnostic body substring (#20 gap) */
+};
 
 static const struct rejrow reject_rows[] = {
 	{ "one_comma",
 	  "package main;\n"
-	  "fn main() i32 = { let t = (5,); return 0; };\n" },
+	  "fn main() i32 = { let t = (5,); return 0; };\n",
+	  "unexpected token in expression", "expected expression" },
 };
 
 struct okrow { const char *label; const char *src; int want; };
@@ -161,7 +196,8 @@ main(void)
 			total++;
 			if (build_should_fail(drivers[d].path,
 			    reject_rows[i].label, reject_rows[i].src,
-			    d * 100 + i) != 0)
+			    drivers[d].gated ? reject_rows[i].wdiag
+			    : reject_rows[i].cdiag, d * 100 + i) != 0)
 				fail++;
 		}
 		for (int i = 0; i < nok; i++) {