Store the frontend cache's LLVM placeholder as an empty array (#2051)

packages/compiler/src/native/prepare.test.ts

@@ -176,7 +176,9 @@ test("serialized IR travels as bounded parts through the frontend cache", () =>
   const key = readdirSync(family).find((name) => /^[0-9a-f]{64}$/.test(name))!;
   const payload = JSON.parse(readFileSync(join(family, key), "utf8"));
   expect(payload.schema).toBe("scriptc.native-frontend.v3");
-  expect(payload.input.llvm).toBe("");
+  // The placeholders keep NativeExecutableInput's member types: the native
+  // compiler checks the parsed entry against them when it reads it back.
+  expect(payload.input.llvm).toEqual([]);
   expect(payload.input.ir).toBeNull();
   expect(payload.llvm).toBe(first.llvm.join(""));
   expect(payload.ir).toEqual(first.ir);

packages/compiler/src/native/prepare.ts

@@ -59,7 +59,7 @@ export interface NativeExecutableInput {
 
 /** The two large artifacts follow the input metadata as separate members so
  * an entry can be written as parts instead of one JSON string; the stored
- * input carries empty placeholders (`""`, `null`) for them. */
+ * input carries empty placeholders (`[]`, `null`) for them. */
 interface FrontendCacheEntry {
   schema: "scriptc.native-frontend.v3";
   probes: FrontendInputSnapshot;
@@ -78,7 +78,7 @@ function* frontendCacheParts(
   const metadata = JSON.stringify({
     schema: "scriptc.native-frontend.v3",
     probes,
-    input: { ...input, llvm: "", ir: null },
+    input: { ...input, llvm: [], ir: null },
   });
   yield metadata.slice(0, metadata.length - 1) + ',"llvm":"';
   for (const part of joinedParts(input.llvm))