Reuse lowered units across builds behind SCRIPTC_INCR_LOWERING=1

packages/compiler/src/executable/prepare.ts

@@ -20,6 +20,7 @@ import {
 } from "../backend/target-diagnostics.js";
 import type { LowerResult, LowerStats } from "../frontend/lowering/lowerer.js";
 import type { FrontendFactory } from "../frontend/pipeline.js";
+import type { UnitStore } from "../frontend/lowering/unit-reuse.js";
 import type { CompilationTiming } from "../timing.js";
 import { readProjectConfig } from "../project-config.js";
 import { nodeIslandPolicy } from "../node-islands/policy.js";
@@ -42,7 +43,9 @@ export interface PreparedExecutableModule {
   warnings: ScrDiagnostic[];
 }
 
-/** Release the parser/checker before native object generation begins. */
+/** Release the parser/checker before native object generation begins.
+ * `unitStore` keeps the lowered units of this build for the next one
+ * (unit-reuse.ts). */
 export function prepareExecutableModule(
   entryPath: string,
   opts: CompileRequestOptions,
@@ -52,6 +55,7 @@ export function prepareExecutableModule(
   timing: CompilationTiming = () => {},
   lowerIslands: ReadonlyMap<string, IslandPackage> = new Map(),
   retryFences = true,
+  unitStore: UnitStore | null = null,
 ): PreparedExecutableModule | CompileFailure {
   // The island decision reads the detector's scan unless islands are off.
   const islandScan =
@@ -115,6 +119,7 @@ export function prepareExecutableModule(
           timing,
           grown,
           retryFences,
+          unitStore,
         );
       }
       return fail(fe.preflight);
@@ -127,6 +132,7 @@ export function prepareExecutableModule(
           targetPlatform: buildPlatform,
           ...(ffi !== null ? { ffiImports: ffi.functions } : {}),
           ...(opts.tier !== undefined ? { tierMode: opts.tier } : {}),
+          ...(unitStore !== null ? { unitStore } : {}),
         },
         (module) => validateModule(module).length === 0,
       );
@@ -165,6 +171,7 @@ export function prepareExecutableModule(
           timing,
           grown,
           retryFences,
+          unitStore,
         );
       }
       // A failed program whose compiled packages need the runtime or a
@@ -267,6 +274,7 @@ export function prepareExecutableModule(
         timing,
         islands,
         false,
+        unitStore,
       );
       if (islanded.ok) return islanded;
       firstError ??= islanded.diagnostics[0] ?? null;