perf(compiler): return nothing from visitors that only read

src/__tests__/compiler/visitor-returns.test.ts

@@ -0,0 +1,75 @@
+import { compile } from "react-native-css/compiler";
+
+interface VisitorCall {
+  pass: number;
+  visitor: string;
+  returned: unknown;
+}
+
+const calls: VisitorCall[] = [];
+
+jest.mock("../../compiler/lightningcss-loader", () => {
+  const actual = jest.requireActual<
+    typeof import("../../compiler/lightningcss-loader")
+  >("../../compiler/lightningcss-loader");
+
+  return {
+    lightningcssLoader: () => {
+      const loaded = actual.lightningcssLoader();
+      let pass = 0;
+
+      return {
+        ...loaded,
+        lightningcss: (options: Parameters<typeof loaded.lightningcss>[0]) => {
+          pass++;
+          const currentPass = pass;
+          const visitor = Object.fromEntries(
+            Object.entries(options.visitor ?? {}).map(([name, visit]) => [
+              name,
+              typeof visit === "function"
+                ? (...args: unknown[]) => {
+                    const returned: unknown = (
+                      visit as (...visitArgs: unknown[]) => unknown
+                    )(...args);
+                    calls.push({ pass: currentPass, visitor: name, returned });
+                    return returned;
+                  }
+                : visit,
+            ]),
+          );
+
+          return loaded.lightningcss({ ...options, visitor });
+        },
+      };
+    },
+  };
+});
+
+beforeEach(() => {
+  calls.length = 0;
+});
+
+test("the second pass hands lightningcss nothing back", () => {
+  compile(`
+.a { color: var(--color); width: 10px; }
+@media (min-width: 640px) { .b { color: red; } }
+`);
+
+  const secondPass = calls.filter((call) => call.pass === 2);
+
+  expect(secondPass.map((call) => call.visitor)).toContain("StyleSheetExit");
+  expect(secondPass.filter((call) => call.returned !== undefined)).toEqual([]);
+});
+
+test("the first pass hands back only the lengths it converts", () => {
+  compile(`.a { width: 10px; height: 2rem; }`);
+
+  const lengths = calls.filter(
+    (call) => call.pass === 1 && call.visitor === "Length",
+  );
+
+  expect(lengths.map((call) => call.returned)).toEqual([
+    undefined,
+    { unit: "px", value: 28 },
+  ]);
+});

src/compiler/compiler.ts

@@ -135,7 +135,7 @@ export function compile(code: Buffer | string, options: CompilerOptions = {}) {
     const remMultiplier = effectiveRem;
     firstPassVisitor.Length = (length) => {
       if (length.unit !== "rem") {
-        return length;
+        return undefined;
       }
 
       return {
@@ -203,7 +203,6 @@ export function compile(code: Buffer | string, options: CompilerOptions = {}) {
   const visitor: Visitor<typeof customAtRules> = {
     Rule(rule) {
       maybeMutateReactNativeOptions(rule, builder);
-      return rule;
     },
     StyleSheetExit(sheet) {
       if (isLoggerEnabled) {
@@ -220,7 +219,6 @@ export function compile(code: Buffer | string, options: CompilerOptions = {}) {
       }
 
       logger(`Exiting lightningcss`);
-      return sheet;
     },
   };