compile().warnings() is unreachable from a real build: no Metro consumer

Start in src/metro/metro-transformer.ts, where the build calls .stylesheet() and ignores the warnings, then look at compile().warnings() in src/compiler/compiler.ts near line 202 and at how src/jest/index.ts consumes them. Pull request #430 is already open against this issue, so check it before starting. The issue leaves the design open (log through Metro's reporter, or gate it behind an option), so confirm the approach with a maintainer first. Done means a real Metro build prints the recorded warnings.

Before submitting a new issue

• I tested using the latest version of the library, as the bug might be already fixed.
• I tested using a supported version of react native.
• I checked for possible duplicate issues, with possible answers.

Bug summary

StylesheetBuilder.addWarning records each declaration the compiler cannot convert, an unsupported property or a value with no React Native equivalent, and compile(css).warnings() returns them. Nothing in a real build reads them. The transformer withReactNativeCSS installs, src/metro/metro-transformer.ts, calls .stylesheet() alone:

const productionJS = compile(css, {
  ...config.reactNativeCSS,
  filename: filePath,
  projectRoot: projectRoot,
}).stylesheet();

and the one other reader, src/jest/index.ts, prints them only when REACT_NATIVE_CSS_TEST_DEBUG is set and NODE_OPTIONS carries --inspect. So a declaration with no React Native equivalent is dropped with nothing printed: the class name is present, the rule compiled, and one declaration did nothing, which is the class of problem a developer cannot diagnose from the outside.

This is how the channel has always worked, so it is not a regression. The shapes I see:

1. metro-transformer.ts reads .warnings() after .stylesheet() and logs a summary through Metro's reporter.
2. The same, off by default behind a reactNativeCSS option, so an existing project does not gain a wall of output.
3. Nothing, if the channel is meant only as a compiler API for tooling, in which case it is worth saying so, because the call sites read as user-facing diagnostics.

#430 does (1) with (2)'s option to silence it.

Library version

main at 2421ea7 (0.0.0-nightly.2421ea7)

Environment info

System:
  OS: Windows 11 10.0.26300
  CPU: "(32) x64 AMD Ryzen 9 9950X3D 16-Core Processor          "
  Memory: 155.26 GB / 255.67 GB
Binaries:
  Node:
    version: 26.12.1
    path: C:\Program Files\nodejs\node.EXE
  Yarn:
    version: 4.9.2
    path: C:\Users\<user>\AppData\Roaming\npm\yarn.CMD
  npm:
    version: 12.2.1
    path: C:\Program Files\nodejs\npm.CMD
  Watchman: Not Found
SDKs:
  Android SDK:
    API Levels:
      - "33"
      - "35"
      - "35"
      - "35"
      - "36"
      - "37"
    Build Tools:
      - 35.0.0
      - 36.0.0
      - 36.1.0
      - 37.0.0
    System Images:
      - android-26 | Google APIs Intel x86_64 Atom
      - android-28 | Google APIs Intel x86_64 Atom
      - android-29 | Google APIs Intel x86_64 Atom
      - android-30 | Google APIs Intel x86_64 Atom
      - android-31 | Google APIs Intel x86_64 Atom
      - android-32 | Google APIs Intel x86_64 Atom
      - android-33 | Google APIs Intel x86_64 Atom
      - android-34 | Google APIs Intel x86_64 Atom
      - android-35 | Google APIs Intel x86_64 Atom
      - android-36.1 | Google Play Intel x86_64 Atom
      - android-36.1 | Pre-Release 16 KB Page Size Google Play Intel x86_64 Atom
      - android-37.0 | Google APIs Intel x86_64 Atom
    Android NDK: Not Found
  Windows SDK: Not Found
IDEs:
  Android Studio: AI-262.9437.185.2621.16467767
  Visual Studio: Not Found
Languages:
  Java:
    version: 17.0.20.1
    path: C:\Program Files\Microsoft\jdk-17.0.20.101-hotspot\bin\javac.EXE
  Ruby: Not Found
npmPackages:
  "@react-native-community/cli": Not Found
  react:
    installed: 19.2.3
    wanted: 19.2.3
  react-native:
    installed: 0.86.3
    wanted: 0.86.3
  react-native-windows: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: Not found
  newArchEnabled: Not found
iOS:
  hermesEnabled: Not found
  newArchEnabled: Not found

Steps to reproduce

1. Clone the reproducer branch, which is main at 2421ea7 with two files added in src/__tests__/metro: the test, and a stand-in for Expo's transform worker that hands a stylesheet back as CSS and anything else as code, so the test runs the transformer with no app around it. Install:

   git clone --branch repro/424-compiler-warnings-unreachable https://github.com/YevheniiKotyrlo/react-native-css
   cd react-native-css
   yarn install
   

2. Run yarn jest src/__tests__/metro/compiler-warnings-reach-the-build.test.ts.

src/__tests__/metro/compiler-warnings-reach-the-build.test.ts:

// A declaration the compiler drops is reported where a developer reads a build: the terminal running Metro
import { join } from "path";

import type { JsTransformOptions } from "metro-transform-worker";

import { compile } from "../../compiler";
import { transform } from "../../metro/metro-transformer";

jest.mock("@expo/metro-config", () => ({
  unstable_transformerPath: require.resolve("./_transform-worker"),
}));

type TransformerConfig = Parameters<typeof transform>[0];

const css = `.a { float: left; }`;

afterEach(() => {
  jest.restoreAllMocks();
});

test("the compiler records the dropped declaration", () => {
  expect(compile(css).warnings()).toStrictEqual({ properties: ["float"] });
});

test("the transformer Metro runs for a native stylesheet reports it", async () => {
  const written: string[] = [];
  for (const method of ["log", "info", "warn", "error"] as const) {
    jest.spyOn(console, method).mockImplementation((...args: unknown[]) => {
      written.push(args.map((argument) => String(argument)).join(" "));
    });
  }

  await transform(
    {} as TransformerConfig,
    process.cwd(),
    join(process.cwd(), "global.css"),
    Buffer.from(css),
    { type: "module", platform: "ios" } as JsTransformOptions,
  );

  expect(written.join("\n")).toContain("float");
});

src/__tests__/metro/_transform-worker.ts:

import type { JsTransformOptions } from "metro-transform-worker";

// Stands in for Expo's transform worker: on web a stylesheet comes back as CSS, and anything else as code
export function transform(
  _config: unknown,
  _projectRoot: string,
  filename: string,
  data: Buffer,
  options: JsTransformOptions,
) {
  return Promise.resolve(
    options.platform === "web" && filename.endsWith(".css")
      ? { output: [{ data: { css: { code: data } } }] }
      : { output: [{ data: { code: data.toString() } }] },
  );
}

Expected: transforming a native stylesheet with .a { float: left; } reports the dropped float where the bundler's output is read, as compile().warnings() records it.

Actual: compile().warnings() returns { properties: ["float"] }, and the transformer writes nothing to the console.

FAIL src/__tests__/metro/compiler-warnings-reach-the-build.test.ts (7.134 s)
  √ the compiler records the dropped declaration (11 ms)
  × the transformer Metro runs for a native stylesheet reports it (2 ms)
  ● the transformer Metro runs for a native stylesheet reports it
    expect(received).toContain(expected) // indexOf
    Expected substring: "float"
    Received string:    ""
      39 |   );
      40 |
    > 41 |   expect(written.join("\n")).toContain("float");
         |                              ^
      42 | });
      43 |
      at Object.toContain (src/__tests__/metro/compiler-warnings-reach-the-build.test.ts:41:30)
      at asyncGeneratorStep (node_modules/@babel/runtime/helpers/asyncToGenerator.js:3:17)
      at _next (node_modules/@babel/runtime/helpers/asyncToGenerator.js:17:9)
Test Suites: 1 failed, 1 total
Tests:       1 failed, 1 passed, 2 total
Snapshots:   0 total
Time:        8.177 s
Ran all test suites matching /src\\__tests__\\metro\\compiler-warnings-reach-the-build.test.ts/i.

Reproducible example repository

https://github.com/YevheniiKotyrlo/react-native-css/tree/repro/424-compiler-warnings-unreachable