Tree shaking not working with scss modules

[jordanlatimer]

Bug report

Tree shaking is not working with SCSS modules.

I have an application with ComponentA and ComponentB using classA, but not classB.

// ComponentA
import React from "react";
import styles from "./styles.module.scss";

export function ComponentA() {
  return <div className={styles.classA}>ComponentA</div>;
}

// ComponentB
import React from "react";
import styles from "./styles.module.scss";

export function ComponentB() {
  return <div className={styles.classA}>ComponentB</div>;
}

// styles.module.scss
.classA {
  color: green;
}

.classB {
  color: purple;
}

I have sideEffects: true in my package.json and the following webpack configuration:

/* eslint-env node */
const path = require("path");
const HtmlWebpackPlugin = require("html-webpack-plugin");
const MiniCssExtractPlugin = require("mini-css-extract-plugin");

module.exports = {
  mode: "production",
  entry: {
    index: path.resolve(__dirname, "./src/index.tsx"),
  },
  output: {
    path: path.resolve(__dirname, "dist"),
    filename: "[name].bundle.js",
  },
  resolve: {
    extensions: [".ts", ".tsx", ".js", ".jsx", ".scss", ".css"],
  },
  module: {
    rules: [
      {
        test: /\.m?js/u,
        resolve: {
          fullySpecified: false,
        },
      },
      {
        test: /\.tsx?$/u,
        use: "ts-loader",
      },
      {
        test: /\.s[ac]ss$/u,
        use: [
          {
            loader: MiniCssExtractPlugin.loader,
            options: {
              esModule: true,
            },
          },
          {
            loader: "css-loader",
            options: {
              esModule: true,
              modules: {
                namedExport: true,
              },
            },
          },
          "sass-loader",
        ],
      },
      {
        test: /\.css$/u,
        use: [MiniCssExtractPlugin.loader, "css-loader"],
      },
    ],
  },
  plugins: [
    new HtmlWebpackPlugin({
      template: path.resolve(__dirname, "src/index.html"),
    }),
    new MiniCssExtractPlugin(),
  ],
};

Actual Behavior

After building, css for classB exists in bundled CSS:

// dist/index.css
.lbEYKkO5_LPzVhlhmgOW{color:green}.HrcP_c2cJ59oMgw7mdme{color:purple}

Expected Behavior

After building, css for classB would not exist in bundled CSS:

// dist/index.css
.lbEYKkO5_LPzVhlhmgOW{color:green}

How Do We Reproduce?

https://github.com/jordanjlatimer/css-modules-css-loader-tree-shaking

Please paste the results of npx webpack-cli info here, and mention other relevant information

System:
OS: macOS 12.6.8
CPU: (4) x64 Intel(R) Core(TM) i5-5350U CPU @ 1.80GHz
Memory: 285.48 MB / 8.00 GB
Binaries:
Node: 16.6.2 - ~/.nvm/versions/node/v16.6.2/bin/node
npm: 7.20.3 - ~/.nvm/versions/node/v16.6.2/bin/npm
Browsers:
Chrome: 117.0.5938.149
Safari: 16.6
Packages:
css-loader: ^6.8.1 => 6.8.1
html-webpack-plugin: ^5.5.3 => 5.5.3
sass-loader: ^13.3.2 => 13.3.2
ts-loader: ^9.5.0 => 9.5.0
webpack: ^5.88.2 => 5.88.2
webpack-cli: ^5.1.4 => 5.1.4

[alexander-akait]

Sorry, tree shaking for CSS is not supported right now - #506