Another “Serializing big strings” warning with Webpack 5 filesystem cache and source maps

[andersk]

• Operating System: Linux
• Node Version: 14.15.1
• NPM Version: 6.14.8
• webpack Version: 5.10.0
• mini-css-extract-plugin Version: 1.3.2

Expected Behavior

No warnings.

Actual Behavior

(This is a sequel to #643, where the case without source maps was fixed.)

When using Webpack 5 with cache: { type: "filesystem" } and css-loader with sourceMap: true, mini-css-extract-plugin triggers this warning on sufficiently large CSS files: <w> [webpack.cache.PackFileCacheStrategy] Serializing big strings (175kiB) impacts deserialization performance (consider using Buffer instead and decode when needed).

Code

// webpack.config.js
const MiniCssExtractPlugin = require("mini-css-extract-plugin");

module.exports = {
  cache: { type: "filesystem" },
  entry: "bootstrap/dist/css/bootstrap.css",
  module: {
    rules: [
      {
        test: /\.css$/,
        use: [
          MiniCssExtractPlugin.loader,
          { loader: "css-loader", options: { sourceMap: true } },
        ],
      },
    ],
  },
  plugins: [new MiniCssExtractPlugin()],
};

How Do We Reproduce?

$ npm i bootstrap css-loader mini-css-extract-plugin webpack webpack-cli
…
+ mini-css-extract-plugin@1.3.2
+ bootstrap@4.5.3
+ css-loader@5.0.1
+ webpack-cli@4.2.0
+ webpack@5.10.0
added 167 packages from 171 contributors and audited 167 packages in 9.398s
…
$ rm -rf .cache node_modules/.cache
$ npx webpack
[webpack-cli] Compilation finished
<w> [webpack.cache.PackFileCacheStrategy] Serializing big strings (175kiB) impacts deserialization performance (consider using Buffer instead and decode when needed)
asset main.css 194 KiB [compared for emit] (name: main)
asset main.js 0 bytes [compared for emit] [minimized] (name: main)
Entrypoint main 194 KiB = main.css 194 KiB main.js 0 bytes
./node_modules/bootstrap/dist/css/bootstrap.css 50 bytes [built] [code generated]
css ./node_modules/css-loader/dist/cjs.js??ruleSet[1].rules[0].use[1]!./node_modules/bootstrap/dist/css/bootstrap.css 194 KiB [code generated]
webpack 5.10.0 compiled successfully in 565 ms