cacheIdentifier option as a function?

[YoonjiJang]

BEFORE YOU SUBMIT please read the following:

I'm submitting a feature request

Webpack Version:
5.x

Babel Core Version:
7.18.x

Babel Loader Version:
8.2.x

Please tell us about your environment:
OSX 13.x

Current behavior:
cacheIdentifier is an option that defaults to stringified json with @babel/core version, babel-loader version, and "options" object.

https://github.com/babel/babel-loader/blob/v8.2.5/src/index.js#L189-L193

Expected/desired behavior:
Want to be able to set it to a function whose argument includes values used in the default value.

// webpack.config.js

{
  loader: 'babel-loader',
  options: {
    cacheIdentifier({  options, babelCoreVersion, babelLoaderVersion }) {
      // returns string
      return JSON.stringify({ options, babelCoreVersion, babelLoaderVersion, someOtherKey: 'cache-busting value' });
    },
  },
},

Maybe something like this could work.

// babel-loader/src/index.js

const cacheIdentifierOptions = {
  options,
  "@babel/core": transform.version,
  "@babel/loader": version,
};

let {
  // ...
  cacheIdentifier = JSON.stringify(cacheIdentifierOptions),
  // ...
} = loaderOptions;

if (cacheIdentifier === 'function') {
  cacheIdentifier = cacheIdentifier(cacheIdentifierOptions);
} 

• If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem along with a gist/jsbin of your webpack configuration.

• What is the expected behavior?

• What is the motivation / use case for changing the behavior?
  The default value makes sense for most use cases and includes info that depends on the file being transpiled, such as filename, but I can't access it :/

[JLHwung]

As a workaround, you can create cacheIdentifier in your webpack config and pass it down to the babel-loader.

const babelConfig = fs.readFileSync("path/to/babel/Config");
const { version: babelVersion } = require("@babel/core");
const { version: babelLoaderVersion } = require("@babel/loader");

{
  loader: 'babel-loader',
  options: {
    cacheIdentifier: [babelConfig, babelVersion, babelLoaderVersion, other_variables].join('\0');
  }
}

It should be even faster than the defaults because the config is not serialized.