Allow tooling authors to provide custom postcss factory through options

README.md

@@ -593,6 +593,37 @@ module.exports = {
 };
 ```
 
+### `implementation`
+
+Type: `Function`
+
+The special `implementation` option determines which implementation of PostCSS to use. Overrides the locally installed `peerDependency` version of `postcss`.
+
+**This option is only really useful for downstream tooling authors to ease the PostCSS 7-to-8 transition.**
+
+**webpack.config.js**
+
+```js
+module.exports = {
+  module: {
+    rules: [
+      {
+        test: /\.css$/i,
+        use: [
+          { loader: "style-loader" },
+          { loader: "css-loader" },
+          {
+            loader: "postcss-loader",
+            options: { implementation: require("postcss") },
+          },
+          { loader: "sass-loader" },
+        ],
+      },
+    ],
+  },
+};
+```
+
 ## Examples
 
 ### SugarSS

src/index.js

@@ -46,6 +46,8 @@ export default async function loader(content, sourceMap, meta) {
       ? true
       : options.postcssOptions.config;
 
+  const postcssFactory = options.implementation || postcss;
+
   let loadedConfig;
 
   if (configOption) {
@@ -105,7 +107,10 @@ export default async function loader(content, sourceMap, meta) {
   let result;
 
   try {
-    result = await postcss(plugins).process(root || content, processOptions);
+    result = await postcssFactory(plugins).process(
+      root || content,
+      processOptions
+    );
   } catch (error) {
     if (error.file) {
       this.addDependency(error.file);

src/options.json

@@ -35,6 +35,10 @@
     "sourceMap": {
       "description": "Enables/Disables generation of source maps (https://github.com/postcss/postcss-loader#sourcemap)",
       "type": "boolean"
+    },
+    "implementation": {
+      "description": "The implementation of postcss to use, instead of the locally installed version (https://github.com/postcss/postcss-loader#implementation)",
+      "instanceof": "Function"
     }
   },
   "additionalProperties": false

test/__snapshots__/implementation.test.js.snap

@@ -0,0 +1,52 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`"implementation" option should work with a custom instance of PostCSS: css 1`] = `
+"a {
+  color: black;
+}
+
+a {
+  color: red;
+}
+
+a {
+  color: green;
+}
+
+a {
+  color: blue;
+}
+
+.class {
+  -x-border-color: blue blue *;
+  -x-color: * #fafafa;
+}
+
+.class-foo {
+  -z-border-color: blue blue *;
+  -z-color: * #fafafa;
+}
+
+.phone {
+  &_title {
+    width: 500px;
+
+    @media (max-width: 500px) {
+      width: auto;
+    }
+
+    body.is_dark & {
+      color: white;
+    }
+  }
+
+  img {
+    display: block;
+  }
+}
+"
+`;
+
+exports[`"implementation" option should work with a custom instance of PostCSS: errors 1`] = `Array []`;
+
+exports[`"implementation" option should work with a custom instance of PostCSS: warnings 1`] = `Array []`;

test/__snapshots__/validate-options.test.js.snap

@@ -36,6 +36,36 @@ exports[`validate options should throw an error on the "execute" option with "te
    -> Enables/Disables PostCSS parser support in 'CSS-in-JS' (https://github.com/postcss/postcss-loader#execute)"
 `;
 
+exports[`validate options should throw an error on the "implementation" option with "/test/" value 1`] = `
+"Invalid options object. PostCSS Loader has been initialized using an options object that does not match the API schema.
+ - options.implementation should be an instance of function.
+   -> The implementation of postcss to use, instead of the locally installed version (https://github.com/postcss/postcss-loader#implementation)"
+`;
+
+exports[`validate options should throw an error on the "implementation" option with "[]" value 1`] = `
+"Invalid options object. PostCSS Loader has been initialized using an options object that does not match the API schema.
+ - options.implementation should be an instance of function.
+   -> The implementation of postcss to use, instead of the locally installed version (https://github.com/postcss/postcss-loader#implementation)"
+`;
+
+exports[`validate options should throw an error on the "implementation" option with "{}" value 1`] = `
+"Invalid options object. PostCSS Loader has been initialized using an options object that does not match the API schema.
+ - options.implementation should be an instance of function.
+   -> The implementation of postcss to use, instead of the locally installed version (https://github.com/postcss/postcss-loader#implementation)"
+`;
+
+exports[`validate options should throw an error on the "implementation" option with "1" value 1`] = `
+"Invalid options object. PostCSS Loader has been initialized using an options object that does not match the API schema.
+ - options.implementation should be an instance of function.
+   -> The implementation of postcss to use, instead of the locally installed version (https://github.com/postcss/postcss-loader#implementation)"
+`;
+
+exports[`validate options should throw an error on the "implementation" option with "something" value 1`] = `
+"Invalid options object. PostCSS Loader has been initialized using an options object that does not match the API schema.
+ - options.implementation should be an instance of function.
+   -> The implementation of postcss to use, instead of the locally installed version (https://github.com/postcss/postcss-loader#implementation)"
+`;
+
 exports[`validate options should throw an error on the "postcssOptions" option with "{"config":[]}" value 1`] = `
 "Invalid options object. PostCSS Loader has been initialized using an options object that does not match the API schema.
  - options.postcssOptions should be one of these:

test/implementation.test.js

@@ -0,0 +1,26 @@
+import postcss from "postcss";
+
+import {
+  compile,
+  getCompiler,
+  getErrors,
+  getCodeFromBundle,
+  getWarnings,
+} from "./helpers";
+
+describe('"implementation" option', () => {
+  it("should work with a custom instance of PostCSS", async () => {
+    const spy = jest.fn(postcss);
+    const compiler = getCompiler("./css/index.js", {
+      // Wrap the spy so it is an instanceof Function
+      implementation: (...args) => spy(...args),
+    });
+    const stats = await compile(compiler);
+    const codeFromBundle = getCodeFromBundle("style.css", stats);
+
+    expect(spy).toHaveBeenCalledTimes(1);
+    expect(codeFromBundle.css).toMatchSnapshot("css");
+    expect(getWarnings(stats)).toMatchSnapshot("warnings");
+    expect(getErrors(stats)).toMatchSnapshot("errors");
+  });
+});

test/validate-options.test.js

@@ -50,6 +50,10 @@ describe("validate options", () => {
       success: [true, false],
       failure: [1, /test/, [], {}, "something"],
     },
+    implementation: {
+      success: [require("postcss")],
+      failure: [1, /test/, [], {}, "something"],
+    },
   };
 
   function stringifyValue(value) {