feat: allow String value for the "implementation" option

README.md

@@ -597,12 +597,14 @@ module.exports = {
 
 ### `implementation`
 
-Type: `Function`
+Type: `Function | String`
 
 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.**
 
+#### Function
+
 **webpack.config.js**
 
 ```js
@@ -626,6 +628,31 @@ module.exports = {
 };
 ```
 
+#### String
+
+**webpack.config.js**
+
+```js
+module.exports = {
+  module: {
+    rules: [
+      {
+        test: /\.css$/i,
+        use: [
+          { loader: "style-loader" },
+          { loader: "css-loader" },
+          {
+            loader: "postcss-loader",
+            options: { implementation: require.resolve("postcss") },
+          },
+          { loader: "sass-loader" },
+        ],
+      },
+    ],
+  },
+};
+```
+
 ## Examples
 
 ### SugarSS

src/index.js

@@ -1,6 +1,5 @@
 import path from "path";
 
-import postcss from "postcss";
 import { satisfies } from "semver";
 import postcssPackage from "postcss/package.json";
 
@@ -14,6 +13,7 @@ import {
   normalizeSourceMap,
   normalizeSourceMapAfterPostcss,
   findPackageJSONDir,
+  getPostcssImplementation,
 } from "./utils";
 
 let hasExplicitDependencyOnPostCSS = false;
@@ -40,7 +40,17 @@ export default async function loader(content, sourceMap, meta) {
       ? true
       : options.postcssOptions.config;
 
-  const postcssFactory = options.implementation || postcss;
+  const postcssFactory = getPostcssImplementation(this, options.implementation);
+
+  if (!postcssFactory) {
+    callback(
+      new Error(
+        `The Postcss implementation "${options.implementation}" not found`
+      )
+    );
+
+    return;
+  }
 
   let loadedConfig;
 

src/options.json

@@ -39,7 +39,14 @@
     },
     "implementation": {
       "description": "The implementation of postcss to use, instead of the locally installed version (https://github.com/postcss/postcss-loader#implementation)",
-      "instanceof": "Function"
+      "anyOf": [
+        {
+          "type": "string"
+        },
+        {
+          "instanceof": "Function"
+        }
+      ]
     }
   },
   "additionalProperties": false

src/utils.js

@@ -433,11 +433,33 @@ function findPackageJSONDir(cwd, statSync) {
   return dir;
 }
 
+function getPostcssImplementation(loaderContext, implementation) {
+  let resolvedImplementation = implementation;
+
+  if (!implementation || typeof implementation === "string") {
+    const postcssImplPkg = implementation || "postcss";
+
+    try {
+      // eslint-disable-next-line import/no-dynamic-require, global-require
+      resolvedImplementation = require(postcssImplPkg);
+    } catch (error) {
+      loaderContext.emitError(error);
+
+      // eslint-disable-next-line consistent-return
+      return;
+    }
+  }
+
+  // eslint-disable-next-line consistent-return
+  return resolvedImplementation;
+}
+
 export {
   loadConfig,
   getPostcssOptions,
   exec,
   normalizeSourceMap,
   normalizeSourceMapAfterPostcss,
   findPackageJSONDir,
+  getPostcssImplementation,
 };

test/__snapshots__/implementation.test.js.snap

@@ -1,5 +1,17 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`"implementation" option should throw error when unresolved package: errors 1`] = `
+Array [
+  "ModuleBuildError: Module build failed (from \`replaced original path\`):
+Error: The Postcss implementation \\"unresolved\\" not found
+    at Object.loader (/src/index.js:47:7)",
+  "ModuleError: Module Error (from \`replaced original path\`):
+(Emitted value instead of an instance of Error) Error: Cannot find module 'unresolved' from 'src/utils.js'",
+]
+`;
+
+exports[`"implementation" option should throw error when unresolved package: warnings 1`] = `Array []`;
+
 exports[`"implementation" option should work with a custom instance of PostCSS: css 1`] = `
 "a {
   color: black;
@@ -50,3 +62,54 @@ a {
 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 []`;
+
+exports[`"implementation" option should work with implementation is string: 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 implementation is string: errors 1`] = `Array []`;
+
+exports[`"implementation" option should work with implementation is string: warnings 1`] = `Array []`;

test/__snapshots__/loader.test.js.snap

@@ -5,7 +5,7 @@ Array [
   "ModuleBuildError: Module build failed (from \`replaced original path\`):
 Error: Something went wrong.
     at Processor.process (/test/loader.test.js:216:26)
-    at Object.loader (/src/index.js:106:30)",
+    at Object.loader (/src/index.js:116:30)",
 ]
 `;
 
@@ -21,7 +21,7 @@ Array [
   "ModuleBuildError: Module build failed (from \`replaced original path\`):
 Error: Something went wrong.
     at Processor.process (/test/loader.test.js:300:26)
-    at Object.loader (/src/index.js:106:30)",
+    at Object.loader (/src/index.js:116:30)",
 ]
 `;
 
@@ -32,7 +32,7 @@ Array [
   "ModuleBuildError: Module build failed (from \`replaced original path\`):
 Error: Something went wrong.
     at Processor.process (/test/loader.test.js:245:26)
-    at Object.loader (/src/index.js:106:30)",
+    at Object.loader (/src/index.js:116:30)",
 ]
 `;
 
@@ -43,7 +43,7 @@ Array [
   "ModuleBuildError: Module build failed (from \`replaced original path\`):
 Error: Something went wrong.
     at Processor.process (/test/loader.test.js:274:26)
-    at Object.loader (/src/index.js:106:30)",
+    at Object.loader (/src/index.js:116:30)",
 ]
 `;
 

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

@@ -38,32 +38,42 @@ exports[`validate options should throw an error on the "execute" option with "te
 
 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)"
+ - options.implementation should be one of these:
+   string | function
+   -> The implementation of postcss to use, instead of the locally installed version (https://github.com/postcss/postcss-loader#implementation)
+   Details:
+    * options.implementation should be a string.
+    * options.implementation should be an instance of function."
 `;
 
 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)"
+ - options.implementation should be one of these:
+   string | function
+   -> The implementation of postcss to use, instead of the locally installed version (https://github.com/postcss/postcss-loader#implementation)
+   Details:
+    * options.implementation should be a string.
+    * options.implementation should be an instance of function."
 `;
 
 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)"
+ - options.implementation should be one of these:
+   string | function
+   -> The implementation of postcss to use, instead of the locally installed version (https://github.com/postcss/postcss-loader#implementation)
+   Details:
+    * options.implementation should be a string.
+    * options.implementation should be an instance of function."
 `;
 
 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)"
+ - options.implementation should be one of these:
+   string | function
+   -> The implementation of postcss to use, instead of the locally installed version (https://github.com/postcss/postcss-loader#implementation)
+   Details:
+    * options.implementation should be a string.
+    * options.implementation should be an instance of function."
 `;
 
 exports[`validate options should throw an error on the "postcssOptions" option with "{"config":[]}" value 1`] = `

test/implementation.test.js

@@ -9,6 +9,28 @@ import {
 } from "./helpers";
 
 describe('"implementation" option', () => {
+  it("should work with implementation is string", async () => {
+    const compiler = getCompiler("./css/index.js", {
+      implementation: require.resolve("postcss"),
+    });
+    const stats = await compile(compiler);
+    const codeFromBundle = getCodeFromBundle("style.css", stats);
+
+    expect(codeFromBundle.css).toMatchSnapshot("css");
+    expect(getWarnings(stats)).toMatchSnapshot("warnings");
+    expect(getErrors(stats)).toMatchSnapshot("errors");
+  });
+
+  it("should throw error when unresolved package", async () => {
+    const compiler = getCompiler("./css/index.js", {
+      implementation: "unresolved",
+    });
+    const stats = await compile(compiler);
+
+    expect(getWarnings(stats)).toMatchSnapshot("warnings");
+    expect(getErrors(stats)).toMatchSnapshot("errors");
+  });
+
   it("should work with a custom instance of PostCSS", async () => {
     const spy = jest.fn(postcss);
     const compiler = getCompiler("./css/index.js", {

test/validate-options.test.js

@@ -51,8 +51,8 @@ describe("validate options", () => {
       failure: [1, /test/, [], {}, "something"],
     },
     implementation: {
-      success: [require("postcss")],
-      failure: [1, /test/, [], {}, "something"],
+      success: [require("postcss"), "postcss"],
+      failure: [1, /test/, [], {}],
     },
   };