feat: Make this package implementation-agnostic (#573)

Rather than always loading Node Sass, this now requires users to pass in
either Dart Sass or Node Sass as an option to the loader.

Closes #435

README.md

@@ -27,8 +27,14 @@ Looking for the webpack 1 loader? Check out the [archive/webpack-1 branch](https
 npm install sass-loader node-sass webpack --save-dev
 ```
 
-The sass-loader requires [node-sass](https://github.com/sass/node-sass) and [webpack](https://github.com/webpack)
-as [`peerDependency`](https://docs.npmjs.com/files/package.json#peerdependencies). Thus you are able to control the versions accurately.
+The sass-loader requires [webpack](https://github.com/webpack) as a
+[`peerDependency`](https://docs.npmjs.com/files/package.json#peerdependencies)
+and it requires you to install either [Node Sass][] or [Dart Sass][] on your
+own. This allows you to control the versions of all your dependencies, and to
+choose which Sass implementation to use.
+
+[Node Sass]: https://github.com/sass/node-sass
+[Dart Sass]: http://sass-lang.com/dart-sass
 
 <h2 align="center">Examples</h2>
 
@@ -48,14 +54,14 @@ module.exports = {
             use: [
                 "style-loader", // creates style nodes from JS strings
                 "css-loader", // translates CSS into CommonJS
-                "sass-loader" // compiles Sass to CSS
+                "sass-loader" // compiles Sass to CSS, using Node Sass by default
             ]
         }]
     }
 };
 ```
 
-You can also pass options directly to [node-sass](https://github.com/andrew/node-sass) by specifying an `options` property like this:
+You can also pass options directly to [Node Sass][] or [Dart Sass][]:
 
 ```js
 // webpack.config.js
@@ -79,7 +85,54 @@ module.exports = {
 };
 ```
 
-See [node-sass](https://github.com/andrew/node-sass) for all available Sass options.
+See [the Node Sass documentation](https://github.com/sass/node-sass/blob/master/README.md#options) for all available Sass options.
+
+The special `implementation` option determines which implementation of Sass to
+use. It takes either a [Node Sass][] or a [Dart Sass][] module. For example, to
+use Dart Sass, you'd pass:
+
+```js
+// ...
+    {
+        loader: "sass-loader",
+        options: {
+            implementation: require("sass")
+        }
+    }
+// ...
+```
+
+Note that when using Dart Sass, **synchronous compilation is twice as fast as
+asynchronous compilation** by default, due to the overhead of asynchronous
+callbacks. To avoid this overhead, you can use the
+[`fibers`](https://www.npmjs.com/package/fibers) package to call asynchronous
+importers from the synchronous code path. To enable this, pass the `Fiber` class
+to the `fiber` option:
+
+```js
+// webpack.config.js
+const Fiber = require('fibers');
+
+module.exports = {
+	...
+    module: {
+        rules: [{
+            test: /\.scss$/,
+            use: [{
+                loader: "style-loader"
+            }, {
+                loader: "css-loader"
+            }, {
+                loader: "sass-loader",
+                options: {
+                    implementation: require("sass"),
+                    fiber: Fiber
+                }
+            }]
+        }]
+    }
+};
+```
 
 ### In production
 
@@ -116,7 +169,7 @@ module.exports = {
 
 ### Imports
 
-webpack provides an [advanced mechanism to resolve files](https://webpack.js.org/concepts/module-resolution/). The sass-loader uses node-sass' custom importer feature to pass all queries to the webpack resolving engine. Thus you can import your Sass modules from `node_modules`. Just prepend them with a `~` to tell webpack that this is not a relative import:
+webpack provides an [advanced mechanism to resolve files](https://webpack.js.org/concepts/module-resolution/). The sass-loader uses Sass's custom importer feature to pass all queries to the webpack resolving engine. Thus you can import your Sass modules from `node_modules`. Just prepend them with a `~` to tell webpack that this is not a relative import:
 
 ```css
 @import "~bootstrap/dist/css/bootstrap";

lib/loader.js

@@ -6,12 +6,9 @@ const formatSassError = require("./formatSassError");
 const webpackImporter = require("./webpackImporter");
 const normalizeOptions = require("./normalizeOptions");
 const pify = require("pify");
+const semver = require("semver");
 
-// This queue makes sure node-sass leaves one thread available for executing
-// fs tasks when running the custom importer code.
-// This can be removed as soon as node-sass implements a fix for this.
-const threadPoolSize = process.env.UV_THREADPOOL_SIZE || 4;
-let asyncSassJobQueue = null;
+let nodeSassJobQueue = null;
 
 /**
  * The sass-loader makes node-sass available to webpack modules.
@@ -20,19 +17,6 @@ let asyncSassJobQueue = null;
  * @param {string} content
  */
 function sassLoader(content) {
-    if (asyncSassJobQueue === null) {
-        const sass = require("node-sass");
-        const sassVersion = /^(\d+)/.exec(require("node-sass/package.json").version).pop();
-
-        if (Number(sassVersion) < 4) {
-            throw new Error(
-                "The installed version of `node-sass` is not compatible (expected: >= 4, actual: " + sassVersion + ")."
-            );
-        }
-
-        asyncSassJobQueue = async.queue(sass.render, threadPoolSize - 1);
-    }
-
     const callback = this.async();
     const isSync = typeof callback !== "function";
     const self = this;
@@ -59,8 +43,9 @@ function sassLoader(content) {
         return;
     }
 
-    // start the actual rendering
-    asyncSassJobQueue.push(options, (err, result) => {
+    const render = getRenderFuncFromSassImpl(options.implementation || require("node-sass"));
+
+    render(options, (err, result) => {
         if (err) {
             formatSassError(err, this.resourcePath);
             err.file && this.dependency(err.file);
@@ -92,4 +77,48 @@ function sassLoader(content) {
     });
 }
 
+/**
+ * Verifies that the implementation and version of Sass is supported by this loader.
+ *
+ * @param {Object} module
+ * @returns {Function}
+ */
+function getRenderFuncFromSassImpl(module) {
+    const info = module.info;
+    const components = info.split("\t");
+
+    if (components.length < 2) {
+        throw new Error("Unknown Sass implementation \"" + info + "\".");
+    }
+
+    const implementation = components[0];
+    const version = components[1];
+
+    if (!semver.valid(version)) {
+        throw new Error("Invalid Sass version \"" + version + "\".");
+    }
+
+    if (implementation === "dart-sass") {
+        if (!semver.satisfies(version, "^1.3.0")) {
+            throw new Error("Dart Sass version " + version + " is incompatible with ^1.3.0.");
+        }
+        return module.render.bind(module);
+    } else if (implementation === "node-sass") {
+        if (!semver.satisfies(version, "^4.0.0")) {
+            throw new Error("Node Sass version " + version + " is incompatible with ^4.0.0.");
+        }
+        // There is an issue with node-sass when async custom importers are used
+        // See https://github.com/sass/node-sass/issues/857#issuecomment-93594360
+        // We need to use a job queue to make sure that one thread is always available to the UV lib
+        if (nodeSassJobQueue === null) {
+            const threadPoolSize = Number(process.env.UV_THREADPOOL_SIZE || 4);
+
+            nodeSassJobQueue = async.queue(module.render.bind(module), threadPoolSize - 1);
+        }
+
+        return nodeSassJobQueue.push.bind(nodeSassJobQueue);
+    }
+    throw new Error("Unknown Sass implementation \"" + implementation + "\".");
+}
+
 module.exports = sassLoader;

package.json

@@ -30,7 +30,8 @@
     "loader-utils": "^1.0.1",
     "lodash.tail": "^4.1.1",
     "neo-async": "^2.5.0",
-    "pify": "^3.0.0"
+    "pify": "^3.0.0",
+    "semver": "^5.5.0"
   },
   "devDependencies": {
     "bootstrap-sass": "^3.3.5",
@@ -44,6 +45,7 @@
     "node-sass": "^4.5.0",
     "nyc": "^11.0.2",
     "raw-loader": "^0.5.1",
+    "sass": "^1.3.0",
     "should": "^11.2.0",
     "standard-version": "^4.2.0",
     "style-loader": "^0.18.2",