fix: restore loader object in postcss config context

lib/index.js

@@ -90,6 +90,8 @@ module.exports = function loader (css, map, meta) {
       }
     }
 
+    rc.ctx.webpack = this;
+
     return postcssrc(rc.ctx, rc.path, { argv: false })
   }).then((config) => {
     if (!config) config = {}

test/fixtures/config/context/plugin.js

@@ -0,0 +1,15 @@
+'use strict'
+
+const postcss = require('postcss')
+
+// This plugin creates asset file in webpack compilation
+module.exports = postcss.plugin('plugin', (ctx) => {
+  ctx.webpack._compilation.assets['asset.txt'] = {
+    source() {
+      return '123';
+    },
+    size() {
+      return 0;
+    }
+  }
+})

test/fixtures/config/context/postcss.config.js

@@ -0,0 +1,5 @@
+module.exports = (ctx) => ({
+  plugins: [
+    require('./plugin')(ctx)
+  ]
+})

test/options/config.test.js

@@ -75,4 +75,24 @@ describe('Options', () => {
         expect(src).toMatchSnapshot()
       })
   })
+
+  test('Pass loader object to config context', () => {
+    const config = {
+      loader: {
+        options: {
+          config: {
+            path: 'test/fixtures/config/context/postcss.config.js'
+          }
+        }
+      }
+    }
+
+    return webpack('css/index.js', config).then((stats) => {
+      const assets = stats.compilation.assets;
+      const expectedAssetName = 'asset.txt';
+
+      expect(expectedAssetName in assets).toBeTruthy();
+      expect(assets[expectedAssetName].source()).toBe('123');
+    })
+  })
 })