Closely imitate postcss-loader's config logic

This will make css-customs-loader behave more like postcss-loader.

Author: silvenon

lib/index.js

@@ -50,9 +50,24 @@ module.exports = async function(content, sourceMap, meta) {
   let postcssPlugins
   let postcssOptions
 
-  if (postcssLoader.options == null) {
+  // imitating postcss-loader behavior
+  // https://github.com/postcss/postcss-loader/blob/master/src/index.js
+
+  if (
+    Object.keys(postcssLoader.options || {}).filter(
+      option => !['ident', 'config', 'sourceMap'].includes(option)
+    ).length > 0
+  ) {
+    const { options, plugins } = await parsePostcssLoaderOptions.call(
+      this,
+      postcssLoader.options
+    )
+    postcssOptions = options
+    postcssPlugins = plugins
+  } else {
     const file = this.resourcePath
     const rc = {
+      path: path.dirname(file),
       ctx: {
         cwd: this.context,
         file: {
@@ -61,8 +76,18 @@ module.exports = async function(content, sourceMap, meta) {
           basename: path.basename(file),
         },
         options: {},
+        webpack: this,
       },
-      path: path.dirname(file),
+    }
+
+    if (postcssLoader.options != null) {
+      const { config = {} } = postcssLoader.options
+      if (config.path != null) {
+        rc.path = path.resolve(config.path)
+      }
+      if (config.ctx != null) {
+        rc.ctx.options = config.ctx
+      }
     }
 
     try {
@@ -72,14 +97,14 @@ module.exports = async function(content, sourceMap, meta) {
     } catch (err) {
       return callback(err)
     }
-  } else {
-    const { options, plugins } = await parsePostcssLoaderOptions(
-      postcssLoader.options
-    )
-    postcssOptions = options
-    postcssPlugins = plugins
   }
 
+  ;['parser', 'syntax', 'stringifier'].forEach(option => {
+    if (typeof postcssOptions[option] === 'string') {
+      postcssOptions[option] = require(option)
+    }
+  })
+
   postcssPlugins = postcssPlugins.map(plugin =>
     plugin.postcssPlugin == null ? plugin() : plugin
   )
@@ -106,6 +131,7 @@ module.exports = async function(content, sourceMap, meta) {
     ]).process(css, {
       ...postcssOptions,
       from: this.resourcePath,
+      to: undefined,
     })
 
     const exportContent = [