Ported from https://github.com/egoist/rollup-plugin-postcss/pull/226

diff --git a/node_modules/rollup-plugin-postcss/dist/index.js b/node_modules/rollup-plugin-postcss/dist/index.js
index 3d04b2e..01dda37 100644
--- a/node_modules/rollup-plugin-postcss/dist/index.js
+++ b/node_modules/rollup-plugin-postcss/dist/index.js
@@ -789,15 +789,14 @@ var index = ((options = {}) => {
     },
 
     generateBundle(opts, bundle) {
-      return _asyncToGenerator(function* () {
-        if (extracted.size === 0) return; // TODO: support `[hash]`
+      var _this2 = this;
 
+      return _asyncToGenerator(function* () {
+        if (extracted.size === 0) return;
         const dir = opts.dir || path.dirname(opts.file);
         const file = opts.file || path.join(opts.dir, Object.keys(bundle).find(fileName => bundle[fileName].isEntry));
 
         const getExtracted = () => {
-          const fileName = typeof postcssLoaderOptions.extract === 'string' ? normalizePath(path.relative(dir, postcssLoaderOptions.extract)) : `${path.basename(file, path.extname(file))}.css`;
-          const concat = new Concat(true, fileName, '\n');
           const entries = Array.from(extracted.values());
           const modules = bundle[normalizePath(path.relative(dir, file))].modules;
 
@@ -806,6 +805,32 @@ var index = ((options = {}) => {
             entries.sort((a, b) => fileList.indexOf(a.id) - fileList.indexOf(b.id));
           }
 
+          let referenceId;
+
+          if (typeof postcssLoaderOptions.extract === 'string') {
+            referenceId = _this2.emitFile({
+              type: 'asset',
+              source: '',
+              // init
+              fileName: normalizePath(path.relative(dir, postcssLoaderOptions.extract))
+            });
+          } else {
+            const name = `${path.basename(file, path.extname(file))}.css`; // Base hash digest on concatenation of extracted code...
+
+            const source = entries.reduce((acc, {
+              code
+            }) => acc + code, '');
+            referenceId = _this2.emitFile({
+              type: 'asset',
+              source,
+              name
+            });
+          }
+
+          const fileName = _this2.getFileName(referenceId);
+
+          const concat = new Concat(true, fileName, '\n');
+
           for (var _i = 0; _i < entries.length; _i++) {
             const res = entries[_i];
             const relative = normalizePath(path.relative(dir, res.id));
@@ -872,20 +897,14 @@ var index = ((options = {}) => {
           }
         }
 
-        const codeFile = {
-          fileName: codeFileName,
-          isAsset: true,
-          source: code
-        };
-        bundle[codeFile.fileName] = codeFile;
+        bundle[codeFileName].source = code; // update
 
         if (map) {
-          const mapFile = {
-            fileName: mapFileName,
-            isAsset: true,
-            source: map
-          };
-          bundle[mapFile.fileName] = mapFile;
+          _this2.emitFile({
+            type: 'asset',
+            source: map,
+            fileName: mapFileName
+          });
         }
       })();
     }