feat: support supressing empty CSS files in rollup plugin

package.json

@@ -27,25 +27,26 @@
     "@tivac/eslint-config": "^2.3.1",
     "browserify": "^16.2.3",
     "cli-tester": "2.0.0",
+    "cssnano": "^4.1.10",
     "dedent": "0.7.0",
     "dentist": "1.0.3",
     "env-cmd": "^8.0.2",
-    "eslint": "^5.15.1",
-    "eslint-plugin-jest": "^22.3.0",
+    "eslint": "^5.15.3",
+    "eslint-plugin-jest": "^22.4.1",
     "factor-bundle": "2.5.0",
     "from2-string": "1.1.0",
-    "jest": "^24.1.0",
-    "jest-cli": "^24.1.0",
+    "jest": "^24.5.0",
+    "jest-cli": "^24.5.0",
     "jest-runner-eslint": "^0.7.3",
     "lerna": "^3.13.1",
     "pegjs": "0.10.0",
     "read-dir-deep": "^4.0.2",
-    "rollup": "^1.5.0",
+    "rollup": "^1.7.0",
     "rollup-plugin-svelte": "^5.0.3",
     "shelljs": "^0.8.3",
     "sugarss": "^2.0.0",
     "svelte": "^2.16.1",
-    "watchify": "3.11.0",
+    "watchify": "^3.11.1",
     "webpack": "^4.29.6"
   },
   "dependencies": {

packages/rollup/rollup.js

@@ -28,6 +28,7 @@ module.exports = (opts) => {
         namedExports : true,
         styleExport  : false,
         verbose      : false,
+        empties      : true,
 
         // Regexp to work around https://github.com/rollup/rollup-pluginutils/issues/39
         include : /\.css$/i,
@@ -214,6 +215,9 @@ module.exports = (opts) => {
             // Track specified name -> output name for writing out metadata later
             const names = new Map();
 
+            // Track chunks that don't actually need to be output
+            const duds = new Set();
+
             for(const node of chunked.overallOrder()) {
                 // Only want to deal with CSS currently
                 if(entries.has(node)) {
@@ -223,8 +227,6 @@ module.exports = (opts) => {
                 const ext = ".css";
                 const name = path.basename(node, path.extname(node));
 
-                const id = this.emitAsset(`${name}${ext}`);
-
                 /* eslint-disable-next-line no-await-in-loop */
                 const result = await processor.output({
                     // Can't use this.getAssetFileName() here, because the source hasn't been set yet
@@ -235,8 +237,14 @@ module.exports = (opts) => {
                     files : graph.getNodeData(node),
                 });
 
-
-                this.setAssetSource(id, result.css);
+                // Don't output empty files if empties is falsey
+                if(!options.empties && !result.css.length) {
+                    duds.add(node);
+
+                    continue;
+                }
+
+                const id = this.emitAsset(`${name}${ext}`, result.css);
 
                 // Save off the final name of this asset for later use
                 const dest = this.getAssetFileName(id);
@@ -290,8 +298,10 @@ module.exports = (opts) => {
 
                 // Attach info about this asset to the bundle
                 const { assets = [] } = chunk;
-
-                chunked.dependenciesOf(entry).forEach((dep) => assets.push(names.get(dep)));
+
+                chunked.dependenciesOf(entry)
+                    .filter((dep) => !duds.has(dep))
+                    .forEach((dep) => assets.push(names.get(dep)));
 
                 chunk.assets = assets;
 

packages/rollup/test/__snapshots__/rollup.test.js.snap

@@ -510,6 +510,8 @@ Array [
 ]
 `;
 
+exports[`/rollup.js should write out empty CSS files by default 1`] = `""`;
+
 exports[`/rollup.js shouldn't disable sourcemap generation 1`] = `
 SourceMap {
   "file": "simple.js",

packages/rollup/test/rollup.test.js

@@ -5,6 +5,7 @@ const { rollup } = require("rollup");
 
 const dedent = require("dedent");
 const shell = require("shelljs");
+const cssnano = require("cssnano");
 
 const read    = require("@modular-css/test-utils/read.js")(__dirname);
 const readdir = require("@modular-css/test-utils/read-dir.js")(__dirname);
@@ -595,6 +596,56 @@ describe("/rollup.js", () => {
         expect(result).toMatchRollupSnapshot();
     });
 
+    it("should write out empty CSS files by default", async () => {
+        const bundle = await rollup({
+            input   : require.resolve("./specimens/empty.js"),
+            plugins : [
+                plugin({
+                    namer,
+                    map,
+
+                    done : [
+                        cssnano(),
+                    ],
+                }),
+            ],
+        });
+
+        await bundle.write({
+            format,
+            assetFileNames,
+            file : prefix(`./output/empty-css/empty.js`),
+        });
+
+        expect(read("./empty-css/assets/empty.css")).toMatchSnapshot();
+    });
+
+    it("should not write out empty CSS files when empties is falsey", async () => {
+        const bundle = await rollup({
+            input   : require.resolve("./specimens/empty.js"),
+            plugins : [
+                plugin({
+                    namer,
+                    map,
+                    empties : false,
+
+                    done : [
+                        cssnano(),
+                    ],
+                }),
+            ],
+        });
+
+        await bundle.write({
+            format,
+            assetFileNames,
+            file : prefix(`./output/no-empty-css/empty.js`),
+        });
+
+        expect(exists("./output/no-empty-css/assets/empty.css")).toBe(false);
+    });
+
+
     describe("case sensitivity tests", () => {
         const fs = require("fs");
         let fn = it;

packages/rollup/test/specimens/empty.js

@@ -0,0 +1,3 @@
+import css from "./empty.css";
+
+console.log(css);

packages/www/src/guide/usage-rollup.md

@@ -86,6 +86,10 @@ Enable `styleExport` will also disable the plugin from emitting any assets as we
 
 Pass an already-instantiated `Processor` instance to the rollup plugin. It will then add any files found when traversing the modules to it and both the rollup-discovered and any already-existing files will be output in the final CSS.
 
+##### `empties`
+
+Set to a falsey value to disable outputting of empty CSS files. This is most common when a file contains only `@value`s and then a minifier is used which strips out the file heading comments. Defaults to `true`.
+
 ##### Shared Options
 
 All other options are passed to the underlying `Processor` instance, see the [Processor Options](#processor-options).