fix: improve warning of conflict order (#465)

src/index.js

@@ -90,12 +90,7 @@ class CssModule extends webpack.Module {
 }
 
 class CssModuleFactory {
-  create(
-    {
-      dependencies: [dependency],
-    },
-    callback
-  ) {
+  create({ dependencies: [dependency] }, callback) {
     callback(null, new CssModule(dependency));
   }
 }
@@ -416,6 +411,9 @@ class MiniCssExtractPlugin {
     if (typeof chunkGroup.getModuleIndex2 === 'function') {
       // Store dependencies for modules
       const moduleDependencies = new Map(modules.map((m) => [m, new Set()]));
+      const moduleDependenciesReasons = new Map(
+        modules.map((m) => [m, new Map()])
+      );
 
       // Get ordered list of modules per chunk group
       // This loop also gathers dependencies from the ordered lists
@@ -435,9 +433,14 @@ class MiniCssExtractPlugin {
 
         for (let i = 0; i < sortedModules.length; i++) {
           const set = moduleDependencies.get(sortedModules[i]);
+          const reasons = moduleDependenciesReasons.get(sortedModules[i]);
 
           for (let j = i + 1; j < sortedModules.length; j++) {
-            set.add(sortedModules[j]);
+            const module = sortedModules[j];
+            set.add(module);
+            const reason = reasons.get(module) || new Set();
+            reason.add(cg);
+            reasons.set(module, reason);
           }
         }
 
@@ -489,16 +492,32 @@ class MiniCssExtractPlugin {
           // and emit a warning
           const fallbackModule = bestMatch.pop();
           if (!this.options.ignoreOrder) {
+            const reasons = moduleDependenciesReasons.get(fallbackModule);
             compilation.warnings.push(
               new Error(
-                `chunk ${chunk.name || chunk.id} [${pluginName}]\n` +
-                  'Conflicting order between:\n' +
-                  ` * ${fallbackModule.readableIdentifier(
-                    requestShortener
-                  )}\n` +
-                  `${bestMatchDeps
-                    .map((m) => ` * ${m.readableIdentifier(requestShortener)}`)
-                    .join('\n')}`
+                [
+                  `chunk ${chunk.name || chunk.id} [${pluginName}]`,
+                  'Conflicting order. Following module has been added:',
+                  ` * ${fallbackModule.readableIdentifier(requestShortener)}`,
+                  'despite it was not able to fulfill desired ordering with these modules:',
+                  ...bestMatchDeps.map((m) => {
+                    const goodReasonsMap = moduleDependenciesReasons.get(m);
+                    const goodReasons =
+                      goodReasonsMap && goodReasonsMap.get(fallbackModule);
+                    const failedChunkGroups = Array.from(
+                      reasons.get(m),
+                      (cg) => cg.name
+                    ).join(', ');
+                    const goodChunkGroups =
+                      goodReasons &&
+                      Array.from(goodReasons, (cg) => cg.name).join(', ');
+                    return [
+                      ` * ${m.readableIdentifier(requestShortener)}`,
+                      `   - couldn't fulfill desired order of chunk group(s) ${failedChunkGroups}`,
+                      `   - while fulfilling desired order of chunk group(s) ${goodChunkGroups}`,
+                    ].join('\n');
+                  }),
+                ].join('\n')
               )
             );
           }

test/TestCases.test.js

@@ -25,6 +25,13 @@ function compareDirectory(actual, expected) {
   }
 }
 
+function compareWarning(actual, expectedFile) {
+  if (!fs.existsSync(expectedFile)) return;
+
+  const expected = require(expectedFile); // eslint-disable-line global-require,import/no-dynamic-require
+  expect(actual).toBe(expected);
+}
+
 describe('TestCases', () => {
   const casesDirectory = path.resolve(__dirname, 'cases');
   const outputDirectory = path.resolve(__dirname, 'js');
@@ -93,6 +100,13 @@ describe('TestCases', () => {
 
           compareDirectory(outputDirectoryForCase, expectedDirectory);
 
+          const expectedWarning = path.resolve(directoryForCase, 'warnings.js');
+          const actualWarning = stats.toString({
+            all: false,
+            warnings: true,
+          });
+          compareWarning(actualWarning, expectedWarning);
+
           done();
         });
       }, 10000);

test/cases/ignoreOrderFalse/e3.css

@@ -0,0 +1,3 @@
+body {
+  content: 'e3';
+}

test/cases/ignoreOrderFalse/expected/styles.css

@@ -6,3 +6,7 @@ body {
   content: 'e1';
 }
 
+body {
+  content: 'e3';
+}
+

test/cases/ignoreOrderFalse/index3.js

@@ -0,0 +1,2 @@
+import './e2.css';
+import './e3.css';

test/cases/ignoreOrderFalse/warnings.js

@@ -0,0 +1,12 @@
+const cssLoaderPath = require.resolve('css-loader').replace(/\\/g, '/');
+
+module.exports = [
+  '',
+  'WARNING in chunk styles [mini-css-extract-plugin]',
+  'Conflicting order. Following module has been added:',
+  ` * css ${cssLoaderPath}!./e2.css`,
+  'despite it was not able to fulfill desired ordering with these modules:',
+  ` * css ${cssLoaderPath}!./e1.css`,
+  "   - couldn't fulfill desired order of chunk group(s) entry2",
+  '   - while fulfilling desired order of chunk group(s) entry1',
+].join('\n');

test/cases/ignoreOrderFalse/webpack.config.js

@@ -4,6 +4,7 @@ module.exports = {
   entry: {
     entry1: './index.js',
     entry2: './index2.js',
+    entry3: './index3.js',
   },
   module: {
     rules: [