fix(@schematics/angular): add less dependency in application migration if needed

The application migration schematic will now attempt to detect the usage
of Less stylesheets within a workspace and install the `less` dependency
if required. This will only occur if the migration analysis allows for
the conversion to use the `@angular/build` package instead of the
`@angular-devkit/build-angular` package. The Less usage detection may
not detect Less stylesheets within Node.js packages and currently does
not handle the case where a project only has inline Less stylesheets.
More complex analysis may be added in the future. However, a build time
error message will be presented with instructions to install `less` in
these cases if the stylesheet preprocessor is required and not present.

(cherry picked from commit 3dae3c0)

Author: clydin

packages/schematics/angular/migrations/use-application-builder/migration.ts

@@ -245,17 +245,58 @@ function updateProjects(tree: Tree, context: SchematicContext) {
       rules.push(
         addDependency('@angular/build', latestVersions.DevkitBuildAngular, {
           type: DependencyType.Dev,
+          // Always is set here since removePackageJsonDependency below does not automatically
+          // trigger the package manager execution.
           install: InstallBehavior.Always,
           existing: ExistingBehavior.Replace,
         }),
       );
       removePackageJsonDependency(tree, '@angular-devkit/build-angular');
+
+      // Add less dependency if any projects contain a Less stylesheet file.
+      // This check does not consider Node.js packages due to the performance
+      // cost of searching such a large directory structure. A build time error
+      // will provide instructions to install the package in this case.
+      if (hasLessStylesheets(tree)) {
+        rules.push(
+          addDependency('less', latestVersions['less'], {
+            type: DependencyType.Dev,
+            existing: ExistingBehavior.Skip,
+          }),
+        );
+      }
     }
 
     return chain(rules);
   });
 }
 
+/**
+ * Searches the schematic tree for files that have a `.less` extension.
+ *
+ * @param tree A Schematics tree instance to search
+ * @returns true if Less stylesheet files are found; otherwise, false
+ */
+function hasLessStylesheets(tree: Tree) {
+  const directories = [tree.getDir('/')];
+
+  let current;
+  while ((current = directories.pop())) {
+    for (const path of current.subfiles) {
+      if (path.endsWith('.less')) {
+        return true;
+      }
+    }
+
+    for (const path of current.subdirs) {
+      if (path === 'node_modules' || path.startsWith('.')) {
+        continue;
+      }
+      directories.push(current.dir(path));
+    }
+  }
+}
+
 function* visit(
   directory: DirEntry,
 ): IterableIterator<[fileName: string, contents: string, sass: boolean]> {

packages/schematics/angular/migrations/use-application-builder/migration_spec.ts

@@ -267,4 +267,22 @@ describe(`Migration to use the application builder`, () => {
     const { stylePreprocessorOptions } = app.architect['build'].options;
     expect(stylePreprocessorOptions).toEqual({ includePaths: ['.'] });
   });
+
+  it('should add "less" dependency when converting to "@angular/build" and a ".less" file is present', async () => {
+    tree.create('/project/app/src/styles.less', '');
+
+    const newTree = await schematicRunner.runSchematic(schematicName, {}, tree);
+
+    const { devDependencies } = JSON.parse(newTree.readContent('/package.json'));
+
+    expect(devDependencies['less']).toBeDefined();
+  });
+
+  it('should not add "less" dependency when converting to "@angular/build" and a ".less" file is present', async () => {
+    const newTree = await schematicRunner.runSchematic(schematicName, {}, tree);
+
+    const { devDependencies } = JSON.parse(newTree.readContent('/package.json'));
+
+    expect(devDependencies['less']).toBeUndefined();
+  });
 });

packages/schematics/angular/utility/latest-versions/package.json

@@ -16,6 +16,7 @@
     "karma-jasmine-html-reporter": "~2.1.0",
     "karma-jasmine": "~5.1.0",
     "karma": "~6.4.0",
+    "less": "^4.2.0",
     "ng-packagr": "^18.0.0-next.0",
     "protractor": "~7.0.0",
     "rxjs": "~7.8.0",