fix(material/schematics): don't drop prebuilt imports in theming API migration

src/material/schematics/ng-update/migrations/theming-api-v12/migration.ts

@@ -41,15 +41,17 @@ interface ExtraSymbols {
  *   matched, the prefix would be `~@angular/cdk/`.
  * @param newMaterialImportPath New import to the Material theming API (e.g. `~@angular/material`).
  * @param newCdkImportPath New import to the CDK Sass APIs (e.g. `~@angular/cdk`).
+ * @param excludedImports Pattern that can be used to exclude imports from being processed.
  */
 export function migrateFileContent(content: string,
                                    oldMaterialPrefix: string,
                                    oldCdkPrefix: string,
                                    newMaterialImportPath: string,
                                    newCdkImportPath: string,
-                                   extraMaterialSymbols: ExtraSymbols = {}): string {
-  const materialResults = detectImports(content, oldMaterialPrefix);
-  const cdkResults = detectImports(content, oldCdkPrefix);
+                                   extraMaterialSymbols: ExtraSymbols = {},
+                                   excludedImports?: RegExp): string {
+  const materialResults = detectImports(content, oldMaterialPrefix, excludedImports);
+  const cdkResults = detectImports(content, oldCdkPrefix, excludedImports);
 
   // Try to migrate the symbols even if there are no imports. This is used
   // to cover the case where the Components symbols were used transitively.
@@ -77,8 +79,10 @@ export function migrateFileContent(content: string,
  * Counts the number of imports with a specific prefix and extracts their namespaces.
  * @param content File content in which to look for imports.
  * @param prefix Prefix that the imports should start with.
+ * @param excludedImports Pattern that can be used to exclude imports from being processed.
  */
-function detectImports(content: string, prefix: string): DetectImportResult {
+function detectImports(content: string, prefix: string,
+                       excludedImports?: RegExp): DetectImportResult {
   if (prefix[prefix.length - 1] !== '/') {
     // Some of the logic further down makes assumptions about the import depth.
     throw Error(`Prefix "${prefix}" has to end in a slash.`);
@@ -95,6 +99,10 @@ function detectImports(content: string, prefix: string): DetectImportResult {
   while (match = pattern.exec(content)) {
     const [fullImport, type] = match;
 
+    if (excludedImports?.test(fullImport)) {
+      continue;
+    }
+
     if (type === 'use') {
       const namespace = extractNamespaceFromUseStatement(fullImport);
 

src/material/schematics/ng-update/migrations/theming-api-v12/theming-api-migration.ts

@@ -22,7 +22,8 @@ export class ThemingApiMigration extends DevkitMigration<null> {
     if (extname(stylesheet.filePath) === '.scss') {
       const content = stylesheet.content;
       const migratedContent = content ? migrateFileContent(content,
-        '~@angular/material/', '~@angular/cdk/', '~@angular/material', '~@angular/cdk') : content;
+        '~@angular/material/', '~@angular/cdk/', '~@angular/material', '~@angular/cdk',
+        undefined, /material\/prebuilt-themes|cdk\/.*-prebuilt/) : content;
 
       if (migratedContent && migratedContent !== content) {
         this.fileSystem.edit(stylesheet.filePath)

src/material/schematics/ng-update/test-cases/v12/misc/theming-api-v12.spec.ts

@@ -716,4 +716,22 @@ describe('v12 theming API migration', () => {
       `@include mat.mdc-button-theme();`,
     ].join('\n'));
   });
+
+  it('should not drop imports of prebuilt styles', async () => {
+    writeLines(THEME_PATH, [
+      `@import '~@angular/material/prebuilt-themes/indigo-pink.css';`,
+      `@import '~@angular/material/theming';`,
+      `@import '~@angular/cdk/overlay-prebuilt.css';`,
+      `@include mat-core();`,
+    ]);
+
+    await runMigration();
+
+    expect(splitFile(THEME_PATH)).toEqual([
+      `@use '~@angular/material' as mat;`,
+      `@import '~@angular/material/prebuilt-themes/indigo-pink.css';`,
+      `@import '~@angular/cdk/overlay-prebuilt.css';`,
+      `@include mat.core();`,
+    ]);
+  });
 });