scripts/api-extractor: warn when introducing new warnings

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

scripts/api-extractor.ts

@@ -281,6 +281,35 @@ async function createTemporaryTsConfig(includedPackageDirs: string[]) {
   return path;
 }
 
+async function countApiReportWarnings(projectFolder: string) {
+  const path = resolvePath(projectFolder, 'api-report.md');
+  try {
+    const content = await fs.readFile(path, 'utf8');
+    const lines = content.split('\n');
+
+    const lineWarnings = lines.filter(line =>
+      line.includes('// Warning:'),
+    ).length;
+
+    const trailerStart = lines.findIndex(
+      line => line === '// Warnings were encountered during analysis:',
+    );
+    const trailerWarnings =
+      trailerStart === -1
+        ? 0
+        : lines.length -
+          trailerStart -
+          4; /* 4 lines at the trailer and after are not warnings */
+
+    return lineWarnings + trailerWarnings;
+  } catch (error) {
+    if (error.code === 'ENOENT') {
+      return 0;
+    }
+    throw error;
+  }
+}
+
 async function getTsDocConfig() {
   const tsdocConfigFile = await TSDocConfigFile.loadFile(
     require.resolve('@microsoft/api-extractor/extends/tsdoc-base.json'),
@@ -331,11 +360,15 @@ async function runApiExtraction({
 
   let compilerState: CompilerState | undefined = undefined;
 
+  const warnings = new Array<string>();
+
   for (const packageDir of packageDirs) {
     console.log(`## Processing ${packageDir}`);
     const projectFolder = resolvePath(__dirname, '..', packageDir);
     const packageFolder = resolvePath(__dirname, '../dist-types', packageDir);
 
+    const warningCountBefore = await countApiReportWarnings(projectFolder);
+
     const extractorConfig = ExtractorConfig.prepare({
       configObject: {
         mainEntryPointFilePath: resolvePath(packageFolder, 'src/index.d.ts'),
@@ -470,6 +503,22 @@ async function runApiExtraction({
           ` and ${extractorResult.warningCount} warnings`,
       );
     }
+
+    const warningCountAfter = await countApiReportWarnings(projectFolder);
+    if (warningCountAfter > warningCountBefore) {
+      warnings.push(
+        `The API Report for ${packageDir} introduces new warnings. ` +
+          'Please fix these warnings in order to keep the API Reports tidy.',
+      );
+    }
+  }
+
+  if (warnings.length > 0) {
+    console.warn();
+    for (const warning of warnings) {
+      console.warn(warning);
+    }
+    console.warn();
   }
 }