[heft-lint-plugin] Lint files selected by ESLint flat config

build-tests/eslint-9-test/.eslint-bulk-suppressions.json

@@ -4,6 +4,11 @@
       "file": "src/index.ts",
       "scopeId": ".",
       "rule": "@typescript-eslint/naming-convention"
+    },
+    {
+      "file": "src/non-program.custom",
+      "scopeId": ".",
+      "rule": "no-undef"
     }
   ]
 }

build-tests/eslint-9-test/eslint.config.js

@@ -7,6 +7,9 @@ const nodeTrustedToolProfile = require('local-node-rig/profiles/default/includes
 const friendlyLocalsMixin = require('local-node-rig/profiles/default/includes/eslint/flat/mixins/friendly-locals');
 
 module.exports = [
+  {
+    ignores: ['coverage/**']
+  },
   ...nodeTrustedToolProfile,
   ...friendlyLocalsMixin,
   {
@@ -25,5 +28,11 @@ module.exports = [
         tsconfigRootDir: __dirname
       }
     }
+  },
+  {
+    files: ['**/*.custom'],
+    rules: {
+      'no-undef': 'warn'
+    }
   }
 ];

build-tests/eslint-9-test/src/__snapshots__/sarif.test.ts.snap

@@ -16,6 +16,11 @@ Object {
             "uri": "src/sarif.test.ts",
           },
         },
+        Object {
+          "location": Object {
+            "uri": "src/non-program.custom",
+          },
+        },
       ],
       "results": Array [
         Object {
@@ -78,6 +83,36 @@ Object {
             },
           ],
         },
+        Object {
+          "level": "warning",
+          "locations": Array [
+            Object {
+              "physicalLocation": Object {
+                "artifactLocation": Object {
+                  "index": 2,
+                  "uri": "src/non-program.custom",
+                },
+                "region": Object {
+                  "endColumn": 14,
+                  "endLine": 1,
+                  "startColumn": 1,
+                  "startLine": 1,
+                },
+              },
+            },
+          ],
+          "message": Object {
+            "text": "'missingGlobal' is not defined.",
+          },
+          "ruleId": "no-undef",
+          "ruleIndex": 2,
+          "suppressions": Array [
+            Object {
+              "justification": "",
+              "kind": "external",
+            },
+          ],
+        },
       ],
       "tool": Object {
         "driver": Object {
@@ -100,6 +135,14 @@ Object {
                 "text": "Enforce naming conventions for everything across a codebase",
               },
             },
+            Object {
+              "helpUri": "https://eslint.org/docs/latest/rules/no-undef",
+              "id": "no-undef",
+              "properties": Object {},
+              "shortDescription": Object {
+                "text": "Disallow the use of undeclared variables unless mentioned in \`/*global */\` comments",
+              },
+            },
           ],
           "version": "9.37.0",
         },

build-tests/eslint-9-test/src/non-program.custom

@@ -0,0 +1 @@
+missingGlobal;

common/changes/@rushstack/heft-lint-plugin/eslint-flat-config-files_2026-09-01-12-00-00.json

@@ -0,0 +1,9 @@
+{
+  "changes": [
+    {
+      "packageName": "@rushstack/heft-lint-plugin",
+      "comment": "Lint files selected by ESLint flat config even when they are not part of the TypeScript program.",
+      "type": "minor"
+    }
+  ]
+}

common/config/subspaces/build-tests-subspace/repo-state.json

@@ -1,6 +1,6 @@
 // DO NOT MODIFY THIS FILE MANUALLY BUT DO COMMIT IT. It is generated and used by Rush.
 {
-  "pnpmShrinkwrapHash": "2f7908424d103b2f677e95bcd5d85a385b75eda2",
+  "pnpmShrinkwrapHash": "dcbcfb259e8b79444cbdcba26d66a607fff9ce96",
   "preferredVersionsHash": "550b4cee0bef4e97db6c6aad726df5149d20e7d9",
   "packageJsonInjectedDependenciesHash": "e8fe4109038ad6e9b1e97cbb83e63d9094d37fe4"
 }

common/config/subspaces/default/repo-state.json

@@ -1,5 +1,5 @@
 // DO NOT MODIFY THIS FILE MANUALLY BUT DO COMMIT IT. It is generated and used by Rush.
 {
-  "pnpmShrinkwrapHash": "09068e8d50f8dae26938555320be968d7ed7861d",
+  "pnpmShrinkwrapHash": "f7863419452aa95244db5c362d58201a9620ee23",
   "preferredVersionsHash": "029c99bd6e65c5e1f25e2848340509811ff9753c"
 }

heft-plugins/heft-lint-plugin/src/Eslint.ts

@@ -11,10 +11,10 @@ import type * as TEslintLegacy from 'eslint-8';
 import * as semver from 'semver';
 import stableStringify from 'json-stable-stringify-without-jsonify';
 
-import { FileError, FileSystem } from '@rushstack/node-core-library';
+import { FileError, FileSystem, Path } from '@rushstack/node-core-library';
 import type { HeftConfiguration } from '@rushstack/heft';
 
-import { LinterBase, type ILinterBaseOptions } from './LinterBase';
+import { LinterBase, type IAdditionalLintFile, type ILinterBaseOptions } from './LinterBase';
 import type { IExtendedSourceFile } from './internalTypings/TypeScriptInternals';
 import { name as pluginName, version as pluginVersion } from '../package.json';
 
@@ -81,20 +81,29 @@ const ESLINT_LEGACY_CONFIG_FILENAMES: Set<string> = new Set([
   LEGACY_ESLINTRC_JS_FILENAME,
   LEGACY_ESLINTRC_CJS_FILENAME
 ]);
+const ESLINT_DEFAULT_EXTENSIONS: Set<string> = new Set(['.js', '.mjs', '.cjs']);
 
-export class Eslint extends LinterBase<TEslint.ESLint.LintResult | TEslintLegacy.ESLint.LintResult> {
+export class Eslint extends LinterBase<
+  TEslint.ESLint.LintResult | TEslintLegacy.ESLint.LintResult,
+  IAdditionalLintFile
+> {
   private readonly _eslintPackage: typeof TEslint | typeof TEslintLegacy;
   private readonly _eslintPackageVersion: semver.SemVer;
   private readonly _linter: TEslint.ESLint | TEslintLegacy.ESLint;
+  private readonly _additionalFilesLinter: TEslint.ESLint | undefined;
   private readonly _eslintTimings: Map<string, number> = new Map();
   private readonly _currentFixMessages: (TEslint.Linter.LintMessage | TEslintLegacy.Linter.LintMessage)[] =
     [];
   private readonly _fixMessagesByResult: Map<
     TEslint.ESLint.LintResult | TEslintLegacy.ESLint.LintResult,
     (TEslint.Linter.LintMessage | TEslintLegacy.Linter.LintMessage)[]
   > = new Map();
+  private readonly _additionalLintResults: Set<TEslint.ESLint.LintResult | TEslintLegacy.ESLint.LintResult> =
+    new Set();
   private readonly _sarifLogPath: string | undefined;
   private readonly _configHashMap: WeakMap<object, string> = new WeakMap();
+  private readonly _fileEnumerator: TEslint.ESLint | undefined;
+  private readonly _typeScriptFilenames: ReadonlySet<string>;
 
   protected constructor(options: IEslintOptions) {
     super('eslint', options);
@@ -106,7 +115,8 @@ export class Eslint extends LinterBase<TEslint.ESLint.LintResult | TEslintLegacy
       tsProgram,
       eslintTimings,
       fix,
-      sarifLogPath
+      sarifLogPath,
+      additionalFileIgnorePatterns
     } = options;
     this._eslintPackage = eslintPackage;
     this._eslintPackageVersion = new semver.SemVer(eslintPackage.ESLint.version);
@@ -195,6 +205,32 @@ export class Eslint extends LinterBase<TEslint.ESLint.LintResult | TEslintLegacy
       // eslint-disable-next-line @typescript-eslint/no-explicit-any
       fix: fixFn as any
     });
+
+    this._typeScriptFilenames = new Set(
+      tsProgram.getRootFileNames().map((filePath: string) => path.resolve(filePath))
+    );
+    if (this._eslintPackageVersion.major >= 9) {
+      const typeScriptIgnorePatterns: string[] = Array.from(this._typeScriptFilenames, (filePath: string) =>
+        Path.convertToSlashes(path.relative(buildFolderPath, filePath))
+      ).filter((relativePath: string) => relativePath !== '..' && !relativePath.startsWith('../'));
+      const flatEslintPackage: typeof TEslint = eslintPackage as typeof TEslint;
+      this._additionalFilesLinter = new flatEslintPackage.ESLint({
+        cwd: buildFolderPath,
+        overrideConfigFile: linterConfigFilePath,
+        fix: fixFn
+      });
+      this._fileEnumerator = new flatEslintPackage.ESLint({
+        cwd: buildFolderPath,
+        errorOnUnmatchedPattern: false,
+        overrideConfigFile: linterConfigFilePath,
+        overrideConfig: {
+          name: `${pluginName}/ignore-typescript-program-files`,
+          ignores: [...typeScriptIgnorePatterns, ...(additionalFileIgnorePatterns || [])]
+        },
+        ruleFilter: () => false
+      });
+    }
+
     this._eslintTimings = eslintTimings;
   }
 
@@ -250,12 +286,45 @@ export class Eslint extends LinterBase<TEslint.ESLint.LintResult | TEslintLegacy
     }
   }
 
+  public async getAdditionalLintFilesAsync(): Promise<ReadonlySet<IAdditionalLintFile>> {
+    if (!this._fileEnumerator) {
+      return new Set();
+    }
+
+    const lintResults: TEslint.ESLint.LintResult[] = await this._fileEnumerator.lintFiles(['.']);
+    lintResults.sort((left: TEslint.ESLint.LintResult, right: TEslint.ESLint.LintResult) =>
+      left.filePath.localeCompare(right.filePath)
+    );
+
+    const additionalLintFiles: IAdditionalLintFile[] = await Promise.all(
+      lintResults
+        .filter(
+          (lintResult: TEslint.ESLint.LintResult) =>
+            !this._typeScriptFilenames.has(path.resolve(lintResult.filePath)) &&
+            !ESLINT_DEFAULT_EXTENSIONS.has(path.extname(lintResult.filePath))
+        )
+        .map(async (lintResult: TEslint.ESLint.LintResult): Promise<IAdditionalLintFile> => {
+          return {
+            kind: 'additional',
+            fileName: lintResult.filePath,
+            text: await FileSystem.readFileAsync(lintResult.filePath),
+            version: ''
+          };
+        })
+    );
+
+    return new Set(additionalLintFiles);
+  }
+
   protected override async getCacheVersionAsync(): Promise<string> {
     return `${this._eslintPackageVersion.version}_${process.version}`;
   }
 
-  protected override async getSourceFileHashAsync(sourceFile: IExtendedSourceFile): Promise<string> {
-    const sourceFileEslintConfiguration: TEslint.Linter.Config = await this._linter.calculateConfigForFile(
+  protected override async getSourceFileHashAsync(
+    sourceFile: IExtendedSourceFile | IAdditionalLintFile
+  ): Promise<string> {
+    const linter: TEslint.ESLint | TEslintLegacy.ESLint = this._getLinterForSourceFile(sourceFile);
+    const sourceFileEslintConfiguration: TEslint.Linter.Config = await linter.calculateConfigForFile(
       sourceFile.fileName
     );
 
@@ -272,10 +341,11 @@ export class Eslint extends LinterBase<TEslint.ESLint.LintResult | TEslintLegacy
   }
 
   protected override async lintFileAsync(
-    sourceFile: TTypescript.SourceFile
+    sourceFile: TTypescript.SourceFile | IAdditionalLintFile
   ): Promise<TEslint.ESLint.LintResult[] | TEslintLegacy.ESLint.LintResult[]> {
+    const linter: TEslint.ESLint | TEslintLegacy.ESLint = this._getLinterForSourceFile(sourceFile);
     const lintResults: TEslint.ESLint.LintResult[] | TEslintLegacy.ESLint.LintResult[] =
-      await this._linter.lintText(sourceFile.text, { filePath: sourceFile.fileName });
+      await linter.lintText(sourceFile.text, { filePath: sourceFile.fileName });
 
     // Map the fix messages to the results. This API should only return one result per file, so we can be sure
     // that the fix messages belong to the returned result. If we somehow receive multiple results, we will
@@ -286,6 +356,12 @@ export class Eslint extends LinterBase<TEslint.ESLint.LintResult | TEslintLegacy
       this._fixMessagesByResult.set(lintResults[0], fixMessages);
     }
 
+    if (linter === this._additionalFilesLinter) {
+      for (const lintResult of lintResults) {
+        this._additionalLintResults.add(lintResult);
+      }
+    }
+
     this._fixesPossible ||=
       !this._fix &&
       lintResults.some((lintResult: TEslint.ESLint.LintResult | TEslintLegacy.ESLint.LintResult) => {
@@ -349,8 +425,19 @@ export class Eslint extends LinterBase<TEslint.ESLint.LintResult | TEslintLegacy
 
     const sarifLogPath: string | undefined = this._sarifLogPath;
     if (sarifLogPath) {
-      const rulesMeta: TEslint.ESLint.LintResultData['rulesMeta'] =
-        this._linter.getRulesMetaForResults(lintResults);
+      const primaryLintResults: TEslint.ESLint.LintResult[] = [];
+      const additionalLintResults: TEslint.ESLint.LintResult[] = [];
+      for (const lintResult of lintResults) {
+        const resultSet: TEslint.ESLint.LintResult[] = this._additionalLintResults.has(lintResult)
+          ? additionalLintResults
+          : primaryLintResults;
+        resultSet.push(lintResult);
+      }
+
+      const rulesMeta: TEslint.ESLint.LintResultData['rulesMeta'] = {
+        ...this._linter.getRulesMetaForResults(primaryLintResults),
+        ...this._additionalFilesLinter?.getRulesMetaForResults(additionalLintResults)
+      };
       const { formatEslintResultsAsSARIF } = await import('./SarifFormatter');
       const sarifString: string = JSON.stringify(
         formatEslintResultsAsSARIF(lintResults, rulesMeta, {
@@ -380,6 +467,20 @@ export class Eslint extends LinterBase<TEslint.ESLint.LintResult | TEslintLegacy
     });
   }
 
+  private _getLinterForSourceFile(
+    sourceFile: TTypescript.SourceFile | IAdditionalLintFile
+  ): TEslint.ESLint | TEslintLegacy.ESLint {
+    if (sourceFile.kind === 'additional') {
+      if (!this._additionalFilesLinter) {
+        throw new Error('The ESLint instance for additional files has not been initialized.');
+      }
+
+      return this._additionalFilesLinter;
+    }
+
+    return this._linter;
+  }
+
   private _getLintFileError(
     lintResult: TEslint.ESLint.LintResult | TEslintLegacy.ESLint.LintResult,
     lintMessage: TEslint.Linter.LintMessage | TEslintLegacy.Linter.LintMessage,