fix(linter): support lib as standalone src path (#26263)

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## Current Behavior
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Root projects are considered standalone projects which shouldn't lint everything if the `src` directory exists.

## Expected Behavior
<!-- This is the behavior we should expect with the changes in this PR
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Root projects which have a `lib` directory should be treated the same way.

## Related Issue(s)
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merged. -->

Fixes #

packages/eslint/src/plugins/plugin.spec.ts

@@ -120,6 +120,47 @@ describe('@nx/eslint/plugin', () => {
       `);
     });
 
+    it('should create a node for just a package.json and root level eslint config if accompanied by a lib directory', async () => {
+      createFiles({
+        '.eslintrc.json': `{}`,
+        'package.json': `{}`,
+        'lib/index.ts': `console.log('hello world')`,
+      });
+      // NOTE: The command is specifically targeting the src directory in the case of a standalone Nx workspace
+      expect(await invokeCreateNodesOnMatchingFiles(context, 'lint'))
+        .toMatchInlineSnapshot(`
+        {
+          "projects": {
+            ".": {
+              "targets": {
+                "lint": {
+                  "cache": true,
+                  "command": "eslint ./lib",
+                  "inputs": [
+                    "default",
+                    "^default",
+                    "{projectRoot}/eslintrc.json",
+                    "{workspaceRoot}/tools/eslint-rules/**/*",
+                    {
+                      "externalDependencies": [
+                        "eslint",
+                      ],
+                    },
+                  ],
+                  "options": {
+                    "cwd": ".",
+                  },
+                  "outputs": [
+                    "{options.outputFile}",
+                  ],
+                },
+              },
+            },
+          },
+        }
+      `);
+    });
+
     it('should not create a node for just a package.json and root level eslint config if accompanied by a src directory when all files are ignored (.eslintignore)', async () => {
       createFiles({
         '.eslintrc.json': `{}`,

packages/eslint/src/plugins/plugin.ts

@@ -128,12 +128,18 @@ function getProjectsUsingESLintConfig(
 
   // Add a lint target for each child project without an eslint config, with the root level config as an input
   for (const projectRoot of childProjectRoots) {
-    // If there's no src folder, it's not a standalone project, do not add the target at all
-    const isStandaloneWorkspace =
+    let standaloneSrcPath: string | undefined;
+    if (
       projectRoot === '.' &&
-      existsSync(join(context.workspaceRoot, projectRoot, 'src')) &&
-      existsSync(join(context.workspaceRoot, projectRoot, 'package.json'));
-    if (projectRoot === '.' && !isStandaloneWorkspace) {
+      existsSync(join(context.workspaceRoot, projectRoot, 'package.json'))
+    ) {
+      if (existsSync(join(context.workspaceRoot, projectRoot, 'src'))) {
+        standaloneSrcPath = 'src';
+      } else if (existsSync(join(context.workspaceRoot, projectRoot, 'lib'))) {
+        standaloneSrcPath = 'lib';
+      }
+    }
+    if (projectRoot === '.' && !standaloneSrcPath) {
       continue;
     }
 
@@ -150,7 +156,7 @@ function getProjectsUsingESLintConfig(
         projectRoot,
         context.workspaceRoot,
         options,
-        isStandaloneWorkspace
+        standaloneSrcPath
       ),
     };
   }
@@ -164,14 +170,16 @@ function buildEslintTargets(
   projectRoot: string,
   workspaceRoot: string,
   options: EslintPluginOptions,
-  isStandaloneWorkspace = false
+  standaloneSrcPath?: string
 ) {
   const isRootProject = projectRoot === '.';
 
   const targets: Record<string, TargetConfiguration> = {};
 
   const targetConfig: TargetConfiguration = {
-    command: `eslint ${isRootProject && isStandaloneWorkspace ? './src' : '.'}`,
+    command: `eslint ${
+      isRootProject && standaloneSrcPath ? `./${standaloneSrcPath}` : '.'
+    }`,
     cache: true,
     options: {
       cwd: projectRoot,