Add linter rule: no function reassignment

packages/@romejs/js-compiler/__rtests__/lint.ts

@@ -10,6 +10,7 @@ import lint from '../api/lint';
 import {parseJS} from '@romejs/js-parser';
 import {createUnknownFilePath} from '@romejs/path';
 import {DEFAULT_PROJECT_CONFIG, ProjectConfig} from '@romejs/project';
+import {PartialDiagnostic} from '@romejs/diagnostics/types';
 import {ConstSourceType} from '@romejs/js-ast';
 
 const LINT_ENABLED_FORMAT_DISABLED_CONFIG: ProjectConfig = {
@@ -315,6 +316,48 @@ test('no duplicate keys', async t => {
   ]);
 });
 
+test('no function reassignment', async t => {
+  function checkCategory(diagnostic: PartialDiagnostic): Boolean {
+    return diagnostic.category === 'lint/noFunctionAssign';
+  }
+
+  const validTestCases = [
+    'function foo() { var foo = bar; }',
+    'function foo(foo) { foo = bar; }',
+    'function foo() { var foo; foo = bar; }',
+    'var foo = () => {}; foo = bar;',
+    'var foo = function() {}; foo = bar;',
+    'var foo = function() { foo = bar; };',
+    `import bar from 'bar'; function foo() { var foo = bar; }`,
+  ];
+
+  const invalidTestCases = [
+    'function foo() {}; foo = bar;',
+    'function foo() { foo = bar; }',
+    'foo = bar; function foo() { };',
+    '[foo] = bar; function foo() { };',
+    '({x: foo = 0} = bar); function foo() { };',
+    'function foo() { [foo] = bar; }',
+    '(function() { ({x: foo = 0} = bar); function foo() { }; })();',
+  ];
+
+  for (const testCase of validTestCases) {
+    const {diagnostics} = await testLint(
+      testCase,
+      LINT_ENABLED_FORMAT_DISABLED_CONFIG,
+    );
+    t.falsy(diagnostics.find(checkCategory));
+  }
+
+  for (const testCase of invalidTestCases) {
+    const {diagnostics} = await testLint(
+      testCase,
+      LINT_ENABLED_FORMAT_DISABLED_CONFIG,
+    );
+    t.truthy(diagnostics.find(checkCategory));
+  }
+});
+
 test('no duplicated args allowed', async t => {
   const duplicatedArgs = await testLint(
     `

packages/@romejs/js-compiler/transforms/lint/index.ts

@@ -18,6 +18,7 @@ import noDeleteVars from './noDeleteVars';
 import noDebugger from './noDebugger';
 import noDupeArgs from './noDupeArgs';
 import noDuplicateKeys from './noDuplicateKeys';
+import noFunctionAssign from './noFunctionAssign';
 import noImportAssign from './noImportAssign';
 import noLabelVar from './noLabelVar';
 import noTemplateCurlyInString from './noTemplateCurlyInString';
@@ -41,6 +42,7 @@ export const lintTransforms = [
   noDeleteVars,
   noDupeArgs,
   noDuplicateKeys,
+  noFunctionAssign,
   noImportAssign,
   noLabelVar,
   noTemplateCurlyInString,

packages/@romejs/js-compiler/transforms/lint/noFunctionAssign.ts

@@ -0,0 +1,27 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import {Path} from '@romejs/js-compiler';
+import {AnyNode} from '@romejs/js-ast';
+import {FunctionBinding} from '@romejs/js-compiler/scope/bindings';
+
+export default {
+  name: 'noFunctionAssign',
+  enter(path: Path): AnyNode {
+    const {node, scope} = path;
+    if (
+      node.type === 'AssignmentIdentifier' &&
+      scope.getBinding(node.name) instanceof FunctionBinding
+    ) {
+      path.context.addNodeDiagnostic(node, {
+        category: 'lint/noFunctionAssign',
+        message: 'Reassignment of function declaration',
+      });
+    }
+    return node;
+  },
+};