fix(eslint-plugin): [no-unused-vars] correct detection of unused vars…

… in a declared module with `export =` (#2505)

If a `declare module` has an `export =` in its body, then TS will only export that.
If it doesn't have an `export =`, then all things are ambiently exported.

This adds handling to correctly detect this case.

packages/eslint-plugin/src/rules/adjacent-overload-signatures.ts

@@ -10,7 +10,10 @@ type RuleNode =
   | TSESTree.TSModuleBlock
   | TSESTree.TSTypeLiteral
   | TSESTree.TSInterfaceBody;
-type Member = TSESTree.ClassElement | TSESTree.Statement | TSESTree.TypeElement;
+type Member =
+  | TSESTree.ClassElement
+  | TSESTree.ProgramStatement
+  | TSESTree.TypeElement;
 
 export default util.createRule({
   name: 'adjacent-overload-signatures',

packages/eslint-plugin/src/rules/indent.ts

@@ -410,7 +410,7 @@ export default util.createRule<Options, MessageIds>({
         // transform it to a BlockStatement
         return rules['BlockStatement, ClassBody']({
           type: AST_NODE_TYPES.BlockStatement,
-          body: node.body,
+          body: node.body as any,
 
           // location data
           parent: node.parent,

packages/eslint-plugin/src/rules/no-unused-vars.ts

@@ -29,6 +29,7 @@ export default util.createRule<Options, MessageIds>({
   create(context) {
     const rules = baseRule.create(context);
     const filename = context.getFilename();
+    const MODULE_DECL_CACHE = new Map<TSESTree.TSModuleDeclaration, boolean>();
 
     /**
      * Gets a list of TS module definitions for a specified variable.
@@ -209,7 +210,7 @@ export default util.createRule<Options, MessageIds>({
       },
 
       // declaration file handling
-      [declarationSelector(AST_NODE_TYPES.Program, true)](
+      [ambientDeclarationSelector(AST_NODE_TYPES.Program, true)](
         node: DeclarationSelectorNode,
       ): void {
         if (!util.isDefinitionFile(filename)) {
@@ -219,14 +220,44 @@ export default util.createRule<Options, MessageIds>({
       },
 
       // declared namespace handling
-      [declarationSelector(
+      [ambientDeclarationSelector(
         'TSModuleDeclaration[declare = true] > TSModuleBlock',
         false,
       )](node: DeclarationSelectorNode): void {
+        const moduleDecl = util.nullThrows(
+          node.parent?.parent,
+          util.NullThrowsReasons.MissingParent,
+        ) as TSESTree.TSModuleDeclaration;
+
+        // declared modules with an `export =` statement will only export that one thing
+        // all other statements are not automatically exported in this case
+        if (checkModuleDeclForExportEquals(moduleDecl)) {
+          return;
+        }
+
         markDeclarationChildAsUsed(node);
       },
     };
 
+    function checkModuleDeclForExportEquals(
+      node: TSESTree.TSModuleDeclaration,
+    ): boolean {
+      const cached = MODULE_DECL_CACHE.get(node);
+      if (cached != null) {
+        return cached;
+      }
+
+      for (const statement of node.body?.body ?? []) {
+        if (statement.type === AST_NODE_TYPES.TSExportAssignment) {
+          MODULE_DECL_CACHE.set(node, true);
+          return true;
+        }
+      }
+
+      MODULE_DECL_CACHE.set(node, false);
+      return false;
+    }
+
     type DeclarationSelectorNode =
       | TSESTree.TSInterfaceDeclaration
       | TSESTree.TSTypeAliasDeclaration
@@ -236,7 +267,7 @@ export default util.createRule<Options, MessageIds>({
       | TSESTree.TSEnumDeclaration
       | TSESTree.TSModuleDeclaration
       | TSESTree.VariableDeclaration;
-    function declarationSelector(
+    function ambientDeclarationSelector(
       parent: string,
       childDeclare: boolean,
     ): string {

packages/eslint-plugin/tests/rules/no-unused-vars.test.ts

@@ -848,6 +848,18 @@ export type Test<U> = U extends (arg: {
         jsxFragmentName: 'Fragment',
       },
     },
+    `
+declare module 'foo' {
+  type Test = 1;
+}
+    `,
+    `
+declare module 'foo' {
+  type Test = 1;
+  const x: Test = 1;
+  export = x;
+}
+    `,
   ],
 
   invalid: [
@@ -1424,5 +1436,25 @@ export const ComponentFoo = () => {
         },
       ],
     },
+    {
+      code: `
+declare module 'foo' {
+  type Test = any;
+  const x = 1;
+  export = x;
+}
+      `,
+      errors: [
+        {
+          messageId: 'unusedVar',
+          line: 3,
+          data: {
+            varName: 'Test',
+            action: 'defined',
+            additional: '',
+          },
+        },
+      ],
+    },
   ],
 });

packages/eslint-plugin/typings/eslint-rules.d.ts

@@ -48,7 +48,6 @@ declare module 'eslint/lib/rules/camelcase' {
 declare module 'eslint/lib/rules/indent' {
   import { TSESLint, TSESTree } from '@typescript-eslint/experimental-utils';
 
-  type Listener = (node: TSESTree.Node) => void;
   type ElementList = number | 'first' | 'off';
   const rule: TSESLint.RuleModule<
     'wrongIndentation',

packages/scope-manager/src/scope/ScopeBase.ts

@@ -15,6 +15,7 @@ import {
   ReferenceTypeFlag,
 } from '../referencer/Reference';
 import { Variable } from '../variable';
+import { TSModuleScope } from './TSModuleScope';
 
 /**
  * Test if scope is strict
@@ -124,6 +125,14 @@ function registerScope(scopeManager: ScopeManager, scope: Scope): void {
 
 const generator = createIdGenerator();
 
+type VariableScope = GlobalScope | FunctionScope | ModuleScope | TSModuleScope;
+const VARIABLE_SCOPE_TYPES = new Set([
+  ScopeType.global,
+  ScopeType.function,
+  ScopeType.module,
+  ScopeType.tsModule,
+]);
+
 type AnyScope = ScopeBase<ScopeType, TSESTree.Node, Scope | null>;
 abstract class ScopeBase<
   TType extends ScopeType,
@@ -209,11 +218,11 @@ abstract class ScopeBase<
    */
   public readonly variables: Variable[] = [];
   /**
-   * For 'global', 'function', and 'module' scopes, this is a self-reference.
+   * For scopes that can contain variable declarations, this is a self-reference.
    * For other scope types this is the *variableScope* value of the parent scope.
    * @public
    */
-  public readonly variableScope: GlobalScope | FunctionScope | ModuleScope;
+  public readonly variableScope: VariableScope;
 
   constructor(
     scopeManager: ScopeManager,
@@ -228,12 +237,9 @@ abstract class ScopeBase<
     this.#dynamic =
       this.type === ScopeType.global || this.type === ScopeType.with;
     this.block = block;
-    this.variableScope =
-      this.type === 'global' ||
-      this.type === 'function' ||
-      this.type === 'module'
-        ? (this as AnyScope['variableScope'])
-        : upperScopeAsScopeBase.variableScope;
+    this.variableScope = this.isVariableScope()
+      ? this
+      : upperScopeAsScopeBase.variableScope;
     this.upper = upperScope;
 
     /**
@@ -252,6 +258,10 @@ abstract class ScopeBase<
     registerScope(scopeManager, this as Scope);
   }
 
+  private isVariableScope(): this is VariableScope {
+    return VARIABLE_SCOPE_TYPES.has(this.type);
+  }
+
   public shouldStaticallyClose(): boolean {
     return !this.#dynamic;
   }

packages/types/src/ts-estree.ts

@@ -457,6 +457,20 @@ export type PrimaryExpression =
   | TemplateLiteral
   | ThisExpression
   | TSNullKeyword;
+export type ProgramStatement =
+  | ClassDeclaration
+  | ExportAllDeclaration
+  | ExportDefaultDeclaration
+  | ExportNamedDeclaration
+  | ImportDeclaration
+  | Statement
+  | TSDeclareFunction
+  | TSEnumDeclaration
+  | TSExportAssignment
+  | TSImportEqualsDeclaration
+  | TSInterfaceDeclaration
+  | TSNamespaceExportDeclaration
+  | TSTypeAliasDeclaration;
 export type Property = PropertyComputedName | PropertyNonComputedName;
 export type PropertyName = PropertyNameComputed | PropertyNameNonComputed;
 export type PropertyNameComputed = Expression;
@@ -1146,7 +1160,7 @@ export interface ObjectPattern extends BaseNode {
 
 export interface Program extends BaseNode {
   type: AST_NODE_TYPES.Program;
-  body: Statement[];
+  body: ProgramStatement[];
   sourceType: 'module' | 'script';
   comments?: Comment[];
   tokens?: Token[];
@@ -1473,7 +1487,7 @@ export interface TSMethodSignatureNonComputedName
 
 export interface TSModuleBlock extends BaseNode {
   type: AST_NODE_TYPES.TSModuleBlock;
-  body: Statement[];
+  body: ProgramStatement[];
 }
 
 export interface TSModuleDeclaration extends BaseNode {