feat(eslint-plugin): add no-meaningless-void-operator rule (#3641)

packages/eslint-plugin/README.md

@@ -128,6 +128,7 @@ Pro Tip: For larger codebases you may want to consider splitting our linting int
 | [`@typescript-eslint/no-implicit-any-catch`](./docs/rules/no-implicit-any-catch.md)                                   | Disallow usage of the implicit `any` type in catch clauses                                                              |                    | :wrench: |                   |
 | [`@typescript-eslint/no-inferrable-types`](./docs/rules/no-inferrable-types.md)                                       | Disallows explicit type declarations for variables or parameters initialized to a number, string, or boolean            | :white_check_mark: | :wrench: |                   |
 | [`@typescript-eslint/no-invalid-void-type`](./docs/rules/no-invalid-void-type.md)                                     | Disallows usage of `void` type outside of generic or return types                                                       |                    |          |                   |
+| [`@typescript-eslint/no-meaningless-void-operator`](./docs/rules/no-meaningless-void-operator.md)                     | Disallow the `void` operator except when used to discard a value                                                        |                    | :wrench: | :thought_balloon: |
 | [`@typescript-eslint/no-misused-new`](./docs/rules/no-misused-new.md)                                                 | Enforce valid definition of `new` and `constructor`                                                                     | :white_check_mark: |          |                   |
 | [`@typescript-eslint/no-misused-promises`](./docs/rules/no-misused-promises.md)                                       | Avoid using promises in places not designed to handle them                                                              | :white_check_mark: |          | :thought_balloon: |
 | [`@typescript-eslint/no-namespace`](./docs/rules/no-namespace.md)                                                     | Disallow the use of custom TypeScript modules and namespaces                                                            | :white_check_mark: |          |                   |

packages/eslint-plugin/docs/rules/no-meaningless-void-operator.md

@@ -0,0 +1,50 @@
+# Disallow the `void` operator except when used to discard a value (`no-meaningless-void-operator`)
+
+Disallow the `void` operator when its argument is already of type `void` or `undefined`.
+
+## Rule Details
+
+The `void` operator is a useful tool to convey the programmer's intent to discard a value. For example, it is recommended as one way of suppressing [`@typescript-eslint/no-floating-promises`](./no-floating-promises.md) instead of adding `.catch()` to a promise.
+
+This rule helps an author catch API changes where previously a value was being discarded at a call site, but the callee changed so it no longer returns a value. When combined with [no-unused-expressions](https://eslint.org/docs/rules/no-unused-expressions), it also helps _readers_ of the code by ensuring consistency: a statement that looks like `void foo();` is **always** discarding a return value, and a statement that looks like `foo();` is **never** discarding a return value.
+
+Examples of **incorrect** code for this rule:
+
+```ts
+void (() => {})();
+
+function foo() {}
+void foo();
+```
+
+Examples of **correct** code for this rule:
+
+```ts
+(() => {})();
+
+function foo() {}
+foo(); // nothing to discard
+
+function bar(x: number) {
+  void x; // discarding a number
+  return 2;
+}
+void bar(); // discarding a number
+```
+
+### Options
+
+This rule accepts a single object option with the following default configuration:
+
+```json
+{
+  "@typescript-eslint/no-meaningless-void-operator": [
+    "error",
+    {
+      "checkNever": false
+    }
+  ]
+}
+```
+
+- `checkNever: true` will suggest removing `void` when the argument has type `never`.

packages/eslint-plugin/src/configs/all.ts

@@ -78,6 +78,7 @@ export = {
     '@typescript-eslint/no-loss-of-precision': 'error',
     'no-magic-numbers': 'off',
     '@typescript-eslint/no-magic-numbers': 'error',
+    '@typescript-eslint/no-meaningless-void-operator': 'error',
     '@typescript-eslint/no-misused-new': 'error',
     '@typescript-eslint/no-misused-promises': 'error',
     '@typescript-eslint/no-namespace': 'error',

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

@@ -50,6 +50,7 @@ import noInvalidVoidType from './no-invalid-void-type';
 import noLoopFunc from './no-loop-func';
 import noLossOfPrecision from './no-loss-of-precision';
 import noMagicNumbers from './no-magic-numbers';
+import noMeaninglessVoidOperator from './no-meaningless-void-operator';
 import noMisusedNew from './no-misused-new';
 import noMisusedPromises from './no-misused-promises';
 import noNamespace from './no-namespace';
@@ -170,6 +171,7 @@ export default {
   'no-loop-func': noLoopFunc,
   'no-loss-of-precision': noLossOfPrecision,
   'no-magic-numbers': noMagicNumbers,
+  'no-meaningless-void-operator': noMeaninglessVoidOperator,
   'no-misused-new': noMisusedNew,
   'no-misused-promises': noMisusedPromises,
   'no-namespace': noNamespace,

packages/eslint-plugin/src/rules/no-meaningless-void-operator.ts

@@ -0,0 +1,100 @@
+import {
+  ESLintUtils,
+  TSESLint,
+  TSESTree,
+} from '@typescript-eslint/experimental-utils';
+import * as tsutils from 'tsutils';
+import * as util from '../util';
+import * as ts from 'typescript';
+
+type Options = [
+  {
+    checkNever: boolean;
+  },
+];
+
+export default util.createRule<
+  Options,
+  'meaninglessVoidOperator' | 'removeVoid'
+>({
+  name: 'no-meaningless-void-operator',
+  meta: {
+    type: 'suggestion',
+    docs: {
+      description:
+        'Disallow the `void` operator except when used to discard a value',
+      category: 'Best Practices',
+      recommended: false,
+      suggestion: true,
+      requiresTypeChecking: true,
+    },
+    fixable: 'code',
+    messages: {
+      meaninglessVoidOperator:
+        "void operator shouldn't be used on {{type}}; it should convey that a return value is being ignored",
+      removeVoid: "Remove 'void'",
+    },
+    schema: [
+      {
+        type: 'object',
+        properties: {
+          checkNever: {
+            type: 'boolean',
+            default: false,
+          },
+        },
+        additionalProperties: false,
+      },
+    ],
+  },
+  defaultOptions: [{ checkNever: false }],
+
+  create(context, [{ checkNever }]) {
+    const parserServices = ESLintUtils.getParserServices(context);
+    const checker = parserServices.program.getTypeChecker();
+    const sourceCode = context.getSourceCode();
+
+    return {
+      'UnaryExpression[operator="void"]'(node: TSESTree.UnaryExpression): void {
+        const fix = (fixer: TSESLint.RuleFixer): TSESLint.RuleFix => {
+          return fixer.removeRange([
+            sourceCode.getTokens(node)[0].range[0],
+            sourceCode.getTokens(node)[1].range[0],
+          ]);
+        };
+
+        const argTsNode = parserServices.esTreeNodeToTSNodeMap.get(
+          node.argument,
+        );
+        const argType = checker.getTypeAtLocation(argTsNode);
+        const unionParts = tsutils.unionTypeParts(argType);
+        if (
+          unionParts.every(
+            part => part.flags & (ts.TypeFlags.Void | ts.TypeFlags.Undefined),
+          )
+        ) {
+          context.report({
+            node,
+            messageId: 'meaninglessVoidOperator',
+            data: { type: checker.typeToString(argType) },
+            fix,
+          });
+        } else if (
+          checkNever &&
+          unionParts.every(
+            part =>
+              part.flags &
+              (ts.TypeFlags.Void | ts.TypeFlags.Undefined | ts.TypeFlags.Never),
+          )
+        ) {
+          context.report({
+            node,
+            messageId: 'meaninglessVoidOperator',
+            data: { type: checker.typeToString(argType) },
+            suggest: [{ messageId: 'removeVoid', fix }],
+          });
+        }
+      },
+    };
+  },
+});

packages/eslint-plugin/tests/rules/no-meaningless-void-operator.test.ts

@@ -0,0 +1,90 @@
+import rule from '../../src/rules/no-meaningless-void-operator';
+import { RuleTester, getFixturesRootDir } from '../RuleTester';
+
+const rootDir = getFixturesRootDir();
+
+const ruleTester = new RuleTester({
+  parserOptions: {
+    ecmaVersion: 2018,
+    tsconfigRootDir: rootDir,
+    project: './tsconfig.json',
+  },
+  parser: '@typescript-eslint/parser',
+});
+
+ruleTester.run('no-meaningless-void-operator', rule, {
+  valid: [
+    `
+(() => {})();
+
+function foo() {}
+foo(); // nothing to discard
+
+function bar(x: number) {
+  void x;
+  return 2;
+}
+void bar(); // discarding a number
+    `,
+    `
+function bar(x: never) {
+  void x;
+}
+    `,
+  ],
+  invalid: [
+    {
+      code: 'void (() => {})();',
+      output: '(() => {})();',
+      errors: [
+        {
+          messageId: 'meaninglessVoidOperator',
+          line: 1,
+          column: 1,
+        },
+      ],
+    },
+    {
+      code: `
+function foo() {}
+void foo();
+      `,
+      output: `
+function foo() {}
+foo();
+      `,
+      errors: [
+        {
+          messageId: 'meaninglessVoidOperator',
+          line: 3,
+          column: 1,
+        },
+      ],
+    },
+    {
+      options: [{ checkNever: true }],
+      code: `
+function bar(x: never) {
+  void x;
+}
+      `.trimRight(),
+      errors: [
+        {
+          messageId: 'meaninglessVoidOperator',
+          line: 3,
+          column: 3,
+          suggestions: [
+            {
+              messageId: 'removeVoid',
+              output: `
+function bar(x: never) {
+  x;
+}
+              `.trimRight(),
+            },
+          ],
+        },
+      ],
+    },
+  ],
+});