fix(eslint-plugin): [promise-function-async] bad fixer with computed and literal methods

packages/eslint-plugin/src/rules/promise-function-async.ts

@@ -1,7 +1,9 @@
 import {
   AST_NODE_TYPES,
+  AST_TOKEN_TYPES,
   TSESTree,
 } from '@typescript-eslint/experimental-utils';
+import { last } from 'lodash';
 import * as ts from 'typescript';
 import * as util from '../util';
 
@@ -156,8 +158,39 @@ export default util.createRule<Options, MessageIds>({
               (node.parent.type === AST_NODE_TYPES.Property &&
                 node.parent.method))
           ) {
-            return fixer.insertTextBefore(node.parent.key, 'async ');
+            // this function is a class method or object function property shorthand
+            const method = node.parent;
+
+            /** the token to put `async` before */
+            let keyToken = sourceCode.getFirstToken(method)!;
+
+            // if there are decorators then skip past them
+            if (
+              method.type === AST_NODE_TYPES.MethodDefinition &&
+              method.decorators
+            ) {
+              const lastDecorator = last(method.decorators)!;
+              keyToken = sourceCode.getTokenAfter(lastDecorator)!;
+            }
+
+            // if current token is a keyword like `static` or `public` then skip it
+            while (keyToken.type === AST_TOKEN_TYPES.Keyword) {
+              keyToken = sourceCode.getTokenAfter(keyToken)!;
+            }
+
+            // check if there is a space between key and previous token
+            const insertSpace = !sourceCode.isSpaceBetween!(
+              sourceCode.getTokenBefore(keyToken)!,
+              keyToken,
+            );
+
+            let code = 'async ';
+            if (insertSpace) {
+              code = ` ${code}`;
+            }
+            return fixer.insertTextBefore(keyToken, code);
           }
+
           return fixer.insertTextBefore(node, 'async ');
         },
       });

packages/eslint-plugin/tests/rules/promise-function-async.test.ts

@@ -1,3 +1,4 @@
+import { noFormat } from '@typescript-eslint/experimental-utils/src/eslint-utils';
 import rule from '../../src/rules/promise-function-async';
 import { getFixturesRootDir, RuleTester } from '../RuleTester';
 
@@ -630,6 +631,41 @@ class Test {
   public async test() {
     return Promise.resolve(123);
   }
+}
+      `,
+    },
+    {
+      code: noFormat`
+class Test {
+  @decorator(async () => {})
+  static protected[(1)]() {
+    return Promise.resolve(1);
+  }
+  public'bar'() {
+    return Promise.resolve(2);
+  }
+  private['baz']() {
+    return Promise.resolve(3);
+  }
+}
+      `,
+      errors: [
+        { line: 4, column: 3, messageId },
+        { line: 7, column: 3, messageId },
+        { line: 10, column: 3, messageId },
+      ],
+      output: noFormat`
+class Test {
+  @decorator(async () => {})
+  static protected async [(1)]() {
+    return Promise.resolve(1);
+  }
+  public async 'bar'() {
+    return Promise.resolve(2);
+  }
+  private async ['baz']() {
+    return Promise.resolve(3);
+  }
 }
       `,
     },