Add no-async-fn-without-await rule (fixes #132) (#140)

Author: jfmengels

docs/rules/no-async-fn-without-await.md

@@ -0,0 +1,33 @@
+# Ensure that async tests use `await`
+
+Translations: [Français](https://github.com/avajs/ava-docs/blob/master/fr_FR/related/eslint-plugin-ava/docs/rules/no-async-fn-without-await.md)
+
+AVA comes with built-in support for [async functions](http://www.2ality.com/2016/02/async-functions.html) (async/await). This allows you to write shorter and clearer tests.
+
+Declaring an async test without using the `await` keyword means that either a Promise is not awaited on as intended, or that the function could have been declared as a regular function, which is confusing and slower.
+
+This rule will report an error when it finds an async test which does not use the `await` keyword.
+
+
+## Fail
+
+```js
+import test from 'ava';
+
+test(async t => {
+	return foo().then(res => {
+		t.is(res, 1);
+	});
+});
+```
+
+
+## Pass
+
+```js
+import test from 'ava';
+
+test(async t => {
+	t.is(await foo(), 1);
+});
+```

index.js

@@ -16,6 +16,7 @@ module.exports = {
 				'ava/assertion-arguments': 'error',
 				'ava/assertion-message': ['off', 'always'],
 				'ava/max-asserts': ['off', 5],
+				'ava/no-async-fn-without-await': 'error',
 				'ava/no-cb-test': 'off',
 				'ava/no-duplicate-modifiers': 'error',
 				'ava/no-identical-title': 'error',

package.json

@@ -73,6 +73,7 @@
   },
   "devDependencies": {
     "ava": "*",
+    "babel-eslint": "^6.1.2",
     "coveralls": "^2.11.9",
     "eslint": "^3.0.1",
     "eslint-ava-rule-tester": "^2.0.0",

readme.md

@@ -34,6 +34,7 @@ Configure it in `package.json`.
 			"ava/assertion-arguments": "error",
 			"ava/assertion-message": ["off", "always"],
 			"ava/max-asserts": ["off", 5],
+			"ava/no-async-fn-without-await": "error",
 			"ava/no-cb-test": "off",
 			"ava/no-duplicate-modifiers": "error",
 			"ava/no-identical-title": "error",
@@ -68,6 +69,7 @@ The rules will only activate in test files.
 - [assertion-arguments](docs/rules/assertion-arguments.md) - Enforce passing correct arguments to assertions.
 - [assertion-message](docs/rules/assertion-message.md) - Enforce or disallow assertion messages.
 - [max-asserts](docs/rules/max-asserts.md) - Limit the number of assertions in a test.
+- [no-async-fn-without-await](docs/rules/no-async-fn-without-await.md) - Ensure that async tests use `await`.
 - [no-cb-test](docs/rules/no-cb-test.md) - Ensure no `test.cb()` is used.
 - [no-duplicate-modifiers](docs/rules/no-duplicate-modifiers.md) - Ensure tests do not have duplicate modifiers.
 - [no-identical-title](docs/rules/no-identical-title.md) - Ensure no tests have the same title.

rules/no-async-fn-without-await.js

@@ -0,0 +1,42 @@
+'use strict';
+const visitIf = require('enhance-visitors').visitIf;
+const createAvaRule = require('../create-ava-rule');
+
+const create = context => {
+	const ava = createAvaRule();
+	let testIsAsync = false;
+	let testUsed = false;
+
+	return ava.merge({
+		'CallExpression': visitIf([
+			ava.isInTestFile,
+			ava.isTestNode
+		])(node => {
+			const implementationFn = node.arguments[0];
+			testIsAsync = implementationFn && implementationFn.async;
+		}),
+		'YieldExpression': () => {
+			if (testIsAsync) {
+				testUsed = true;
+			}
+		},
+		'CallExpression:exit': visitIf([
+			ava.isInTestFile,
+			ava.isTestNode
+		])(node => {
+			if (testIsAsync && !testUsed) {
+				context.report({
+					node,
+					message: 'Function was declared as `async` but doesn\'t use `await`'
+				});
+			}
+			testIsAsync = false;
+			testUsed = false;
+		})
+	});
+};
+
+module.exports = {
+	create,
+	meta: {}
+};

test/no-async-fn-without-await.js

@@ -0,0 +1,53 @@
+import test from 'ava';
+import avaRuleTester from 'eslint-ava-rule-tester';
+import rule from '../rules/no-async-fn-without-await';
+
+const ruleTester = avaRuleTester(test, {
+	parser: 'babel-eslint',
+	env: {
+		es6: true
+	}
+});
+
+const error = {
+	ruleId: 'no-async-fn-without-await',
+	message: 'Function was declared as `async` but doesn\'t use `await`'
+};
+const header = `const test = require('ava');\n`;
+
+ruleTester.run('no-async-fn-without-await', rule, {
+	valid: [
+		`${header} test(fn);`,
+		`${header} test(t => {});`,
+		`${header} test(function(t) {});`,
+		`${header} test(async t => { await foo(); });`,
+		`${header} test(async t => { t.is(await foo(), 1); });`,
+		`${header} test(async function(t) { await foo(); });`,
+		`${header} test(async t => { if (bar) { await foo(); } });`,
+		`${header} test(async t => { if (bar) {} else { await foo(); } });`,
+		// shouldn't be triggered since it's not a test file
+		'test(async t => {});'
+	],
+	invalid: [
+		{
+			code: `${header} test(async t => {});`,
+			errors: [error]
+		},
+		{
+			code: `${header} test(async function(t) {});`,
+			errors: [error]
+		},
+		{
+			code: `${header} test(async t => {}); test(async t => {});`,
+			errors: [error, error]
+		},
+		{
+			code: `${header} test(async t => {}); test(async t => { await foo(); });`,
+			errors: [error]
+		},
+		{
+			code: `${header} test(async t => { await foo(); }); test(async t => {});`,
+			errors: [error]
+		}
+	]
+});