Add no-inline-assertions rule (#262)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

docs/rules/no-inline-assertions.md

@@ -0,0 +1,25 @@
+# Ensure assertions are not called from inline arrow functions
+
+The test implementation should not purely consist of an inline assertion as assertions do not return a value and having them inline also makes the tests less readable.
+
+This rule is fixable. It will wrap the assertion in braces `{}`. It will not do any whitespace or style changes.
+
+
+## Fail
+
+```js
+import test from 'ava';
+
+test('foo', t => t.true(fn()));
+```
+
+
+## Pass
+
+```js
+import test from 'ava';
+
+test('foo', t => {
+	t.true(fn());
+});
+```

index.js

@@ -28,6 +28,7 @@ module.exports = {
 				'ava/no-identical-title': 'error',
 				'ava/no-ignored-test-files': 'error',
 				'ava/no-import-test-files': 'error',
+				'ava/no-inline-assertions': 'error',
 				'ava/no-invalid-end': 'error',
 				'ava/no-nested-tests': 'error',
 				'ava/no-only-test': 'error',

readme.md

@@ -46,6 +46,7 @@ Configure it in `package.json`.
 			"ava/no-identical-title": "error",
 			"ava/no-ignored-test-files": "error",
 			"ava/no-import-test-files": "error",
+			"ava/no-inline-assertions": "error",
 			"ava/no-invalid-end": "error",
 			"ava/no-nested-tests": "error",
 			"ava/no-only-test": "error",
@@ -83,6 +84,7 @@ The rules will only activate in test files.
 - [no-identical-title](docs/rules/no-identical-title.md) - Ensure no tests have the same title.
 - [no-ignored-test-files](docs/rules/no-ignored-test-files.md) - Ensure no tests are written in ignored files.
 - [no-import-test-files](docs/rules/no-import-test-files.md) - Ensure no test files are imported anywhere.
+- [no-inline-assertions](docs/rules/no-inline-assertions.md) - Ensure assertions are not called from inline arrow functions. *(fixable)*
 - [no-invalid-end](docs/rules/no-invalid-end.md) - Ensure `t.end()` is only called inside `test.cb()`.
 - [no-nested-tests](docs/rules/no-nested-tests.md) - Ensure no tests are nested.
 - [no-only-test](docs/rules/no-only-test.md) - Ensure no `test.only()` are present. *(fixable)*

rules/no-inline-assertions.js

@@ -0,0 +1,46 @@
+'use strict';
+const {visitIf} = require('enhance-visitors');
+const createAvaRule = require('../create-ava-rule');
+const util = require('../util');
+
+const create = context => {
+	const ava = createAvaRule();
+
+	return ava.merge({
+		CallExpression: visitIf([
+			ava.isInTestFile,
+			ava.isTestNode
+		])(node => {
+			const functionArgIndex = node.arguments.length - 1;
+			if (functionArgIndex > 1) {
+				return;
+			}
+
+			const functionArg = node.arguments[functionArgIndex];
+
+			if (!util.isFunctionExpression(functionArg)) {
+				return;
+			}
+
+			const {body} = functionArg;
+			if (body.type === 'CallExpression') {
+				context.report({
+					node,
+					message: 'The test implementation should not be an inline arrow function.',
+					fix: fixer => [fixer.insertTextBefore(body, '{'), fixer.insertTextAfter(body, '}')]
+				});
+			}
+		})
+	});
+};
+
+module.exports = {
+	create,
+	meta: {
+		docs: {
+			url: util.getDocsUrl(__filename)
+		},
+		type: 'suggestion',
+		fixable: 'code'
+	}
+};

test/no-inline-assertions.js

@@ -0,0 +1,47 @@
+import test from 'ava';
+import avaRuleTester from 'eslint-ava-rule-tester';
+import rule from '../rules/no-inline-assertions';
+
+const ruleTester = avaRuleTester(test, {
+	env: {
+		es6: true
+	}
+});
+
+const errors = [{ruleId: 'no-inline-assertions'}];
+const header = 'const test = require(\'ava\');\n';
+
+ruleTester.run('no-todo-test', rule, {
+	valid: [
+		// Shouldn't be triggered as the test implementation is not an inline arrow function
+		header + 'test("my test name", t => {\n t.true(fn()); \n});',
+		header + 'test("my test name", function (t) { foo(); });',
+		// Shouldn't be triggered since test body is empty
+		header + 'test("my test name", () => {});',
+		header + 'test("my test name", t => {});',
+		// Shouldn't be triggered since test body is ill-defined
+		header + 'test("my test name", t => "foo");',
+		// Shouldn't be triggered since it's not a test file
+		'test.todo("my test name");',
+		// Shouldn't be triggered since the signature is incorrect
+		header + 'test.todo("my test name", "bar");',
+		header + 'test.todo("my test name", undefined, t => {})'
+	],
+	invalid: [
+		{
+			code: header + 'test("my test name", t => t.skip());',
+			errors,
+			output: header + 'test("my test name", t => {t.skip()});'
+		},
+		{
+			code: header + 'test("my test name", t => t.true(fn()));',
+			errors,
+			output: header + 'test("my test name", t => {t.true(fn())});'
+		},
+		{
+			code: header + 'test("my test name", t => \n t.true(fn()));',
+			errors,
+			output: header + 'test("my test name", t => \n {t.true(fn())});'
+		}
+	]
+});